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Counterfeit Deterrent Features for the Next-Generation Currency Design (1993)

Chapter: 4 Description and Assessment of Deterrent Features

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Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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4

DESCRIPTION AND ASSESSMENT OF DETERRENT FEATURES

A number of features are now being used in U.S. currency to deter counterfeiting, and many additional ones are being considered for future use to respond to the projected threats. This chapter contains a description of and results from the committee's assessment of categories of counterfeit-deterrence features that represent a wide variety of technologies. In line with the objectives of this study, the emphasis is on those features that are visible to the unaided eye. But less obvious features that can be easily inspected using relatively inexpensive aids have also been included. The first section discusses the features that are currently incorporated in U.S. banknotes. The following section discusses candidate features, grouped by generic classes. The last section is a summary of the recommendations.

Discussion of two types of visible deterrent features is contained within this chapter: passive and active. A passive feature is one that is difficult to reproduce or simulate in its own right; if it is copied or simulated, the fake can be detected by examining the quality of the reproduction. An active feature may not be an obvious feature on a genuine note; however it interacts with the reprographic process in some way, resulting in an obvious indication on the duplication attempt. The security thread is an example of a passive feature; an aliasing pattern is an example of an active feature.

Within the discussion of each feature class is a description of the feature, its significant advantages and limitations, and the committee 's assessment. The committee was guided by evaluation framework (presented in the previous chapter) in performing the assessment.

In some instances, a strong synergistic deterrent effect was found between features that were placed in different classes. Each class was assessed separately, but where appropriate the discussion points out the possible increase in effectiveness if one feature is used in conjunction with another. For instance, color by itself was not assessed to be a particularly effective deterrent; but the effectiveness of moiré patterns can be enhanced through the appropriate use of color. Suggested criteria for integrating multiple features are further discussed in the next chapter.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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CURRENTLY USED OVERT U.S. COUNTERFEIT-DETERRENCE FEATURES

U.S. currency presently in circulation has a number of features that serve to deter counterfeiting. Some of the features can be authenticated at points of sale through unaided visual inspection by inspection using a low-power magnifier or similar simple device. Other features require relatively sophisticated instrumental aids and will not be discussed further. This report focuses on overt, visible features that serve as counterfeit deterrents.

Paper

U.S. currency is printed on a special paper composed of cotton and linen fibers with no wood fibers or starch. Stringent specifications describe the fiber configuration, thickness, weight, color, and reflectance to ensure uniformity and durability of the paper (BEP, 1991). It has a distinctive feel that is readily detectable by many people who handle large amounts of currency; they can easily identify counterfeits that are not printed on the appropriate paper. Many of the counterfeits are detected at points of transaction by having a wrong “feel.” This distinctive paper, in combination with intaglio printing, provides significant counterfeit deterrence at a moderate cost. The BEP requires strict control of currency paper to ensure that it is not available to potential counterfeiters. Unauthorized possession or control of this or similar paper is a criminal offense. While it is theoretically possible that counterfeiters could make paper that is identical to the U.S. currency paper, it would be a major effort, requiring significant technical expertise, equipment, and monetary resources.

Red and Blue Fibers

U.S. currency paper contains red and blue fibers that are added to the paper slurry and become randomly dispersed throughout the paper. These fibers are observable visually; however, it requires close inspection in good lighting to detect them. The fibers represent a deterrent to the professional counterfeiter who must add two colors to his palette for a high-quality forgery. However, the red and blue fibers can be simulated by drawing red and blue lines with a pen or pencil or, possibly, by printing them. Such simulations would most likely pass casual visual inspection but not careful inspection with a magnifier. The casual counterfeiter will most likely find the color copy reproduction of the fiber quite adequate. The red and blue fibers do not provide a high level of counterfeit deterrence for detection at points of sale, but they are relatively inexpensive. They could also be enhanced, as discussed in a later section.

Intaglio Printing and Fine-Line Engraving

The intricate designs used in existing U.S. banknotes represent a significant deterrent feature that is already in place. The variations in the fineness and depth of the line work,

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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which are produced by master engravers, give an intaglio-printed note its characteristic embossed or raised “feel” (Buckley, 1992). The variation of line width in a portrait or a sculptured border gives an appearance of gray-level and depth to the imagery that is very difficult to reproduce—most counterfeit notes simply do not look “right.”

Intaglio is a complex printing process commonly used for printing high-security documents. The process starts with the fabrication of master printing plates containing the fine-line engraving, incised by hand by skilled engravers. The master plates are used to produce the production plates that are used in the printing presses. Intaglio printing presses require a specially formulated high-viscosity ink that is applied to the grooves in the plate. The plate is wiped to remove all of the ink except what is captured in the grooves. The plate is then pressed against the paper under a very high pressure (typically 7,500 to 15,000 psi) to transfer the ink and emboss the paper (Graminski, 1993a). The plates are wiped clean after each impression. (Generally, approximately 15 percent of the ink actually is transferred to the banknote. Graminski, 1993b). The remainder is removed during the wiping operations.)

The embossing effect and the thick layer of printed ink causes the printed lines to be raised, giving the notes a distinctive look and feel. This produces lifelike portraits that cannot be exactly duplicated in counterfeits made by other printing processes or by copiers and printers. Similarly, the complex, unbroken fine-line patterns in the borders of the notes and in the backgrounds of the portraits are not duplicated well by lower-resolution copiers or printers.

The intaglio printing process is used for the black print on the front side of the notes and the green print on the back side. The Treasury seal, Federal Reserve seal, and serial numbers are printed by a typographic or letterpress process. In a letterpress, the characters to be printed are formed by raised surfaces on the printing dies. A roller applies ink to these raised surfaces and then the die is pressed against the paper to transfer the ink.

Microscopic inspection reveals unique characteristics of intaglio and letterpress print that distinguish them from other types of print. For example, the high pressure applied to the ink when the printing plates are pressed against the paper during intaglio printing forces ink into the spaces between the paper fibers, beyond the edges of printed lines. In letterpress printing, the pressure between the plate and the paper tends to squeeze ink to the edges of the raised characters, leaving an obviously thicker layer of ink along the boundaries.

Figures 4-1 through Figures 4-3 contain a series of photomicrographs, taken at 11-power magnification, of the eye in the engraved portrait of Alexander Hamilton on a $10 banknote. Figures 4-1 is the image as printed by the BEP using the intaglio process. The distinctness of the lines and the bleeding of ink (or “feathering ”) along the fibers is evident. Figures 4-2 is a photomicrograph of the same area that has been photocopied. Note the loss of detail and the inability of the process to reproduce the sharp lines. The toner particles are clearly evident.

Figures 4-3 is a photomicrograph of the same area again, this time from a counterfeit note produced using the lithographic process. Again, the image is clearly different from the intaglio one. The raised border and loss of definition can be seen. The amount of detail is greater than that of the photocopies note, but less than that of the intaglio printed note. In addition, the tactile feel of U.S. currency is extremely difficult to exactly reproduce using other printing or duplication methods. The committee judges intaglio printing of fine-line engravings to be an effective deterrent, particularly if a low-power magnifier is used as an aide for detection.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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FIGURE 4-1 Photomicrograph of intaglio printed image.

FIGURE 4-2 Photomicrograph of photocopied image.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
×

Figure 4-3 Photomicrograph of lithographic printed image.

However, the intaglio printing equipment employed by the BEP has several limitations that prevent some potential deterrent features from being incorporated. For instance, those counterfeit-deterrence features that depend on accurate registration of printed features between the front and back sides of the note would not be possible to produce on the existing intaglio equipment, even assuming ideal conditions. The BEP prints currency in 32-note sheets. The green print forming the backs of the notes is printed first and allowed to dry. Then the black print for the front side is applied. The very high pressure produced by the printing plates tends to stretch the paper. This causes errors in registration between the front and back images, even if the sheets have been placed into the press very accurately. Exact registration would require simultaneous, or near simultaneous, printing of the front and back images.

As another example, the current BEP intaglio equipment is limited in its ability to print additional colors. Multiple colors can be printed by selective inking of the printing plates. However, only a restricted number of fountains are available in which to hold additional ink colors.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
×
Serial Numbers

There are two serial numbers printed in the same green ink as the Treasury seal on the face of each note. No two notes of the same series, bank, and denomination have the same serial number. The Federal Reserve banks are designated by a letter and a corresponding numeral. The first character of the serial number is a letter that designates the Federal Reserve Bank and matches the letter in the Federal reserve seal. The corresponding numerical designation of the Federal Reserve Bank is printed in four locations on the face of each note.

The serial numbers and Federal Reserve Bank designators are faithfully copied on counterfeits made by copiers or electronic printers. A large number of copies of the same note, with the same serial number and bank designators, is subject to detection by alert point-of-sale or bank personnel. The serial numbers and bank designators are a more effective deterrent to the printing of notes on printing presses. The relationship among the features is not commonly known, although it is public information. Even professional counterfeiters are not aware of the simple relationship. But neither are personnel at points of transaction. Therefore, it is not normally used as a means of counterfeit detection. Making notes with different serial numbers complicates the printing process for the forger.

The denomination of each note is printed in intaglio in each corner on both sides of the notes and is spelled out in the lower border. It is also printed in black ink where it is overprinted with the green Treasury seal. The large number of denomination indicators is a deterrent to attempts to upgrade notes by simply altering the denomination numbers in the corners of the notes. Also, a distinctive portrait on the front of the note is associated with a particular denomination, as is a distinctive back. (This was one of the important changes made during the last major currency-redesign effort, which occurred in 1929.)

Security Thread

The BEP began incorporating a security thread in the Series 1990 banknotes. It was first introduced in the $100 notes and then subsequently in the $50, $20, and $10 notes. This thread is a thin metallized polyester strip 1.4 to 1.8 mm in width, and 10 to 15 µm in thickness (BEP, 1990). It is placed in the paper during its manufacture. It is located in the clear field between the border of the note and the Federal Reserve seal. The letters “USA” and the denomination of the note are printed on the thread. The thread is contained within the paper substrate so that it is not observable in reflected light and cannot be reproduced by the reflected light of copiers. The thread and its printing can be detected visually in transmitted light. A deliberate action (holding the note up to the light) is required for visual detection of the existence of the thread and especially to read the printing on it. It is the committee's opinion that at the present time, the public is not generally aware of this security feature or how to authenticate it. Therefore the security thread 's deterrence potential has not yet been fully exploited.

The thread can be simulated to pass casual inspection by drawing, printing a white line with an opaque white ink, or pasting on a thin sheet of paper. It would be easy to simulate the printing on the thread using the latter approach. If the simulation was done first, followed by the banknote printing, the simulated thread would be more difficult to detect as a forgery.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
×

Even though the security thread was not originally developed to be easily machine detectable, readers are available that can detect the existence and the location of a metallized thread; these readers can be fooled if the counterfeits contain a metal strip or wire in the appropriate location.

The security thread has been incorporated into 45 percent of the $100 and $50 notes in circulation worldwide; status for lower denominations are not available. It will be incorporated into $5 notes in 1994. When most of the notes in circulation have the security thread (estimated to be 1995) and the public is educated better about its existence and how to authenticate it, the security thread will be a reasonably effective counterfeit deterrent.

Microprinting

Concurrent with the introduction of the security thread in the Series 1990 banknotes, the words “THE UNITED STATES OF AMERICA” have been printed repeatedly around the portrait in a very fine line, 6 to 7 thousands of an inch wide. The print appears as a thin line to the naked eye, but the lettering can easily be read using a low-power magnifier. The resolution of most current copiers is not sufficient to copy this fine print, but equipment beginning to appear in the marketplace has sufficient resolution to copy it. The microprint is at the limit of resolution of the intaglio printing process; therefore, it will not be possible to use intaglio microprint to deter reproduction by higher-resolution copiers and printers.

Color

The light-green tint of authentic currency paper is difficult to reproduce and is one feature distinguishing this paper from commonly available paper. Since currency paper contains none of the fluorescent whiteners that are common in commercial papers, it will not fluoresce under an ultraviolet light. This provides a simple means of detecting suspicious notes, but is not a foolproof method. For example, genuine notes that have been washed might exhibit fluorescence due to whiteners present in laundry detergents.

Overall Assessment of Existing Visible Features

The existing counterfeit-deterrence features cannot be authenticated easily and unobtrusively by inexperienced and untrained personnel at points of sale.

  • Detecting the unique feel of authentic currency paper requires experience in handling currency. But the feel of the paper changes with wear.

  • The distinctness of intaglio printed images can be observed with a low-power magnifier, but this requires experience (and time) to do. And, the richness of the intaglio images becomes harder to discern as the banknotes wear in circulation.

  • The simple relationship between the serial numbers and the Federal Reserve Bank indicators is not known by most cashiers. It does require a certain amount of concentration.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
×

Most people do not pay particular attention to serial numbers and will rarely notice multiple bills with the same serial number.

  • The red and blue fibers can be detected only with a very close inspection in good lighting.

  • Reading the microprinting requires the use of a magnifier; but the advanced copiers and printers that are becoming widely available will be able to satisfactorily reproduce it.

  • The polyester strip can be detected only by holding the note so it can be observed in transmitted light.

In summary intaglio printed fine-line engravings on a yellow-green tinted paper has been very effective in the past in allowing the general public to readily identify notes that “aren't right.” However, with the availability of high-quality color copiers and printers, these deterrents will become much less effective over time. As has happened before in history, the existing overt counterfeit-deterrence features of U.S. currency require upgrading to respond to new counterfeiting threats that are driven by advanced technology.

INNOVATIVE VISIBLE COUNTERFEIT-DETERRENCE FEATURES

A number of innovative counterfeit-deterrence features have been proposed for incorporation into U.S. currency. These features can be categorized as substrate-based, printed, multicolored, design-based, post-printed optically variable, and random pattern with encryption and as deterrents built into copiers and printers.

Substrate-Based Features

The use of a particular, high-quality paper substrate represents the first line of defense of U.S. currency. As currently produced for Series 1990 bills, the paper stock incorporates a number of security features, including a denominated metallized polyester security thread, red and blue fibers, and a distinctive tint. The physical properties, fiber composition, and surface treatment are closely specified and subjected to careful quality control. Substrate-based deterrent features cannot be exactly reproduced and are therefore particularly attractive deterrent features. A large number of additional substrate-based features have been suggested, and several of these are currently employed in foreign currencies. An assessment of the generic types of substrate-based features follows.

Laminated Substrates

The ability to produce currency paper by joining two half-thickness sheets with or without a very thin plastic interleaf provides many possibilities for the introduction of deterrent features to the substrate (James River, 1993). For example, designs, images, or text can be printed on what becomes the interior of the note. Such printed information would not be visible in reflected light but would be apparent on viewing in transmitted light. If a plastic interleaf is employed, transparencies could be introduced. In effect, then, the current features

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
×

of the security thread could be extended over the entire note. The sophisticated paper-making technology that is available to produce and join the half-thickness sheets makes this a potentially attractive deterrent.

The committee views the laminated substrate as a system capable of incorporating multiple security features. These systems in paper/paper, paper/plastic and plastic/plastic laminates have undergone substantial development and are in use for some security document applications (paper/paper) and, in one case for currency (plastic/plastic in Australia). The promise of this technology recommends a careful monitoring of the progress in their development and experience in various applications. The ability of the adhesive that joins the laminates to withstand intentional separation of the layers is a particular area of interest.

Skilled counterfeiters could probably readily simulate these laminates, as well as any of the features contained within the laminate. They would not be concerned with durability of the note, making the task much easier than that of the genuine paper maker. Paper splitting and re-bonding with glue is one possible simulation route; another would involve bonding two thin sheets of paper.

Plastic Substrates

As mentioned above, plastic can be substituted for paper as the substrate material, as has been done in Australia for its $10 note. Plastic provides a smoother surface for microprinting, and hence finer details can be printed. It also makes it relatively easy to introduce transparencies. The plastic may itself incorporate various additives and deterrent features. Some increase in substrate durability may be anticipated but there may be problems with heavy creasing in the bills leading to premature cracking (Haslop, 1993a).

There are a number of problems with using plastic substrates for a note. First, the feel of a plastic note would be substantially different from that of a paper note. It may be more difficult to develop a distinctive feel for a genuine plastic banknote, as has been done for the current paper ones. And thin plastic material is readily available and would be impossible to restrict. Also, plastic is currently more expensive than paper. Many of the security features associated with paper substrate (e.g., watermarks) are not possible with a fully plastic substrate. But similar features are possible; for example, a shadow image can be produced in the plastic coating that can have the same functionality as a watermark. it's also sensitive to the ink composition and printing parameters regarding durability issues such as ink adherence, temperature, and humidity effects.

Most of the benefits associated with a plastic substrate can be achieved with laminated paper structures, or innovative extensions to the current substrate material. All of the U.S. experience and testing procedures are based on the current substrate material. At this time, there does not appear to be a significant enough advantage to plastic substrates to overcome the cost of conversion.

Enhanced Security Thread

A security thread has been introduced in Series 1990 currency in denominations of $10 and higher. As currently specified, the threads do not fluoresce under ultraviolet illumination; they are not visible in reflected light but are clearly readable to the unaided eye in transmitted

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
×

light. The committee recommends that the position or width of the thread, which currently is essentially the same for all denominations, be varied by an obvious amount to permit denomination discrimination on the basis of position or width alone. In addition to their assistance to the casual viewer, such changes would assist in machine recognition.

A change in position would have to be carefully designed to avoid occlusion by another feature, such as a printed image. The center of the note should also be avoided, since that is where bills tend to be folded. As an alternative, the number of threads could be varied depending on the denomination. Both the wider thread and the incorporation of more than one thread raise concerns about creating weak zones in the substrate. Analysis would be required to determine if delamination and reduced note durability would result from these changes.

The committee was shown an example of a forged note with a simulated security thread, complete with microprinting. Though this note was not fully convincing, one cannot assume that the thread as currently employed will deter the dedicated professional counterfeiter. However, it is highly effective against the casual photocopy, which does not copy the thread.

Two enhancements to polyester security threads have been incorporated in foreign currencies. A windowed thread is used in British, German, Turkish, and Bahrainian currency, while Finnish notes use an imbedded thread with holographic printing (Haslop, 1993a). The current supplier of security paper to the BEP could implement a windowed thread following some development effort (Crane, 1993).

The security thread could be enhanced by combining it with other features. For example, if the thread were overprinted with characters using a photoluminescent ink, the characters could potentially be made to appear obvious on a reprographic copy. There are many potential combinations of other features that could be used to enhance the security thread. These should be studied for long-term implementation. The experiences of other countries in similarly enhancing their security threads should also be closely monitored.

Watermarks

Watermarks have been used in paper since handmade papers were produced in Italy at the end of the thirteenth century. It is estimated that some one million different watermarks were used before the introduction of machine-made papers in the nineteenth century. Watermarks have been widely employed for centuries as a means of marking high-value documents. For instance, authentication of rare prints and drawings often involves the study and identification of the watermarks contained in the substrate.

Watermarks may be introduced by bent wire devices in cylinder-mold paper machines, embossed in the wet paper with a dandy roll in a Fourdrinier paper machine, or impressed on the dried paper. Simulated watermarks may be printed on paper with fatty materials (Kühn, 1986).

The image in a watermark is formed by local variations in paper density becoming visible in transmitted light. The watermark may also be detected in transmitted light as a variation in the thickness of the paper. There are two types of watermarks. “Non-localized” ones that are placed in generalized locations throughout the substrate with no particular reference to other features; and “localized” or “registered” watermarks that are placed in a specific location

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
×

within a printed image.

Watermarks are widely used in selected denominations of most of the world's currency. They were used in U.S. currency from 1869 to 1879 as part of a campaign to halt widespread counterfeiting that developed during the Civil War. Though watermarks are highly resistant to copying, they are not easily observed under difficult lighting conditions and can be simulated. However, watermarks can be designed to make high-quality simulations difficult. Should different watermarks be used for different denominations, they would introduce an added deterrent against “raising,” that is, bleaching a low domination bill and printing a higher denomination bill on the bleached substrate. The introduction of watermarks to U.S. currency was recommended in the previous National Materials Advisory Board report. The current supports committee supports that recommendation.

Tinted Substrates

A yellowish-green color tint is currently specified for all U.S. currency denominations. Such pale tints are difficult to reproduce accurately in most reprographic systems, are readily recognized, are easily machine read, and do not add to the cost of the paper. The use in U.S. currency of a pale tint that is difficult to reproduce serves as a visible deterrent. While the use of different tints for the several denominations and the general use of color, or selectively enhanced features in the unprinted stock, is discussed later in this chapter, it should be noted that tints can improve the effectiveness of other deterrent features. Tinted substrates are being used by many other countries, among them Mexico, Thailand, and Japan (Haslop, 1993a).

Paper Furnish Additives

The BEP has been offered a wide range of additives to the paper furnish that have the potential to increase the security of the paper substrate. Among them are planchettes, enhanced fibers, optical fibers, taggants, and particles with special properties. The enhancements for fibers and planchettes include: optically variable iridescence, dichroism, metamerism, microprinting, fluorescence, and phosphorescence. Since the quantities added would be very small (for example, the current BEP specification calls for 0.31 kg of red and blue fibers per 1,000 kilograms of fiber furnish), such additives offer low-cost deterrent possibilities. However, additives in these small quantities would be barely noticeable to the casual observer.

Fibers that are simply colored could be readily reproduced by high-quality copiers. Many other possible enhancements are discussed elsewhere in this chapter. The addition of small amounts of enhanced fibers (e.g., plastic optical fibers), or combinations of variously enhanced fibers, for example, may play a significant role in the random pattern/encryption concept, as discussed in a later section.

Fibers or particles that selectively emit or absorb light at the same wavelengths used for scanning or copying can produce obvious forgeries, because the copier or printer will yield spots or lines on the copy that are white, black, or unevenly colored. These are discussed below. A technical discussion of the chemical aspects of these features is present in the section, Inks for Printed Features later in this chapter.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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Enhanced Fibers

The introduction of randomly distributed fibers with specific responses to ultraviolet, infrared, or visible radiation detectable with simple point of exchange devices (a bright pen-light would be adequate for optical fibers) would enhance the security of the paper substrate with little additional cost. Ultraviolet enhanced fibers are being incorporated by several countries, such as Brazil and Bahrain (Haslop, 1993a).

The nature of these enhancements makes such fibers virtually impossible to copy and difficult to simulate. It is their limited distribution, and in some cases nonvisible character, that makes them unsuitable for public deterrence as presently deployed. Both an increase in concentration and considerable education of the general public would be required to render them generally effective. They may, however, prove quite useful for point-of-sale devices, machine readability, and forgery detection sensors that could be built into copiers (see Counterfeit Deterrence Incorporated in Copiers and Printers later in this chapter).

The committee examined paper made with small lengths of plastic optical fibers added to the furnish. Resistance to copying and simulation would be very high, and the ease of recognition by examination with a simple pen light is impressive. The BEP printed some notes on this paper, using a hand-operated intaglio press (Church, 1993). There was some evidence of ink adherence problems in the area directly above some of the fibers. There was also a concern that the high pressure in the production presses might cause the fibers to crush. The committee believes that these are typical of problems encountered with any new development and may be readily overcome. No fundamental barrier to future progress is evident, and several avenues of research opportunity are possible. For instance, a smaller diameter plastic fiber could be tried. Therefore, the committee recommends the continued development of optical fibers as an enhancement of the counterfeit-deterrent feature of the paper substrate.

Planchettes

Planchettes are colored or reflective pieces of paper or plastic a few millimeters in diameter. They are added in the paper furnish during paper manufacture. They can be enhanced in the same manner as fibers and thus can be optically variable, luminescent, magnetic, iridescent, microprinted, etc. Iridescent planchettes are currently incorporated in the currency of several countries, such as Mexico (Haslop, 1993a). As with fiber additives, cost is not a significant factor. Significant deterrence to routine copying and simulation can be achieved, but many of the enhancements do not present visible deterrence. Some can, however, be easily observed with inexpensive aids, such as a “black” light for the ultraviolet dyed planchettes. The use of reflective planchettes will defeat simple copying, but simulation of the effect appears to be relatively easy. There is also some concern that planchettes could lead to durability problems if some material properties of the planchette (such as stiffness) are significantly different from that of the substrate or are inadequately bonded to the substrate.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
×
Microtaggants™ and Microcapsules

Tiny fragments of a wide range of materials, typically color coded with well-defined chemical composition or specific microwave, ultraviolet, infrared, or other characteristics, may be added to the paper furnish. Microtaggants™ are color-coded, layered polymer particles. These particles are generally of dimensions such that they are not visible to an unaided eye. These types of features would be most effective for machine detection and forensic analysis. They may also prove useful in random encryption applications. A careful assessment of these particles must be conducted to ensure that they are nonhazardous and nontoxic.

Radar Reflectance

The introduction of radar reflective dipoles either randomly distributed in the paper or included in an enhanced security thread was considered as a machine-readable feature (Patton, 1993). Model calculations suggest that the concept is feasible, but an instrumental method operating at radar frequencies is required for authentication. Bekaert, Inc. makes stainless fibers that a French-British company, Arjomari-Wiggins, places in a special paper reportedly used for some high security documents. As an alternative approach, the dipoles could possibly be printed on the paper with metallic ink; this approach would require further study to assess the printing resolutions and methods required, long-term durability, etc. However, the committee is not aware of any currency development effort on this feature. Although producing the paper may not require much development effort, the radar detector will. The radar transmitter will probably have to be separated from the receiver (i.e., the banknote would be examined in transmission). Making a detector that would not be fooled by a simulation that glued wires of the proper length to the outside of the bill might be a considerable effort.

Color
Some Fundamentals

Color is a perception: we see color when a non-white distribution of light is detected by our eyes and interpreted by the brain. The eye has three sets of cone detectors, most sensitive in the blue, green, and red parts of the spectrum. The relative responses of these three sets of cones gives us our color perception (Evans, 1974; Hill, 1987). These responses depend strongly on the nature of the illumination, yet our eye-brain system can compensate for considerable color and huge intensity variations in the illumination with minimal change in the color perceived. What little change in perception does remain is called metamerism and can prove troublesome in subtle color matching. Another complication is produced by color perception defects. The most common defects are deuteranopia (“green blindness”) and deuteranamoly, where there is, respectively, an absence or limitation in the ability to distinguish red from green1.

1  

Deuteranopia or deuteranamoly is present in 6 percent of males and 0.4 percent of females.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
×

By itself, the memory for a specific color is relatively unreliable. Machine reading is preferred for precise identification. However, in proper illumination, when color standards are available for comparison, the eye is an excellent instrument. In the context of colored patterns, the eye is quite sensitive to anomalies and can detect an unusual pattern or color component relatively easily.

Appendix C contains additional background materials relating to the measurement of color and the physical and chemical causes of color.

Uses of Color in Banknotes

Color can serve many functions in banknotes, but it is not a great deterrent by itself2. It can provide general identification of the note, specific identification of the denomination3, and counterfeit deterrence when used in conjunction with another feature and in machine-readable form. An example of color used for deterrence is the use of colored or colorless inks that fluoresce under ultraviolet illumination. Color features lend themselves to instrumental detection.

Color can be utilized several different ways in currency. It can be used to tint the paper itself in a uniform manner; as part of various paper additives such as fibers, threads, planchettes, and the like; in intaglio printed background and foreground designs; and in overprinting.

It is frequently assumed that the essentially two-color nature of U.S. banknotes (black with green overprinting on the face, green back) is dictated by tradition and that the U.S. public would be opposed to any change. The committee found no evidence for this point of view, and some anecdotal evidence indicates the contrary. A small but statistically valid public opinion survey could add needed data to this debate.

There are a number of possible ways in which color can be used effectively on U.S. banknotes. Full-color printing with multiple colors on both the face and back that are different for each denomination is one possibility. This would provide ready denomination recognition and additional counterfeit deterrence against the professional counterfeiter using lithographic equipment (currently the most prevalent method of counterfeiting), who would be forced to make many plates and skillfully maintain precise registration while using several different inks. But printing fully multicolored notes on the existing intaglio printing presses is not a short-term possibility, since the BEP has a restricted number of additional ink fountains available on the current intaglio press equipment with which to apply other colors.

Additional options present a limited use of color that could be incorporated almost immediately on existing equipment. Each denomination could use a different pale color tint in the paper, as described in a previous section. There is the possibility of employing the existing overprinting step, used to apply the seal and serial number, to print additional color

2  

This is more a tribute to the advancements in non-impact color printing than it is a failing of color.

3  

This is not currently used in U.S. currency but is widely used in the currencies of other countries.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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for the face. For instance, the denominations could be overprinted in prominent size, using a different color for each denomination matched to the pale tint of the paper4.

Yet another option is the incorporation of additional color involving an optically variable feature, with much stronger counterfeit deterrence (Brettler, 1992; Haslop, 1993a; Phillips, 1993). These features are described in a following section. Also, overprinting a detailed image, such as a seal, using a reflective metallic ink is possible. Metallic ink does not have to be carefull tilted in light and closely examined under close scrutiny, as do optically variable images, and would therefore be observed more frequently. But metallic ink by itself would not be difficult to simulate, so it must be used to print a detailed pattern. However, the durability of such a pattern would be expected to be poor, similar to that of the metallized holograms (which are described in a following section). As a variation of this option, metallic or ordinary colored or colorless inks that fluoresce in response to ultraviolet light could be used. This would take advantage of two very useful features, and could be easily checked by a cashier or bank teller equipped with an ultraviolet lamp. Such fluorescent inks are used in the currencies of several countries.

Finally, induced moiré and variable-sized dot patterns discussed elsewhere in this chapter are made more effective by being printed in color, using different spatial frequencies for the different primary colors. As a result, the moiré or variable sized dot patterns show up in a different tint juxtaposed against the background; the eye is particularly sensitive to such small changes in color.

Other Aspects of Color in Banknotes

Color combines well with the fine-line engraved intaglio printing with its subtle shading and precision registration. Color anomalies, poor registration, and the absence of the crispness of quality printing are readily detected even by the relatively untrained observer. There are specialized line designs that show abnormal patterns when reproduced on color copiers; this effect is enhanced when subtle color patterns are used (Haslop, 1993a). This is discussed in detail elsewhere in this report.

Several general principles should be followed in using color. First, colors and color combinations should be tasteful and aesthetically pleasing. Second, the color compatibilities and color distinctions should be acceptable in commonly encountered illuminations. Finally, the choice of colors should be made with due consideration for human color deficiencies, particularly the red-green discrimination defect, which is present in about three percent of the population.

Inks for Printed Features

U.S. currency is currently printed using intaglio techniques. Intaglio inks are typically dark powder pigments that are added to oils. The intaglio inks transferred from plate to paper

4  

In addition to the colors needed for the current six denominations ($1,5,10,20,50,100), it might also be desirable to be prepared for at least one additional color, in case the need arises for printing another denomination, such as a $2 bill or a denomination higher than the $100 bill.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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display graphic relief with a thickness of approximately 20 µm. The thick intaglio ink provides a versatile medium to carry other inks or agents. Consequently, printed features on the currency provide many opportunities to incorporate anticounterfeiting measures. To meet the threat of color copiers, printed features are desired that can not easily be reproduced by copying technology. Besides the special images and patterns that can be printed, the optical and materials properties of the inks used to form the printed features are themselves potential anticounterfeiting vehicles.

A number of the dye and pigment substances, both colored and colorless, show photoexcitation; that is, they produce color by photoluminescence (fluorescence) when suitably excited (e.g., with ultraviolet light). For machine reading, the fluorescent emission does not need to be in the visible region.

The various specialty inks that have potential for deterrence include:

  • color-shifting (optically variable) inks;

  • metameric ink pairs;

  • photochromic inks;

  • photoluminescent inks;

  • transparent or absorbing infrared inks; and

  • reflective inks.

As a group, these inks have many different optical and materials properties. Overt features that produce optical effects to the naked eye in sunlight or incandescent light can be achieved by some of these inks. Other inks require external perturbation of light to produce visible indications. In addition, covert features that can only be monitored by an instrument can be incorporated by some of the inks.

Given the emphasis on readily visible and recognizable overt anticounterfeiting features, the color-shifting inks have the highest potential for successful deterrence in the near term of all the inks listed above. They possess many desirable optical characteristics that do not require any perturbations other than sunlight or incandescent light. They can be implemented easily by the BEP. And their manufacture requires a high-technology fabrication processes that would not be easy to replicate or simulate. In contrast, the metameric, photochromic, and photoluminescent ultraviolet and visible inks are active features that require either additional perturbations to realize their effects or special instruments to observe their special properties.

The transparent or absorbing infrared inks offer excellent covert features that would be desirable for machine readers. The reflective inks are analogous to metallic stripes or metallic films in their effect and will not be considered further in this section. Photochromic or photoluminescent inks could produce false colors on reproduced banknotes, thereby thwarting the counterfeiter before an attempt is made to pass the bogus notes. These inks would have to respond to the exposing light wavelengths and intensities used in copiers and scanners. To be effective though, these inks must undergo a relatively rapid noticeable color change in response to light stimulus. In the future, the committee envisions that nonlinear optical material could be developed with high enough sensitivity to respond to the intensity of infrared laser light used in copiers to produce short wavelength response in the visible region.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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Color-shifting Inks

Color-shifting (optically variable) inks reflect various wavelengths in white light differently depending on the angle of incidence to the surface. An unaided eye will observe this effect as a color change as the viewing angle is changed. A color copier or scanner can copy a document only at one fixed angle relative to a document's surface. Therefore, a copier or scanner would record only one color of an image printed in an color-shifting ink, losing information about the reflectivity changes versus angle of incidence. Thus the color copied image will be obvious, since it will remain the same color regardless of the viewing angle.

These inks are composed of color-shifting thin-film flakes suspended in a mixture of regular ink. The thin flakes produce their unique optical effect because of optical interference of light reflected from two parallel interfaces as shown in Figures 4-4. The reflected light from the two parallel interfaces will either constructively or destructively interfere with itself depending on the optical path differences.

FIGURE 4-4 Color-shifting device principle of operation.

For a particular thickness of a thin film with two parallel interfaces, different wavelengths will constructively or destructively interfere at different angles. For example, the thin film may appear green at normal incidence and blue at 45° relative to the surface normal. The particular colors that are observed at normal incidence and at 45° relative to the surface normal will depend critically on the thin-film thickness. This thickness must be carefully controlled in the fabrication process and cannot be easily produced without expensive instrumentation.

The magnitude of the optical effect from the ink depends on the number density of thin-film flakes in the ink. The quality of the optical effect also depends on the precise orientation of the flakes on the surface of the ink. Physical forces cause the flakes to align parallel to the surface as shown in Figures 4-5. Without this preferential orientation, the optical effect would

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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be significantly degraded. The use of surfactants and functional groups on the thin-film flakes may further improve the optical effect. Figures 4-6 is a detailed cross-section of a typical thin film flake used as a pigment.

FIGURE 4-5 Color-shifting ink.

FIGURE 4-6 Cross-section of color-shifting pigment.

The intaglio inks with color-shifting thin-film flakes that are currently available produce a readily observed color change versus viewing angle under conditions of normal illumination5. The colors that are observed at normal incidence and 45° relative to the surface normal depend on the thin film thickness. In addition, dye or pigment filters can be added to the ink mixture to absorb particular light wavelengths. These absorbing dyes and pigments can further intensify and modify the observable color change of the ink versus viewing angle. With different combinations of flake thicknesses and dye or pigment filters, many color combinations are possible such as gold/green, green/magenta, green/blue, and green/black (Phillips, 1990, 1993). The committee's opinion is that the color changes, while noticeable (the committee thought that the last two pairs, of the four mentioned above, seemed to give the

5  

For example, OVI™ is a color-shifting intaglio ink available now from SICPA.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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most noticeable color change, but this was a decidedly nonscientific sample), would be more effective if they were even more dramatic. The BEP should encourage the color-shifting ink manufacturers to continue to explore various combinations of dyes and optical flakes to intensify the optical interference effect.

A brief description of the fabrication process will illustrate the difficulty in reproducing color-shifting inks. Ultrathin films are initially deposited on a flexible planar substrate in high vacuum using electron-beam or sputtering-deposition techniques. The absorber/dielectric/reflector/dielectric/absorber structure is symmetrical, because the thin-film flakes can either be oriented up or down in the ink suspension. Typical materials used to obtain this multilayer structure are Cr/MgF2/Al/MgF2/Cr or Cr/SiO2/Al/SiO2/Cr. Typical thicknesses for the layers are respectively 50 Å, 4,000 Å, 900 Å, 4,000 Å, 50 Å. The flexible planar substrate is dissolved and the thin film broken into flakes, 50 to 200 µm in diameter, which are then ultrasonically agitated to reduce the particle diameters to approximately 2 to 20 µm. Since the flakes are approximately 1 µm thick, they have a “pancake” structure with an aspect ratio that averages 10 to 1. Because typical printed ink thicknesses are between 5 to 30 µm this high aspect ratio helps align the flakes parallel to the surface of the ink.

The high-technology manufacturing process enhances the security of this ink, since the process is far beyond the capabilities of most (but not all) counterfeiters. Simulation is possible using inks that have metallic sheen. However, these inks do not change color. If the public is aware of the color change and knows how to observe it, and if the ink is used to print a design that is somewhat complex, simulation will be very difficult.

Macroscopic thin-film devices, such as interference films, can also produce excellent color-shifting effects. However, these large films are very susceptible to mechanical damage and they typically fail the “crumple” test. The small size of the thin-film flakes in the color-shifting ink precludes significant mechanical damage. They are already broken up and dispersed in the ink, and crumpling does not adversely affect their optical performance. The optical flakes are fairly chemically resistant, because they are composed of relatively stable metallic and dielectric materials. Moreover, they are embedded in the ink mixture, which protects them to some extent from chemical exposure.

The ink is compatible with intaglio and silk screen printing; it cannot, at this time, be applied using a lithographic or offset process. The cost of the ink is extremely high compared with the usual intaglio inks (two orders of magnitude more expensive), but it is not so expensive as to be impractical. However, since this is a new technology, the committee feels that with additional development effort, a significant reduction in the current cost of this ink may be possible.

Test banknotes printed with color-shifting ink have been subjected to the usual gamut of tests. The notes did not perform exceedingly well in the 24- acid soak or alkali soak tests, as was be expected (BEP, 1993). On the other hand, notes printed by other countries using color-shifting inks have not yet shown ink-related durability problems. A potentially fruitful area of research would be investigating ways to enhance the effect of the color shift. The concentration of the ink pigment, the size of the coated area, printing method, and the presence of a reference color can be optimized to enhance the effect. It was also demonstrated to the committee that having a reference color that does not shift color immediately adjacent to the image printed with color-shifting ink makes the color shift more dramatic. And, since

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
×

the color-shift effect does require manipulation by the observer relative to a light source, care must be taken in the design of the printed image to provide a large enough area that the effect can be readily discerned.

Many countries are beginning to apply these inks to print small images on their banknotes. Examples are the German 500 and 1,000 Deutschemark, the 50,000 Italian Lira, and the 10,000 Belgian Franc. Other countries are considering adopting color-shifting ink and may do so in the near future. Their experience should be closely monitored.

Color-shifting ink could be implemented fairly quickly by the BEP if the need arose, and hence is recommended by the committee as a feature for consideration in the short term. Additional research directed at enhancing the color shift should be encouraged.

Metameric, Photochromic, and Photoluminescent Inks

Whereas the effect of color-shifting inks can be seen in sunlight or incandescent light, other specialty inks display effects that require additional perturbations to obtain their unique signatures. Consequently, additional light sources or special viewing instruments are required to see their effects. Despite this disadvantage, these specialty inks can potentially be an effective measure against counterfeiting by color copiers or scanners. Most of these specialty inks are visible or ultraviolet inks.

Metameric ink pairs are designed to appear the same color under a particular illumination. When illuminated with a different light source, the metameric ink pair yields a different color. Metameric inks could have great utility if they induced a color change in the process of replication that subsequently produced a copy that did not accurately duplicate the original color. However, the previous evaluations of metameric inks have judged their color changes to be too subtle to be an effective deterrent. Since the last committee report, there is no new evidence to suggest that this situation has changed. Metameric inks must be judged an intriguing prospect for the future, and research developments should be monitored.

Photochromic inks have color properties that change as a function of light illumination. Well known examples of photochromic materials are the silver halides or spiro compounds found in sunglasses, which darken when exposed to bright light. The principle of photochromic inks is based on the photochemical conversion of the original compound to a photoproduct with different optical properties. For example, exposure to ultraviolet light can lead to molecular bond breakage and ring opening in various aromatic molecules, such as adamantane-2-spiro-naphtopyran. Upon ring opening, the light absorption properties will change, resulting in a change of ink color. After photochemical excitation, the photochromic inks revert back to the original state.

One drawback is that this excitation process is generally time consuming, reported to require seconds to minutes. However, faster response times have been recently reported for some experimental inks (Hepfinger, 1993). Fast reaction times and dramatic color change must be demonstrated before these photochromic materials can be seriously considered.

Another issue with photochromic inks is their long-term chemical stability. They are known to experience photochemical fatigue and will degrade with time. Even the best photochromic inks will degrade after several thousand photochemical conversion cycles, although the degradation rate may be a function of light intensity. The importance of this

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
×

stability for banknote applications must be further assessed, since banknotes would not be expected to be subjected to more than several copying attempts over their lifetime.

Photoluminescent inks display colors that are determined by both the absorption spectrum of the dye molecule and subsequent energy transfer in the excited dye molecule. If the energy transfer leads to the occupation of lower-lying electronic states, these states can relax to produce luminescence that is significantly red-shifted. Assuming that the initial excitation of the dye molecule was in the ultraviolet, this red-shifted light can mix with ambient light reflected in the visible to produce a composite color. Consequently, the color of the ink in the visible will depend on whether the photoluminescent ink is simultaneously excited in the ultraviolet spectral region.

Another feature of some photoluminescent inks is the lifetime of the luminescent dye molecules. The lower-lying electronic state may be “triplet” in nature, and its relaxation may require a spin-forbidden electronic transition in order to return to the “singlet” ground state. This spin-forbidden transition leads to a long phosphorescence lifetime of 10-3 to 10 seconds. Consequently, after the excitation source is removed, the ink will continue to luminescence for a period of time that can be detected either instrumentally or by the naked eye under ideal conditions or instrumentally. Other types of photoluminescent inks will undergo photochemical conversion to excited reaction products that have a different red-shifted emission spectrum. These inks are related to photochromic inks. The difference between photochromic and photoluminescent inks is that the effect of photoluminescent inks is observed in the altered emission spectra, whereas the effect of photochromic inks is observed in the altered absorption spectra.

Chemical and photochemical stability is a key issue with photoluminescent and photochromic inks. Recent work indicates that their chemical fragility can be improved by encasing photoluminescent molecules in small polymer fibers (Hepfinger, 1993). These small polymer fibers could then be incorporated in the ink. Although this technique remedies the chemical stability problem, the photochemical stability of the photoluminescent inks must also be surmounted. Additional research is required before the photoluminescent inks can be confidently deployed.

Photochromic and photoluminescent materials have great potential to be deterrents. Advancements in the field and the experiences of other countries that have implemented features using these materials, such as Japan, should be closely monitored.

Transparent or Absorbing Infrared Inks

In some instances of copying, infrared inks could potentially act as active features. The light emitted by the laser diodes, used in most printers and scanners to form a digital image, is in the range of 800-900 nm. Since many infrared inks absorb light at these wavelengths, the resulting image would be black where it obviously should not be this could be used to spell out a word to draw attention to the copy. For this concept to work, the absorption spectrum of the ink must fall within the emission wavelength of the laser diodes, and the laser illumination must be sufficiently intense.

Infrared inks are a potential source of covert anticounterfeiting features. Because they can not be duplicated by a color copier, they may provide an additional deterrent to augment

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
×

currency security. Their main drawback is that they are primarily not visible and normally require machine reading to detect their presence. Besides their use in anticounterfeiting efforts, the infrared inks would be extremely useful for machines that perform currency verification and denomination determination.

An advantage of infrared inks is that they can absorb very weakly in the visible wavelengths and very strongly in the infrared wavelength. Thus, they can not be observed by the naked eye, but can be detected by common infrared detectors. Most infrared dyes are based on a substituted phthalocyanine structure. This extended aromatic ring acts as an excellent antenna for infrared light. The precise details of the infrared light absorption are dictated by the various substituents attached to the extended aromatic ring. Depending on the substituents, the infrared dyes can absorb light in the range of 700-1100 nm. A comparison between typical dyes in the visible region and narrow and broad-band dyes in the near infrared is displayed in Figures 4-7.

This wavelength range is compatible with a variety of inexpensive infrared light emitters and detectors, so detection technology is readily available to exploit the signature of infrared dyes on currency 6. For example, semiconductor lasers based on GaAlAs emit infrared laser light at wavelengths of 780-840 nm. These diodes are the same ones used in laser printers. Light-emitting diodes also used in electronic printers emit at about 900 nm as well. Common infrared light-emitting diodes also emit at 900 nm. These infrared wavelengths are also efficiently detected by inexpensive silicon photodiodes. Because of their large

FIGURE 4-7 Infrared dye spectrum.

6  

The wavelength of visible light in 400-700 nm, and the infrared range begins above it, starting at 700 nm.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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absorption coefficients, the presence of infrared inks could be easily detected by transmission measurements. But these absorption properties may make it difficult to record detailed pattern information.

The phthalocyanine infrared dyes are also well known for their excellent chemical properties. The phthalocyanines are nonpolar and not water soluble. They are also resistant to aqueous acids and bases and various organic solvents. These phthalocyanine infrared inks are known for their heat resistance. Moreover, in tests of photochemical stability, the phthalocyanine dyes were very robust7. These infrared dyes can potentially provide useful and reliable anticounterfeiting measures as both overt and covert deterrent features.

Design-Based Security Features

Design-based security features involve counterfeit-deterrent methods that can be incorporated into the printed banknote by modifying its design layout. In this section, several design-based features will be described together with an assessment of their effectiveness as a deterrents against counterfeits made using copier/ computer/ scanner systems. Each security feature is described in a separate section below. Design-based security features include: fine-line engraving, line work that induces moiré patterns when photocopied or digitized, variable-sized dot patterns, and latent images. In addition, the use of bar-code technology as a machine-readable, counterfeit-deterrent feature for banknote applications is discussed. Line patterns that produce moiré patterns or variable sized dot structures are examples of active deterrent features, since their presence on a genuine note may not be obvious but would be on a copied note.

Moiré-Inducing Line Structures

Current high-quality color photocopiers, and those that will be produced in the foreseeable future, are based on digital imaging technology. Computer work stations and their associated imaging peripherals (input scanners and printers) are also digital in nature. A digital image can appear to be a very faithful reproduction provided that the digital samples were taken sufficiently close together. However, if the image being digitized possesses spatial detail that is sufficiently fine, spurious patterns will be introduced into the reconstructed image; this effect is known as “aliasing” (Pratt, 1968)8. In the fields of printing and optics, aliasing errors are often referred to as moiré patterns. By using a properly designed pattern on the banknote, whose spatial frequency content is higher than the sampling frequency of digital photocopiers, scanners, and printers, a striking large-scale moiré pattern will be produced in the

7  

After exposure in a xenor fadeometer to the equivalent of about 4 months of daylight, the infrared absorption at 900 nm decreased only nominally (ICI Colours, Inc., 1993).

8  

Moiré patterns often look like ripples on the surface of water.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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reconstructed image that immediately suggests the note is a counterfeit. (Note: these designs will not produce a moiré pattern if copied on the older analog copiers.) Appendix D contains additional technical discussion of the moiré effect.

An example of moiré patterns that are induced by digital sampling is shown in Figures 4-8 and Figures 4-9; this example was used in a presentation to the committee to illustrate the concept of induced moiré (Morris, 1992). Figures 4-8 contains a pattern known as a Fresnel zone plate (Longhurst, 1973). The fundamental spatial frequency of the grating pattern of Fresnel zone plates increases linearly as the radius increases from the center. The resulting image produced when this pattern is sampled using a digital scanner is shown in Figures 4-9. (Note that the resulting sampled image actually contains several moiré or “ripple-like” patterns.) The additional patterns are produced by commensurations between the various spatial harmonic frequencies of the zone plate and the sampling rate of the scanner.

A previous National Research Council report considered the use of moiré-generating patterns as a counterfeit-deterrent feature (NRC, 1987). The report considered the case in which two linear grating patterns were superimposed (or printed) with a small angular misalignment, giving rise to a low-frequency fringe pattern that is readily visible, while the individual gratings were of sufficiently high frequency so as to show no visible fringe pattern by themselves. In the proposed scheme, the presence of a visible moiré fringe pattern would then be used for authentication. It was noted that the printing resolution of the BEP exceeded the resolution capability of then available copiers and scanners. The committee reasoned that

FIGURE 4-8 Fresnel zone plate pattern.

FIGURE 4-9 Digital image produced by a sampled Fresnel zone plate.

these systems would produce the low-frequency moiré fringe pattern but not the high-frequency grating pattern. Therefore, the visual effect of both the original and the photocopy

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
×

would be the same; hence, they concluded that this did not appear to be a viable deterrent method. While the committee addressed moir é patterns produced between two patterns printed on the note and reproduced by analog copiers, they apparently did not consider aliasing effects that will occur when high-frequency patterns are under-sampled by digital photocopiers and scanners.

Induced moiré (i.e., moiré patterns due to aliasing in a sampled image system) has been used as a counterfeit-deterrent feature in various forms since the late 1970s for document security (Wicker, 1991; Canadian Banknote Company, 1966; Kendrick and Jefferson, Ltd. 1988; Thomas De La Rue and Company, Ltd. 1985)9.

Recently another method of using induced moiré for document security has been reported (Spannenburg, 1991). The formation of “alias” or moiré images was investigated using dot- and line-frequency modulation and screen-angle modulation. An example of a frequency-modulated image and a copy of this image made using a digital color copier are shown in Figures 4-10 and Figures 4-11 respectively (Spannenburg, 1991).

Researchers have also investigated “specialized line structures” that are invisible on the genuine document and clearly visible on the counterfeit, thereby confirming its invalidity (Thomas De La Rue and Company, Ltd., 1992).

FIGURE 4-10 Frequency-modulated image. (True original did not have any moiré.)

FIGURE 4-11 Copy of image made using a state-of-the-art color copier.

The committee has found that concentric-circle patterns and zone plate patterns (Figure 4-8, Figure 4-9, and Figure D-1) are effective in defeating accurate reproductions. Due to the alias patterns

9  

Wicker, 1991, a follow-on patent that covers further extensions and embodiments of introduced moiré for document security, has also been granted.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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generated, this approach should be capable of defeating digital systems operating with sample frequencies up to 1,800 dpi or more.

The committee suggests further development of another family of moir é-inducing patterns, referred to as space filling, self-similar patterns, by the BEP. A fractal-like pattern utilizing this concept is shown in Figures Figures 4-12 and Figures 4-13. Initial BEP experiments on this first-generation pattern utilizing an advanced color copier indicate that it will cause moiré

FIGURE 4-12 Space-filling pattern.

FIGURE 4-13 Moiré image of the pattern in Figures 4-12 that had been reduced (50 µm line width).

at the appropriate combination of line spacing and line width. This pattern has the advantage that it naturally has built-in a range of modulation frequencies that will induce moiré over a wide range of copiers and scanners that have different spatial resolutions.

Interestingly, the moiré pattern occurred in color even though the original image was black and white. This indicates that the sampling registry was probably not the same for all the colors in the copier. In fact the latest approach in color copiers is to use color filter arrays in front of the imaging array; this automatically results in a different sampling registry for each color. Hence, it appears that induced moiré can be quite effective as a deterrent against color photocopiers, scanners, and printers.

To eliminate aliasing effects (i.e., to defeat this feature) the image must be optically “pre-filtered” prior to sampling in order to remove the high spatial frequency content. Optical pre-filtering can be accomplished by two different methods-defocusing the image or designing a special “low-pass” optical imaging system prior to sampling. Fortunately, the amount of image defocus that would be required to eliminate the moiré patterns destroys the fidelity of the entire image, and the design of the appropriate imaging optics prior to sampling is beyond the capabilities of the casual counterfeiter, whose tools consist of a photocopier or computer image processing work station. Likewise, to eliminate aliasing when printing one must utilize some type of “anti-aliasing ” filter. Printer vendors are beginning to offer “anti-aliasing” filters implemented in software (Sensors, 1992). The committee recommends that the BEP monitor the status of “anti-alias” filter design, including devices that produce variable dot shapes and

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
×

sizes or an irregularly spaced raster, to minimize these effects. To date, these various resolution enhancements are somewhat limited and at best modestly decrease the visibility of the induced moir é patterns.

A casual counterfeiter would have a difficult time overcoming the moiré effect. The resultant copies would be obvious fakes due the moiré patterns that could not be easily reworked by hand or that were badly out of focus. The petty counterfeiters would likewise be stymied. However, the professional counterfeiters would not find this feature much of an obstacle, since they would have access to higher-resolution systems.

Given the above discussion, it is the committee's opinion that the induced moiré technique represents an effective and low-cost method to deter counterfeits produced by digital color photocopiers and digital scanning systems. In the long run however, manufacturers of high-quality copiers and scanners will probably incorporate optical anti-alaising filters to appropriately “blur” the sharp edges of line patterns to reduce the moiré effect.

Variable-Sized Dot Patterns

By using halftone methods, one can create the appearance of gray-scale images using binary printing techniques. With halftone printing a human observer will perceive two areas as having the same average shade of gray, even when one area is printed with a large number of small dots and the other area with a small number of larger dots. If the dots that make up one gray-scale area are selected to be below the resolution of reprographic systems and the adjacent areas are selected to contain resolvable dots, then the first area will be incorrectly reproduced. It will appear to the eye as a different shade of gray. In this way, a validation pattern, such as the word “VOID” in large letters, can be encoded that will appear upon reproduction. (Note that in digital image systems, such as high-quality photocopiers and computer work stations, a dot pattern that is finer than the resolution limit of the reprographic system is also likely to produce significant alias or moiré patterning.)

Color could enhance the effect of this deterrent. The dots could be printed in color, using a different dot size for each primary color, in close proximity (but not overlapping) so that the the eye would perceive the result as a dark color (e.g., near black). At least one of the primary color dots would be below the resolution limit of a targeted copier or scanner. The resultant copy would not contain all the colored dots, making it appear an obviously different color from the original.

Effectiveness of this deterrent requires that the printing resolution capability of the BEP stay ahead of that of reprographic technology. For example, for the foreseeable future, readily available copiers and printers will not exceed a resolution of 1,200 dpi. This resolution corresponds to a dot pattern with a dot diameter of 31.75 µm or 1.25 mils (this diameter includes the industry standard 50 percent overlap necessary to avoid jagged edges). This is below the practical resolution limit of intaglio printing. The high pressure of intaglio printing results in small dots being printed with feathered edges requiring fairly wide dot separations. In addition, the array of dots would not lie on a flat plane. Also, sufficient color fonts are not available on the BEP presses to print each size dot with a particular color. However, the necessary resolutions are well within the limits that can be achieved by offset printing.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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Therefore, this feature will require an additional offset printing step to be effective against the high-resolution copiers and scanners. However, another possibility exists. A variable dot pattern could be incorporated in the banknote's design, and the pattern photo-etched into an engraving. The fine pattern would be at the limit of the intaglio printing process. Such a feature would be effective against the older copiers and scanners. If an offset printing step was added, the feature could be printed in finer detail. In this way, the feature could evolve over time without requiring a major design change.

The committee concludes that variable dot-pattern-generated gray-scale printing or “void” patterns can be an effective deterrent as long as the BEP's printing capability exceeds the resolution of new reprographic technologies. (A similar conclusion was reached in a previous report; NRC, 1987.) These features also lend themselves to continual improvement over time.

Latent Images

In the context of counterfeit-deterrent methods, the term “latent image” is used to refer to the method in which the variation surface-relief pattern of the ink obtained with the intaglio printing process is used to produce a different image when the image viewing angle is changed10. Normally, the latent image is observed at large angles with respect to the surface normal, that is, at near grazing incidence. Several countries currently utilize latent images in their currency.

It is the committee's opinion that the latent images can be difficult to see, even on new currency under good lighting conditions. Furthermore, the durability of a latent image is fairly low, as demonstrated by standard durability testing. For these reasons, latent images do appear to be as promising as other deterrent methods. The experiences in other countries should, however, be closely monitored.

See-Throughs

“See-throughs” refers to an area on the banknote in which the front and back images are printed in almost perfect registry. The image on the front is printed to complement the image on the back. These images may be in different colors. The design of the images is done so that any slight misalignment would be obvious when viewed in transmission, and hence would be an indication that the note was a counterfeit. Thus, the name “see-through.” The production of this feature requires a press that prints both sides of a note simultaneously so that the necessary high-precision registration is achievable. Such precision is not possible with the current intaglio press equipment installed at the BEP. Therefore, the implementation of such a feature would require a significant capital investment by the federal government. The value of the effectiveness of this feature would have to be closely analyzed before such expenditures were made. However, with the proper equipment, the printing of these features is inexpensive. Many countries are beginning to print such features. Their effectiveness in deterring counterfeiting should be closely monitored.

10  

This “latent image” is not the same as a photographic “latent image.”

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
×
Bar-Code Technology

Bar-code technology provides a reliable method to encode information about a given item in a convenient machine-readable format. Several bar-code formats have been developed. Conventional linear bar codes are widely used to represent product identification numbers; these are used to look up product information contained in a central data base, for example, the product description and price. Similarly, bar codes can represent the denomination and serial number of banknotes to provide a fast and reliable machine-readable method to sort (and potentially track) them (Storch and Van Haage, 1989). In fact, The Netherlands has adopted such a scheme for its banknotes.

Conventional linear bar codes are, however, limited in the amount of information that can be stored. To increase information capacity, two-dimensional bar codes have been devised for example, with stacked bar codes and matrix codes (Pavlidis et al., 1991; Pennisi, 1991). By increasing the information content, the two-dimensional codes offer the potential to create a “portable data file” that can be retrieved without the need to access a central data base.

With regard to counterfeit deterrence, by tracking and processing bar-coded serial numbers, unauthorized or duplicated notes could be detected. But there may be an appreciable cost associated with the information processing task of maintaining a data base of active currency serial numbers.

It has been proposed that the bar code include randomly selected information in a manner somewhat analogous to the random pattern/encryption concept described later in this section (Storch and Van Haage, 1989). The random information that is encoded might be associated with some particular physical characteristics of the note, for example, the orientation of fibers in the paper or the number of fluorescent microtaggants at a given location. A counterfeit could therefore be detected by comparing the encoded, randomly selected information with the actual physical characteristics of the note; this would, however, require a special machine reader to detect the physical characteristic in addition to the required bar-code scanner.

The general use of bar-code technology for U.S. currency is beyond the scope of this report that focuses on counterfeit deterrence. However, visible bar-codes can be copied as easily as any other printed feature. The counterfeit-deterrent aspects associated with bar codes arise from two aspects: the ability to match serial numbers against an external data base, and to encode random patterns. The practicality of the former has yet to be demonstrated, although it would seem that a major benefit is related to the tracking of currency; this feature would provide a technological solution to the possibilities offered by the individual serialization of each banknote. A detailed discussion of encryption of random patterns is presented in a subsequent section of this chapter.

Post-Printed Optically Variable Devices

Post-printed optically variable devices are those that are added after all other printing operations have been completed. They include diffraction-based devices-holograms, kinegrams, and pixelgrams; multiple diffraction gratings; thin-film devices; and hot-stamped metallic

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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security stripes (these stripes can also be applied by the paper manufacturer). The committee's description and analysis of the various devices are discussed below.

Diffraction-Based Holograms

Holograms are widely used as counterfeit-deterrent features for security documents and credit cards. The striking image that can be produced by a hologram provides a good overt security feature. By far, the largest application to date for hologram security devices has been for credit-card security.

A hologram is a recording of the interference pattern formed by two coherent beams of light–the picture beam and the reference beam (see Figure 4-14). When illuminated by the reference beam during readout, the hologram produces (or reconstructs) the picture beam. The

FIGURE 4-14 Optical setup for recording a hologram of an object.

picture beam can take on many different forms: a three-dimensional image, which posses depth information; a two-dimensional image, for example, a presidential portrait, a bar-code pattern for a machine-reading system; and so on.

A unique property of holography is that more than one image can be recorded and subsequently reconstructed from a given hologram. For example, when viewed at normal incidence, one might see the portrait of Andrew Jackson; however, when viewed at a different angle, one would see an image of printed text, for example, “$20.” Multiple-exposure holograms can also be used to produce image motion as the hologram is viewed at different angles.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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Because a hologram possesses extremely fine structural features, it is essentially impossible to copy or duplicate reprographically with even the most sophisticated equipment; hence, it is an excellent deterrent for the casual counterfeiter. However, it is possible to replicate or simulate hologram security devices using more advanced technical methods. (See RandomPattern/Encription Counterfeit Deterrence Concept.)

The use of holograms for document security has been investigated extensively (American Bank Note Company, 1984; Battelle Columbus Laboratories, 1983, 1985, 1990; Bander, 1984; Church and Littman, 1991; Collins, 1986; Fagan, 1990, 1991; Martin, 1983). A major shortcoming is the hologram's lack of durability under even normal usage when placed on a flexible substrate. The image rather quickly becomes unrecognizable, making the job of a counterfeiter much easier, since a “worn” hologram is easy to simulate. The durability of holograms suffers from the wrinkling of the metallic film. If these devices were not metallized, but were instead placed in a “window” on the banknote for viewing in transmission, their durability might be significantly increased.

The BEP should continue to monitor the status of developments for these devices, and promising research directions should be encouraged. Experience in other countries the effectiveness and durability of holograms should be closely followed. These devices have potential for future consideration.

Diffraction-Based Kinegrams and Pixelgrams

In the recent literature two important variations of hologram technology for document security have been reported: the kinegram and the pixelgram. The kinegram, a patented device, was invented at Landis and Gyr. (Antes, 1983). The pixelgrarn, also a patented device, was invented at CSIRO in Australia (Lee, 1988, 1991). The salient features of these devices are summarized briefly below.

Both the kinegram and the pixelgram can be regarded as special types of surface-relief, computer-generated diffractive optical elements. The basic distinction between these two devices is that a kinegram is constructed using vector-addressing methods, whereas the construction of a pixelgram is based on a discrete-pixel (picture element) addressing scheme. The master element for both the kinegram and the pixelgram is typically generated using electron-beam lithography.

When designing a kinegram, one can vary the spacing, angle, and depth of the lines to produce the desired image reconstruction. These line features can also be varied at different spatial locations so that when the kinegram is rotated, the reconstructed image appears to move. For document security applications, the principal advantage offered by the kinegram is that the recording of the holographic master requires additional processing steps and more sophisticated equipment than those required for making a “simple” hologram. Because of the apparent motion, the image produced by a kinegram is both striking and unique; hence, its authenticity can be easily checked by the public.

Like the kinegram, the pixelgram is also capable of producing multiple high-resolution images. To make a pixelgram, one starts by digitizing the desired image into an array of N × M pixels using a high-quality color scanner-the larger the number of pixels one chooses, the higher the resolution of the resulting image. In the second step, each pixel of the image is

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
×

converted into a miniature diffracting grating. The lines of the diffraction grating are generally curved so as to produce a diffracted intensity that is proportional to the average intensity of the pixel associated with the original portrait image. The resulting grating structure is written onto a recording material using electron-beam lithography; this serves as the pixelgram “master” plate. A pixelgram master typically contains over 10 gigabytes of binary data, and it takes between 10 to 20 hours to write a 20-mm by 26-mm pixelgram. Available optical effects include a positive/negative flip; switch on/off effects; specific color flips; and movement effects.

Both the kinegram and pixelgram can be applied to bank notes quickly and at relatively low cost by using hot-stamping, foil-based techniques (Reinhart, 1991). However, the fastest current hot stamping equipment operates at about 3,000 sheets/hr, which is one-third the rate required by the BEP (Church, 1993), thus three such units would be required on the production floor, necessitating a major realignment of the equipment and production flow, plus additional production personnel.

Several countries have used these technologies on their bank notes. Finland and Austria have used the kinegram (Finland: 500 and 1,000 Mark notes; Austria: 5000 Shilling). Australia and Singapore have incorporated pixelgrams into commemorative banknotes (Australia: $10 note; Singapore: $50 note).

The kinegram and pixelgram technologies offer a number of excellent counterfeit-deterrent aspects for document security. Unfortunately, however, for U.S. banknote applications all of the hologram samples that have been tested to date have been deemed to have a severe limitation: durability. The holograms that have been tested have all failed, to various degrees, the BEP's tests for mechanical durability (abrasion, crumpling, etc.) and chemical durability (laundry detergent, bleach, dry-cleaning chemicals, etc.)11 As is the case for holograms, the durability of kinegrams and pixelgrams suffers from the wrinkling of the metallic film. Their durability might be significantly increased if they were not metallized but were placed in a “window” for viewing in transmitted light.

The so-called “crumple test” is particularly detrimental to the integrity of the reconstructed holographic image (Church, 1992). To improve the fidelity of the holographic image after mechanical distortion (creasing, folding, or crumpling) of the banknote, a “multi-redundancy” hologram has been developed (Haslop, 1993b). The multi-redundancy hologram consists of a unique sub-image that has been replicated several times over the area of the reflective holographic foil. Since only a single sub-image is required to detect authenticity, not the full image produced by the holographic foil, the device exhibits improved resistance to mechanical distortions.

At the time of this report, data regarding the durability of the banknotes in circulation, that utilize holographic technology were not available. To assess properly the issue of durability, the BEP should seek information regarding the durability of circulated banknotes, particularly about recognizability, from the other countries currently utilizing holographic security devices and should correlate this information with the test procedures used at the

11  

See Bureau of Engraving and Printing Test Methods BEP-88-02, “Crumple Test;” BEP-88-04, “chemical Resistance;” and BEP-88-05, “Laundering.”

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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BEP. Promising research directions should be encouraged, and the BEP should continue to monitor the status of developments for these devices, which have promise for the future.

Multiple-Diffraction Gratings

A diffraction grating is an optically variable device that consists of a series of finely spaced parallel grooves. Diffraction gratings can be formed on a wide variety of substrate materials, including metals, glass, polyesters, and polymers (or plastics). The optically variable nature of a diffraction grating is controlled by the selection of grating parameters-the groove spacing and the width, depth, and shape of the grooves-so as to produce a particular color and brightness in a specified viewing direction. By combining a collection of gratings (multiple diffraction gratings) with different grating parameters, one can create a detailed multicolor design pattern (lettering or other two-dimensional images). When the multiple diffraction grating is tilted to a different observation angle, both color shifts and design shifts can occur.

Multiple-diffraction-grating structures were investigated in detail in the 1980s by the BEP and several other countries, including Austria, Australia, Canada, the United Kingdom, and Switzerland. The research on multiple diffraction gratings for counterfeit deterrence was summarized in a BEP presentation (Church and Littman, 1991). They were found to be readily recognizable by the general public and cannot be reproduced on advanced reprographic equipment. However, they could be simulated by technically proficient counterfeiters with relatively simple materials. The durability of the devices was also rated low. They are susceptible to severe wear by abrasion, and cannot survive the BEP's crumple test. Australia is using a multiple diffraction grating in its commemorative $10 note; Singapore also is using a multiple-diffraction-grating in a commemorative note.

Thin-Film Interference Filters

A thin-film interference filter (TFIF) consists of one or more layers of vacuum-deposited inorganic materials formed on a substrate. The filter utilizes the wave nature of light to filter selectively a specific color or band of colors. Using TFIFs, it is possible to design a multilayer structure that exhibits a striking and distinctive color change when viewed from different angles, for example, green to gold, blue to red, etc (Berning and Phillips, 1987a,b; Dobrowolski et al., 1989; Phillips, 1990). Color-shifting inks, discussed earlier in this chapter, are a special case of TFIFs.

This variable color change cannot be produced by photocopying, photography, or other reprographic techniques. Furthermore, because one needs sophisticated vacuum-coating equipment, coating designs, and process control to make TFIFs, there are only a limited number of facilities in the world that are capable of making these filters, which further adds to document security. However, the notes must be manipulated correctly with respect to a light source to observe the color shift.

TFIFs can be based on either a dielectric-metal multilayer stack or an all-dielectric multilayer system, which can be affixed to the document using an adhesive. It was found that the performance of the dielectric-metal TFIFs in aging, mechanical, and chemical durability tests was worse than the all-dielectric designs (Dobrowolski et al., 1989). Furthermore, since the metal-dielectric TFIFs don't require as many layers as the all-dielectric designs, they are

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
×

somewhat easier to make; hence, they are more susceptible to attempted counterfeiting. If metals are used for one or more layers, their potential environmental effect should be analyzed.

Researchers have investigated TFIFs consisting of a metal reflector/dielectric/metal absorber structure (Berning and Phillips, 1987a,b). To overcome problems associated with the transfer of the TFIF to the document and to improve the performance of the TFIF with respect to mechanical wear and tear, the researchers found that by removing the multilayer coating from the polyester web, they could produce small flakes of TFIF, which could be used to make an optically variable pigment as a component in an color-shifting (optically variable) ink. Color-shifting inks, discussed earlier in this chapter, exhibit the same type of color shift with angle observed the large-area TFIFs but at a lower apparent intensity, since the optically variable pigment is suspended in ink. On the other hand, color-shifting inks are less sensitive than TFIFs to mechanical deformations, such as crumpling and folding.

The National Research Council of Canada determined that the preferred combination of multilayers were oxide (dielectric) films of ZrO 2 and SiO2 (Dobrowolski et al., 1989). It reported that the films of these materials deposited by evaporation were rather porous and tended to age on exposure to air. However, the aging was found to be quite predictable and could be taken into consideration in the filter design. Measurements of dependence of the spectral performance of the dielectric multilayers with changes in temperature, humidity, mechanical wear and tear, and chemical attack were found to be acceptable for banknote applications. This type of TFIF is currently being used on the Canadian $50 and $100 notes. The Bank of Canada reports that its tests indicate that the optical effect of its thin-film security device can still be recognized after the note has been crumpled, washed, dry-cleaned, and scratched (Church and Littman, 1991). However, these films will not hold up to all of the BEP's exacting durability tests.

These devices have many of the advantages of the color-shifting inks. They are presently more expensive than the color-shifting inks, although the color change is more dramatic. They could be simulated by metallic film, but such a counterfeit would be easy to detect since it would not undergo the characteristic color change. The BEP should stay informed about the Canadian experience with TFIFs.

Hot-Stamped Security Stripe

Metallized, hot-stamped stripes, followed by an overprint step, can also be an effective counterfeit deterrent against photocopying. When a banknote containing a metallized security stripe (normally consisting of metallized segments) is copied, the photocopy turns black at the locations of the metallic segments due to specular reflection, and the overprinting on the original banknote is completely lost in the photocopied one. Furthermore, because of the overprinting, one cannot simply remove a given segment and place it at different locations, which makes it more difficult to produce “raised” notes. The security stripe can contain different regions, some functioning as a simple reflector and some functioning as an optically variable device. Stripe widths typically vary from 3 to 3 mm. Because of its small size, the security stripe is found to be more durable than large-area diffraction-based and thin-film devices described above. Mechanical durability of an optically variable device in the stripe is

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
×

increased because it is small, (Haslop, 1993a) and both the chemical and the mechanical durability of the stripe can be improved, because there is greater latitude in selecting materials that are more flexible and more resistant to chemical attack (Reinhart, 1991). This feature has potential for the long term, and the progress of its development should be monitored by the BEP.

Embedded Zero-Order Diffraction Gratings

Embedded zero-order diffraction gratings represent a new type of microstructure device that may prove to be useful as a counterfeit-deterrent feature. The optical characteristics are similar to those of a thin-film interference filter in that striking color shifts are produced as the structure is rotated or tilted. They consist of extremely fine, surface-relief grating structures embedded in a transparent plastic.

The structure and reflected spectrum of an embedded zero-order diffraction grating structure are shown in Figures 4-15. The reflection spectrum is sharply peaked due to the fact that the operation of the device depends on guided-mode resonances associated with the structure. The optical properties and applications of these structures are being investigated by a number of researchers (Gale, 1991; Gale et al., 1990; Norton et al., 1993; Wang et al., 1990). The design and fabrication of such structures are now possible due to recent advances in the theoretical understanding and computer modeling of light propagation in sub-wavelength structures and advances in microlithography. Because the operation of these devices depends on the fact that the submicron grating structure must be embedded in plastic and the refractive index of the plastic plays a key role in the design of the device, the possibility of copying these structures either mechanically or optically is highly unlikely.

FIGURE 4-15 Structure and reflection spectrum of an embedded lamellar diffraction grating.

Source M.T. Gale, Zurich Switzerland, Diffraction Microstructures for Security Applications (Figure 2).

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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While the application of zero-order gratings as a counterfeit-deterrent feature requires significant further development, it appears promising. Therefore, the committee that the BEP monitor the progress that is being made in the development of these devices.

Random Pattern/Encryption Counterfeit-Deterrence Concept
Implementation Using Two Visible Features

This system has the potential to provide a very high degree of authentication of banknotes that even the determined professional counterfeiter cannot defeat. It does require the use of an automated reader. And leverages a significant amount of development that has been done for arms control verification purposes (Bauder, 1983; Graybeal and McFate, 1989).

This concept is based on tagging each banknote with a unique random three-dimensional pattern as an identifier. Since the unique identifier is three-dimensional, it cannot be reproduced by copiers or printers such as those discussed in this report or by other existing reprographic processes that produce only two-dimensional images. Since the unique identifier (fingerprint) is randomly generated, a counterfeiter cannot duplicate it even by employing the same process used to produce the original12. In order for this method to work for banknotes, three technical issues must be addressed: (1) selection of an appropriate three-dimensional random pattern, (2) encryption of the pattern in such a way that an authenticator can be printed directly on the banknote, and (3) implementation of a method to read the pattern and compare the results with the printed authenticator of the pattern (Church and Littman, 1991). The progress made to date in addresing these issues is summarized below.

The unique identifier can be drawn from the natural three-dimensional features of the paper such as the pattern of the fibers or features that are added to the paper during its manufacture. In a proof-of-concept demonstration for banknotes, short pieces of plastic optical fiber are added to the fiber-water slurry that forms the paper. The optical fibers become distributed randomly throughout the paper. The pattern in a defined area is read by passing the note under a light bar surrounded by an array of photodetectors. Whenever the end of an optical fiber is illuminated, the light transmitted to the other end is detected and its location recorded. The result is a pattern that is a function of the relative locations of the ends of the optical fibers. Other unique identifiers can be used; a choice among them would consider readability, cost, durability, and appearance.

The three-dimensional properties of the unique identifier are read with an imaging device to create a numerical description of the pattern. The description can be encrypted and printed on the note in bar code or other machine-readable pattern to serve as the authenticator. A note is authenticated using a device that reads the unique identifier pattern, reads and decrypts the authenticator, and compares the two; if they match, the note is authentic.

12  

By definition, it is impossible to predict the outcome of a random process; therefore, each pattern that is generated will be different.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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A public key encryption method can be used (Hellman, 1979; Rivest et al., 1979). This method includes a decryption key and an encryption key that are different, and neither one can be derived from the other. For this application, the key required to decrypt the authenticator can be public knowledge. This allows authentication of notes by the public without fear that the secret encryption key would become available to potential counterfeiters who could then generate their own random unique identifiers and the related correct encrypted authenticators.

Fiber-optic patterns on currency have been read and authenticators have been printed at speeds up to 400 in/s (equivalent to more than 40 notes/s in a production application; Baird Corporation, 1989). By careful selection of the random pattern, it would be possible to provide a lower level of authentication at points of sale inexpensively, since special equipment would not be required. A higher level of authentication could be implemented at the Federal Reserve banks, and possibly other locations, using automated high-speed readers. For example, if the fiber-optic pattern described above were implemented, the existence of the fibers themselves could easily be detected visually at points of sale. Further inspection with a small light source (e.g., a pen light) would detect whether or not these fibers conduct light. The highest level of detection would require machine reading to compare the fingerprint (the specific unique pattern of the fibers) with the authenticator.

Initial readings of unique identifiers will differ from subsequent readings taken when notes are authenticated because of unavoidable variations in the location of the reader with respect to the note, variations in the readers, and changes in the pattern due to wear and tear. Methods have been developed that account for these variations and ensure low false-decision probabilities. These methods are described in Appendix E.

Efforts should continue to develop the random-pattern/encryption concept for possible future use in currency. It has the potential to provide a major increase in counterfeit deterrence.

Implementation Using One Visible and One Secret Feature

An alternative approach to implementing this concept would involve a visible and a secret feature. In this case, a unique visible feature, such as the serial number, could be subjected to a public key encryption algorithm as before. But the resulting pattern would be printed in a secret fashion. During authentication, the device would compare the unique overt feature with the covert encryption; if they match, the note would be accepted as genuine.

Such a system suffers from the fact that the visible feature selected is not a random three-dimensional pattern, and hence could be readily copied. The security of the method would then depend on the ability to keep the encrypted pattern secret and thus not able to be copied. The committee thinks that the covertly printed image could be detected by a determined professional. Because of this, the approach is not attractive.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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Counterfeit Deterrence Incorporated in Copiers and Printers

The committee has concluded that color copiers and color printers operated by the casual counterfeiter present the greatest counterfeiting threat. (See Chapter 2 and Chapter 3.) One approach to discouraging such opportunistic counterfeiting involves making the equipment less “friendly” to use. Two strategies being pursued are for the equipment to recognize banknotes and fail to reproduce them and for each copy from a particular machine to print a covert, traceable code in the image area of the reproduction.

Currency Recognition System

One manufacturer of advanced color copiers has developed and installed in its latest copiers, a first-generation system to automatically identify an attempt to copy certain banknotes (Tsujita, 1993). If the equipment detects a forgery attempt, it prints a black or blank copy and can be programmed to then shut down until it is reset by a repairman.

The detailed implementation of this anti-forgery approach has, understandably, not been presented in full detail. It is, after all, a security measure, and complete disclosure would leave the system vulnerable to compromise. Since it is a technological solution to a human problem, the solution is not likely to be perfect. It could however be reasonably effective for some period of time, particularly against the threat posed by the casual counterfeiter. But eventually any purely technological solution is likely to be compromised through development of some countertechnology.

This forgery detection and prevention system is, in effect, an expert system that includes software and hardware components (Canon, 1990) 13. The input information to the software is provided as a byproduct of the successive red, blue, green, and brightness scans. The scans provide a rough location of the position and orientation of the bill, followed by a refinement that includes the location and identification of important features on the bill. Statistical information is computed regarding the distribution of colors and feature sizes (possibly linked with color).

The example stated in the patent involves the detection of a specific Japanese banknote. The present system can reportedly store the data for four samples of each of eight different currencies; that is, 32 separate images. Therefore not all possible denominations of all possible currencies will be contained in the stored detection set. It is not known whether the samples stored in each machine are identical. Features have been included in the design of the electronics to prohibit operation of the copier if the forgery detection feature is disabled. However, a drawback for any technological approach to forgery detection is that once the rules are known, possible countermeasures can be developed to evade the detection algorithm.

13  

The patent describes the form scanning operations used to determine if a forgery is being attempted. The first scan detects the approximate position of the note. The second scan determines the exact position and oventation of the note. The third scan calculates the expected position of the seal using data from the previous scans, and then determines if the seal is present at that position. The fourth scan produces a black full overprint of the note image if the third scan concluded that a forging is being attempted.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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There a number of unknowns. The committee does not know the extent to which this forgery detection process is successful. Reports are that it does work under a number ofunidentified conditions. (Legal considerations prohibited field testing of the feature by individual committee members.) This committee did not receive any information as to the efficiency of the process against simple countermeasures. Since the print engines in color copiers will also function as computer-driven output devices, it is not clear what degree of protection will be provided by the forgery detection feature in computer printers. Also, the degree to which the technology will be made available to other manufactures is not known.

The number of false alarm detections of currency will certainly become an issue. The cost of these machines is significant enough that no user will purchase a system that fails to copy any range of materials that “look like” but are not currency. Market forces will eliminate any feature that diminishes the usefulness of the copier. In addition, it is not known how well the feature can detect attempts to copy an already counterfeit (and thus not perfect) banknote.

In the opinion of the committee, the copier manufacturer is to be congratulated for this technological achievement. In the short term, the currency recognition feature will probably serve as a deterrent for the casual counterfeiter as a result of the knowledge about the anticounterfeiting feature. The committee, however, thinks that this feature will not be a long-term solution to the copying problem. Any well-defined technological detection scheme can probably be unwrapped by a technological solution. It is not likely to deter a technically knowledgeable, determined professional or casual “hacker” counterfeiter.

The present copy-protection scheme is a promising start. As electronics continue to grow cheaper and more capable, it is likely that more sophisticated schemes for recognizing attempts to copy currency will be made. Since the committee's concern is the U.S currency, it is interesting to note that good copying of the colors in the present currency are reported to be more difficult than that of some of the varicolored currency of other countries. Post-copying addition of simulations of such features as metallic or variable inks seems possible and can likely be done successfully. The use of metameric ink to “fool” the copier is not likely to be successful in the long run, as market forces will force development of techniques that are resistant to metameric problems in non-currency copying.

Consideration should be given to requiring the application of this technology to all advanced color copiers and printers, as other countries are considering. That depends, of course, on whether the patent holder is willing to license at least some aspects of the technology or if an alternative approach can be found that would be made widely available. Also, the addition of appropriate electronic components to all advanced color copiers and printers could increase the price of the low-end models significantly enough to be outside the range of the feasible.

The usefulness of this approach could be enhanced if many nations adopted the same unique feature on their banknotes that would trigger the currency recognition system. In addition to easy detection, such a feature should not require extensive pattern-matching algorithms to match-up scale, geometric orientation effects, etc. A fractal-based self-similar pattern, such as is shown in Figures 4-12, may be useful. The U.S. Treasury Department could take the lead in lead in defining the problem and securing international cooperation in this regard.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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It is important to note that the color copier will fail to copy correctly such features as metallic inks, color-shifting inks, and holographic types of features. It is possible to consider machine detection schemes that are capable of discriminating such features on currency and detecting probable attempts to carry out forgeries. The result of a copy operation will be to record whatever optical image of the variable feature is presented through the copier which will necessarily be incomplete and readily detected.

Copier/Printer Identification System

A potentially important feature on some advanced copiers is the encryption of the machine serial number at several locations on all of the copied material. A microdot pattern is used to encode the machine number in every copy that is made. Detection and reading of this encoded microdot pattern require special techniques that must be implemented in the laboratory. The presence of a traceable origin of each copy will probably deter some copying and would make tracing the origin of copied currency possible14.

Consideration should also be given to extending this technique to color printers, not just electophotographic printers (e.g., ink-jet printers). This may require the addition of additional pre-printing computational processing steps, resulting in a requirement for increased computational power. The cost effectiveness of such a requirement must be closely examined.

Features Used by Other Countries

During its deliberations, the committee considered more than forty types of features that were deemed either to offer an aid to detection of counterfeit currency or to act as a deterrent to counterfeiters. The debate had to take place mainly in the domain of the former, since there are relatively easy measures of success for detection —either in the public marketplace or by official government agencies —whereas it is more difficult to prove a negative in the domain of the latter (i.e., whether due to incorporation of any specific feature, counterfeits have not shown up in circulation).

Of the long list of special features (a described in detail throughout this report), all but about a dozen are already incorporated in world currencies, although some have been introduced only relatively recently (e.g., plastic substrates in Australia) or have been announced to be imminent (e.g. kinegrams in Australia now, in Switzerland in 1995). Thus, at first sight, it might appear that there is a considerable body of experience to draw from, but in actuality there are few quantitative facts. Obvious and intuitive special features such as holograms may have defeated color copiers and other forms of electronic printing but could be relatively easy for proficient counterfeiters to simulate otherwise. They may even be counterproductive in the sense that they may reduce the practical life of the note or focus attention on a single feature of the note. On the other hand, color-shifting inks —now used

14  

The committee realizes that other public policy issues will be raised by this capability. The perspective here is purely from the standpoint of counterfeit deterrence.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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by many countries—are more subtle in appearance but defeat color copiers and at this stage would seem to be more difficult to counterfeit by other means. The trade-off would thus seem to favor color-shifting inks, but there are no readily available data to support this conclusion. The situation becomes further complicated by the classification into visible and hidden features. For example, Dutch currency has incorporated visible bar codes, whereas Great Britain uses these codes in hidden form, principally as an aid to the automatic sorting of notes by banks. Most “smart inks” are not obvious unless detected by means of an appropriate physical property, such as fluorescence, phosphorescence, iridescence, and magnetic properties. There are varying degrees of difficulty in copying or simulating these inks using existing technologies. The inks currently used by various countries, but again with little quantitative evidence of effectiveness.

The majority of features deemed attractive for special consideration by the committee have typically already been incorporated in one or more currency (or related documents). On the other hand, technical approaches that appear promising for future research and consideration (e.g., optical fibers in paper) have not yet been incorporated in any known currencies and so there is no practical experience on which to draw.

The overall conclusion is that whereas long lists can be drawn up of currencies incorporating a variety of smart features, there is very little evidence of quantitative effectiveness, or for that matter of sharing any practical experience between countries. Ironically, it is the existence of new emerging global communication technologies (electronic networks and output devices) that presents the opportunity for prospective counterfeiters and that may pose a much more serious problem as these technologies evolve. Just as a single, new high-quality color copier may cause simultaneous counterfeiting problems worldwide, the power of a collective deterrent strategy rather than ad hoc design changes by individual countries will probably become increasingly important.

A handful of nations print their own currency—these are typically the more economically advanced nations—while the remainder are served by a very small number of private printing companies. In that sense, there are only relatively few “independent” blocks of data/experience, and there are existing collaborations between some of these blocks in certain common currency matters. It is important to increase the degree of sharing of counterfeiting/deterrence information.

Other U.S. federal agencies are concerned with secure documents. These include the Postal Service (e.g., postal money orders), State Department (Passport Office), Agricultural Department (e.g., food stamps), and so on. Increased sharing of ideas and experience would be mutually beneficial. Thus it would seem there are obvious avenues for greater sharing of experience and for a collective approach to research in detection and deterrence in the future.

RECOMMENDATIONS

Although there are many new features that can be used to deter counterfeiters, the BEP should continue to utilize intaglio printing, the security thread, and the current substrate material as methods of deterrence against “classical” printing technologies and present day reprographics. In addition, future banknote designs should incorporate additional visible

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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features to serve as deterrents against counterfeiting and as a means for rapid visual authentication. If analysis shows it is cost-effective to do so, some of these visible features could be incorporated into a banknote and their existence not publicly disclosed until they are needed to thwart a new counterfeiting threat.

The BEP should implement a system of complementary features on each banknote that create added complexity for simulation by all levels of counterfeiters. They should not, however, constrain their design by a requirement that the same set of counterfeit-deterrence features be on all denominations of bills. And although multiple features rather than a single dominant feature should be present on each banknote, the number of announced features should not be so great that it overwhelms the user or does not allow space for future feature incorporation.

The BEP should carry out a redesign of U.S. banknotes to include the recommended features, making such changes in appearance as are necessary to produce a new series of notes that effectively and efficiently incorporates these advanced counterfeiting deterrents. By a wide margin, most banknotes in circulation are genuine. All things being equal, deterrent features that would cause a counterfeit note to look significantly different from a genuine note would probably be more useful to the average citizen than a feature that caused a genuine note to be authenticated upon detailed inspection. On the other hand, a specific machine-detectable feature incorporated in a genuine note would be more useful in a currency-authentication device.

The recommended features fall into three categories: near term, intermediate term, and long term. Within the categories, the deterrent features are not prioritized because of insufficient data relating to implementation issues and the realization that no single feature is adequate protection from even casual counterfeiting.

The committee recommends incorporation of at least some of the following visible features in the near term:

  • color-shifting inks for printing;

  • moiré (alias-generating) line structures, with color added as necessary to enhance the effect;

  • security thread modifications—for example, location or width based on the denomination;

  • variable-size dot patterns, with color added to enhance the effect; and

  • localized watermarks.

Incorporation of at least some of the following features, requiring inexpensive visual aids for detection at the point-of-sale, are recommended for the intermediate term:

  • infrared inks for printing;

  • optically active coated fibers and particles embedded in the substrate; and

  • photoluminescent inks for printing.

Longer-term plans for advanced deterrents (listed in alphabetical order) should include additional development and understanding of the following features:

  • diffraction-based holograms and related devices;

  • embedded zero-order diffraction gratings;

  • laminated paper substrates with selected features;

  • metallic or specular woven security features;

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
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  • optical fibers embedded in the substrate; and

  • random pattern encryption methods.

For the far term, the BEP should continually assess fundamental advances in the chemical, applied physical, and biological sciences for developments that are applicable to innovative deterrent features. Assessment of research in psycho-physics would also be pertinent since a better understanding of how people perceive visible features may provide insight into the selection of the “best” features.

Before any new counterfeit-deterrent feature is implemented, it should be evaluated by adversary-analysis experts to determine how readily it can be defeated. This process would be aided by having a means to quickly produce currency with appropriate design changes.

There are other elements of a deterrent strategy that can be implemented. To begin with, counterfeit-detection education should be emphasized for point-of-sale persons as a priority, and then for the public at large. Potential incentives that would encourage the public to turn in counterfeits should be closely studied to determine which would be effective and not subject to abuse15.

Industry should be encouraged to develop effective point-of-sale aids to assist in banknote authentication. Efforts that will lead to a high degree of authentication, particularly for the higher denomination bills, should be continued. These may involve synergistic combinations of visible and hidden covert features that could be related in some way, such as through a public key encryption system.

The Department of Treasury should encourage U.S. legislation to require source identification to be embedded in images produced by new copier and printer systems capable of producing color counterfeit banknotes. In addition, the department should strongly encourage the use of sensors built-into color copier/printer systems that can recognize and inhibit banknote copying. For this approach to be most effective, a unique, high signal-to-noise ratio feature universally applied to currency should be identified and developed, possibly in conjunction with other nations.

15  

The committee believes that counterfeiting should not be a “victimless” crime, since the fear of a loss does provide the public with some incentive to examine banknotes.

Suggested Citation:"4 Description and Assessment of Deterrent Features." National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, DC: The National Academies Press. doi: 10.17226/2267.
×

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Traditionally, counterfeit deterrent features restricted counterfeiting to only the dedicated craftsman. With the advent of highly sophisticated reprographic systems, this is no longer true. Redesign of U.S. banknotes is necessary in order to incorporate additional features aimed at discouraging counterfeiting using advanced copiers-scanners-printers. This volume evaluates a large number of such features while recommending a comprehensive national strategy for anticipating and responding to counterfeiting threats.

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