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4 Display Charactensffcs In this chapter we summarize and evaluate the known relation- ships between characteristics of video display devices and observer visual performance, subjective responses, and physio- logical responses. The chapter is divided into major sections on CRT display variables, pertinent display measurement techniques and associated problems, a comparison of flat-panel and CRT display characteristics, and characteristics and relative effec- tiveness of filters. For each of the pertinent display variables, we consider three categories of effects on human users: physiological effects, the effects of display variables on measurable and objective perfor- mance, and known relationships between display parameters and subjective estimates of display quality or related physical symp- toms. Physiological effects are those in which the display param- eter has a known, direct physiological effect on the human visual or other organic system. Physiological effects typically cannot be controlled by a user and are not necessarily recognized by a user. For the second category of effects, representative performance measures include speed and accuracy of performance. In the third category, the reported symptoms include subjective estimates of blurring of characters, headaches, visual fatigue, and musculo- skeletal discomfort. EFFECTS OF CRT DISPLAY VARIABLES Luminance Increases in display luminance have several direct effects on visual physiological and optical responses and visual performance. 66

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67 Effects on Visual Acuity In general, increases in display luminance will cause decreases in pupil size, which in turn lead to increases in the optical depth of field and improvement in optical quality. Figure 4.1 illustrates reduction in pupil size as a function of retinal illuminance, assuming a uniformly illuminated retina. This increase in retinal illuminance, which causes a decrease in pupil diameter, directly affects the visual acuity of the normal healthy eye, as shown in Figure 4.2. While the differences are not very great over the normal display operating range, an increase from approximately 1 or 2 milliLamberts (mL) to about 60 or 70 mL causes an increase in visual acuity of approximately 50 per- cent. Thus, displays having higher luminance permit an operator to see finer details on the display. The greatest proportional gain in acuity with increasing luminance takes place between approxi- mately 1 and 10 mL. In general, a positive-contrast display (light characters on a dark background) will have a background luminance of about 1 or 2 mL, and a character luminance of about 25 mL, with a character density of approximately 30 percent. This combination produces a display having an average (adapting) luminance of about 6 or 7 mL. By comparison, a negative-contrast display (dark characters on a light background) will have a background luminance on the order 8 7 - _ 6 cr LL LL ~ J I: J 4 3 `jI Tat_ 2 a, Dark -1 0 1 2 3 4 5 OG R ETI NAL I LLUM I NANCE (trolands) -1 0 1 2 FIGURE 4.1 Diameter of the pupil as a function of retinal illuminance. SOURCE: ten Doesschate and Alpern (1967~.

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68 ~ .6 1 .4 1.0 - c' J 0.8 6 - - cn > 0.6 0.4 0.2 o f l- :~t . it. , ~ 1 1 1 1 1 1 1 _5 _4 _3 - 2 -~' 0 1 2 3 LOG L (mL) - FIGURE 4.2 Relationship between visual acuity and adapting luminance. SOURCE: Hecht (1934~. Of 25 mL and a character luminance of about 1 mL, producing an average (adapting) luminance of about 17 mL. Accordingly, one might expect an increase in relative acuity from 1.4 to 1.6, or approximately 15 percent, for a change from positive to negative contrast. This acuity increase, however, is probably neither important nor real. As suggested by Rupp (1981), the adaptation level is probably not a function of either background luminance or inte- grated luminance, but rather a function of the higher luminance of an irregular surface. Thus, Rupp suggests that the lighter of the two items, either the background or the character, will essentially control the adapting luminance level, thereby negating any effect on pupil size due to positive versus negative contrast. Whether this is actually the case has yet to be demonstrated experimen-

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69 tally for VDTs. There is cause for concern over such general- izations because of the lack of direct application of existing literature to VDTs. For example, there is overwhelming evidence that contrast sensitivity, as well as acuity, increases significantly with increases in overall retinal illuminance (see Figure 4.3~; these and other data are based, however, on display fields in which the light and dark elements are approximately equal in area rather than on the unbalanced display typical of a VDT. It is known that people with poorer eyesight benefit more from increased levels of retinal illumination than do people with normal eyesight (Hopkinson and Collins, 1970~. It is also known that maximum acuity is obtained when the surround (the area or sur- face around the display) is equal in luminance to the display (adapting) luminance (Hopkinson and Collins, 1970~. A secondary benefit of higher display luminance is the increase in visual depth of field Cased on a fixed diameter of the "blur circle") as the 1.0 0.5 Zo - ~ 0.05 o - 0.1 ~ 0.01 is o c' 0.005 0.001 Retinal ~ Illuminance (Trolands) 0.0009~ ~ 0.09 - - // //111 /// / /// 0.9 ~ ~90// ~ 90k l l l 1~1111 l 1 1 1 ,,,1 , , 1 1,,,,1 0.1 0.5 5 10 50 100 SPATIAL FREQUENCY (cycles per degree) FIGURE 4.3 Effect of retinal illuminance on contrast threshold. SOURCE: van Nes and Bouman (1967~.

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70 pupil diameter decreases. Assuming that the luminance to which an observer adapts is in fact the space-average luminance of the display, a negative contrast display (higher space-average luminance) would typically yield a pupil diameter of about 4.5 mm while a positive contrast display (lower space~verage luminance) would yield a pupil diameter of about 5.0 mm (see Figure 4.1, above). This difference in pupil diameter corresponds to approxi- mately a 30 percent difference in blur circle diameter (at the 50 percent intensity point), as shown in Figure 4.4. Again, however, application of these data to VDTs in the workplace should be experimentally verified. As with all lenses, aberrations in the eye are greatest in the periphery of the cornea and the lens. Thus, pupil constriction improves the quality of the image formed on the ret me by excluding light that passes through the peripheral portions of the cornea and the lens (i.e., light rays beyond the border of the pupil at its adapted diameter). While pupil constriction is caused by increasing the amount of light in the adapting field, it also occurs synergistically with lens accom- modation (focusing) for near objects. Thus, as the eye focuses on closer objects, such as a VDT at a working distance, the pupil will e.oo _ 7.00 - C' 6.00 . _ - ~ 5.00 LL J Cl: Cal J m ,~ 10% Intensity 1~/ } 12% 4.00 3.00 2.00 1.00 I ~ 1 1 1 1 1 1 1 1.00 50% I ntensity ~ 2.00 3.00 4.00 5.00 6.00 7.00 PUPI L DlAM ETER (mm) FIGURE 4.4 Blur circle diameter as a function of eye pupil diameter. SOURCE: Campbell and Gubisch (1966).

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71 "automatically" constrict to obtain a somewhat sharper image. Thus, there is a significant interrelationship among display lumi- nance, pupil diameter, blur circle, depth of field, and contrast sensitivity (or acuity). Generally, increases in display luminance will improve visual performance and tend to permit greater cancellation of spherical aberrations by the constricted pupil. On the other hand, positive contrast may tend to make the pupil larger, thereby reducing visual acuity (or contrast sensitivity), increasing the blur circle, and permitting greater spherical aberration. Effects on Flicker Threshold Another physiological effect on the visual system resulting from changes in display luminance relates to shifts in the flicker threshold. As illustrated in Figure 4.5, the temporal contrast sensitivity function becomes less sensitive with decreases in retinal illuminance. Thus, as the average (adapting) luminance of a display increases, the eye is more likely to perceive flicker at any particular repetition rate. This effect has been reported in numerous experiments, including those that have included such variables as the wavelength of the light, the wave form of the stimulus, the size and shape of the stimulus, etc. ~ generali- zation from the research of de Lange (1958), which illustrates the relationship between the critical flicker frequency and the Fourier spectrum of the time varying stimulus, is shown in Figure 4.5. In general, de Lange found that the Fourier fundamental of the display could be used to predict the modulation at which flicker is perceived, as a function of repetition rate, irrespective of the wave form of the light. Unfortunately, large-area displays using negative contrast are perceived to flicker at much higher refresh rates than those using positive contrast in a typical VDT environment. Thus, a display with ~ 50 Hz refresh rate that is just at threshold for flicker at 10 cd/m will flicker very noticeably if luminance is increased to 100 cd/m2. This effect is in conformance with the well~stablished Ferry-Porter Law, which suggests that the highest frequency at which flicker is perceived increases linearly with the logarithm of the adapting luminance, or by approximately 10 Hz for each tenfold increase in luminance. The data of Bauer and Cavonius (1980) clearly support this result. Bauer and Cavonius recommend 1 See also the discussion in Chapter 7 of the relationship between pupil size and accommodation in studies of fatigues

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72 Retinal Illuminance (Trolands) 1 0.5 o J o ~ 0.1 o 0.05 0.01 0.005 - Waveforms _ rUL - nurL _ J111J1~ 4.3 43 430 /11 ::j 1 1 1 1 1 1111 1 1 1 1 1 11-11 1 5 10 50 100 FREQUENCY (hertz) FIGURE 4.5 Temporal contrast sensitivity function. SOURCE: de Lange (1958~. Reprinted with permission of the Optical Society of America. a repetition rate of 100 Hz for VDTs with negative contrast. This recommendation appears to be reasonable and probably indicates the main reason that manufacturers have been reluctant to use negative-contrast displays in the past: standard television monitors cannot produce that repetition rate. Effects on Visual Task Performance The effect of display luminance on visual task performance has been investigated in a few studies. Snyder and Taylor (1979) demonstrated that increases in character luminance caused significant increases in individual character legibility in several

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73 different viewing tasks. Unfortunately, in that particular experiment, the background luminance level was held constant, and therefore the character luminance was totally confounded with the contrast of the displayed image. Supporting evidence, however, for the effect of display luminance on performance is offered by Bauer and Cavonius (1980), who found that a higher- luminance negative~ontrast display yielded both greater subjective preference and improved visual performance than a lower-luminance negative-contrast display. Further research on the subject of the effect of display luminance when separated from the influence of contrast and contrast polarity is needed, however, before this issue can be directly resolved. Luminance Uniformity There is very little research in the literature to provide informa- tion on the minimum requirements for the uniformity of visual displays. No studies are known to provide either thresholds of detection or tolerance limits for large~rea nonuniformities. In general, we simply do not know how much large~rea nonuniform- ity is a reasonable design goal. The case for small-area nonuniformity is similar. Unless one applies basic sine-wave sensitivity data to a given form of small- area nonuniformity distribution and attempts to predict the detectability of nonuniformity, there is currently not even a suggested means for evaluation. Except for an initial study by Riley and Barbato (1978), there little knowledge of the effects of line errors (on or off) or of element errors (on or off) on display legibility and utility. Research efforts to fill these data gaps are obviously needed. Contrast and Contrast Polarity As suggested in the preceding discussion, increases in contrast have been shown to produce significant increases in visual task performance. In addition to the study of Snyder and Taylor, Shurtleff (1982) also demonstrated increases in legibility as a result of increases in character/background contrast. Further, negative-contrast displays have been found to yield greater legibility than positive~ontrast displays (Bauer and Cavonius, 1980; Radl, 1980~. These studies should, however, be viewed carefully, because changes in polarity were also combined with changes in ambient illumination and absolute contrast magnitude. Again, further research is indicated to achieve a complete under- standing of the relationship between display image contrast and

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74 the performance of typical workers. In the experiments to date, all the observers used have been young and have had healthy eyes. Since VDT workers often include older workers and workers having some visually limited capability, it is particularly critical that research be conducted with stratified subject populations that include those people representative of typical VDT workers. No physiological effects are known to be pertinent to the variables of contrast or contrast polarity. Further, the only subjective preference data dealing with these variables has been reported by Radl (1980)and by Bauer and Cavonius (1980), who reported a significant preference for the negative-contrast (black on white) display among the several combinations investigated. Whether this preference would exist under other display and illuminance conditions is unknown. Raster Structure Most VDTs produce characters known as in-raster characters. A CRT creates these characters by drawing horizontal lines (scan lines) on the screen. The electron beam that draws these scan lines is turned on or off as required to produce line segments of symbols and characters on the screen. The collection of scan lines is called a raster, and the characters produced within the raster . are Raster characters. Figure 4.6 shows an example of characters produced in this fashion. Stroke characters are those that are produced by a continuous line process so that they do not appear to be composed of a collection of dots. The printing on this page is an example of stroke characters. Note that the in-raster characters shown in the right portion of Figure 4.6 appear continuous because of the close spacing of the scan lines and proper adjustment of the scan line width. In general, stroke-written characters are preferable to characters having a visible dot or element structure. As the spacing between dots or elements increases, the reading time and reading difficulty increase. As Figure 4.7 indicates, reductions in the space between individual dots reduce reading time, and extrapolation of this function to the zero value on the abscissa suggests an adjusted reading time of zero seconds; that is, zero space between dots (i.e., a stroke-written character) causes no elevation in reading time, which is otherwise the result of space between the dots. It must be recognized that most word processing and data processing displays today use either dot-matrix or raster-written characters, either of which can have visible spacing in the ver- tical dimension and, in the case of dot-matrix characters, also in

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- ~ FIGURE 4.6 Characters produced on a VDT screen from a raste' structure.

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100 Decreased Unstressed Luminance Condition A Condition B Decreased Lumi nance & Increased Reading Dist Condition C Background 1.2 0.12 0.12 0.97 (fL) Luminance Reading Dist 18 t8 24 24 (inches) Contrast 7.5 7.5 7.5 3.2 20 UJ Cry CC UJ Z 10 CC o cr: 111 5 o 10 Decreased Co ntrast & Increased Reading Dist Condition D \ ,Condition D ,,L, If \~/ \ Condition C ~ Condition B ,< \ ~ O ~ 20 30 40 50 60 70 80 MEASURED PERCENTAGE OF ACTIVE AREA FIGURE 4.23 Effect of percent active area on character recognition. SOURCE: Stein (1980). Font The legibility of displayed alphanumeric information is greatly dependent on the character style or font. Legibility also interacts with the size of the matrix and the overall character size. As illustrated in Figure 4.22, the Huddleston font is the most legible of those studied for 5 x 7 characters, but the Huddleston and

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101 Lincoln/Mitre fonts are equally legible for either 7 x 9 or 9 x 11 matrix sizes. Since there is absolutely no standardization of fonts across existing systems, care should be taken by designers and users to select fonts that give optimum legibility rather than unique character designs. Generalizations from existing literature pertaining to stroke-written characters (e.g., printed text) appear reasonable and should be followed until more directly related data are generated. Luminance Uniformity Uniformity considerations are similar to those discussed previously in this chapter for CRT displays. Information Density Research relating the minimum, maximum, and optimum densities of information in the vertical and horizontal dimensions is urgently needed. Currently, word processing and data processing displays range from a few lines through a more typical 24 lines per display height to a full page of approximately 60 lines. The displays vary in physical size, and the characters also vary in size. It is clear that full-page displays are desirable for formatting purposes, but they are often very difficult to read because of the resulting small character size. Similarly, it is obvious that large character sizes on partial page displays produce legible characters but that formatting is a difficult and often tiring task. There are no useful guidelines from the literature to suggest optimum levels of display information density, and we strongly recommend research in this area. Dot-Matrix Display Quality Measures While image quality measures have been researched in some depth for CRT displays, very little attention has been given to suitable measures of image quality for flat-panel displays. Although it may at first seem reasonable to assume that such measures should be approximately the same, the very nature of the differences between the two displays suggests that the metrics designed to accommodate continuous information, as is the case with the CRT, cannot often be used to describe information that is presented discretely. This section summarizes briefly the only research done to date that has attempted to summarize image quality for dot-matrix displays.

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102 TABLE 4.3 Pool of Predictor Variables Vertical Horizontal Description VFREQ HFREQ Fundamental spatial frequency (cyc/deg) VFLOG HFLOG Base 10 log of fundamental spatial frequency VSQR HSQR Square of (fundamental spatial frequency minus 14.0) VMOD HMOD Modulation of fundamental spatial frequency VDIV HDIV Fundamental spatial frequency divided by modulation VLOG HLOG Base 10 log of VDIV and HDIV VMTFA HMTFA Pseudo-modulation-transfer-function area VMLOG HMLOG Base 10 log of VMTFA and HMTFA MCROS HCROS Spatial frequency at which modulation curve crosses the threshold curve VRANG HRANG Crossover frequency minus fundamental frequency SOURCE: SnyderandMaddox(1978). In a three-year research program, Snyder and Maddox (1978) summarized the best possible prediction of image quality and visual task performance from a variety of geometric and photometric variables that were measured from flat-panel displays. The pool of predictor variables is shown in Table 4.3. These variables were all measured physically from a variety of flat-panel displays from which human visual task performance data were collected. The data pertained to two visual tasks, a reading task and a visual search task for randomly appearing alphanumerics. The predictor variables shown in Table 4.3 were then entered into a linear stepwise multiple regression equation, to obtain the best prediction equation for both the reading and the visual search tasks. The resulting prediction equations are shown in Table 4.4. From this table it can be seen that the prediction equation predicts reading time to an accuracy of approximately 53 percent of the total variance among display types, and the equation for search time predicts approximately 50 percent of the variability among different displays. It would appear that these predictability proportions can be improved with further research, but it is also clear that it is necessary to make careful and detailed measurements of displays to achieve this level of predictability. Further research is clearly indicated to obtain a greater understanding of the relationship between visual task performance and the design of flat-panel displays.

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103 TABLE 4.4 Extended Predictive Equations Task Metric and Related Information Reading Time Search Time Adjusted Reading Time (s) = 5.74 + 0.3111 (HFREQ) + 2.479(HMOD) + 4.365(HLOG) - 14.973(HFLOG) + 1.112(VMLOG) Correlation Coefficient R = .72 R2 = .525 Asymptotic R2 = .637 Search Time (s) = 7.27 + 0.027(HDIV) + 2.159(HLOG) + 5.916(VFLOG) - 0.339(VMTFA) - 0.054(VRANG) + 5.487(VMLOG) Correlation Coefficient R = .71 R2 = .500 Asymptotic R2 = .575 SOURCE: Snyder and Maddox (1978). Advantages and Disadvantages of Flat- Panel Displays Compared With CRTs A flat-panel display is usually only 1 to 2 in. deep, while the CRT used in most terminals is on the order of 12 to 18 in. deep. Thus, for a given desk size, a flat-panel display can be located farther from an operator than a CRT display and may therefore be helpful in preventing problems with accommodation. The flat-panel display is also usually lighter weight and can therefore be moved more readily. A flat-panel display has a fixed image location, which does not vary with voltage irregularities, and it does not have deflection circuit inadequacies and some of the other ills that plague CRT displays. It has been suggested by some that the better image stability of flat displays may help significantly reduce ocular discomfort reported by users of CRT VDTs; however, there has been no research directly addressing this suggestion. Greater contrast can be obtained on some flat-panel displays in compari- son with CRTs. This is often desirable in an environment that has high ambient illumination. The major disadvantage of a flat-panel display is its extremely high cost relative to a CRT. At the present time, the few flat- panel displays that would meet the requirements of current data processing and word processing terminals cost in excess of $3,000 prohibitive compared with the cost of typical CRT displays. Thus, it may be some time before widespread use of flat-panel displays is seen in the VDT environment.

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104 FILTERS FOR VDTs The contrast-reducing effects of reflections can be partially controlled by the use of various optical and physical techniques. If these techniques are not used and if the ambient lighting conditions cannot be properly controlled, it may be advisable to use a filter over the screen. Many types of filters are available, ranging from less than 35 to more than $100 and having an equal range of effectiveness. The purpose of these filters is to improve the legibility of the display by improving the contrast or reducing glare: in most cases "glare" refers to specular reflections from the front surface of the VDT. Both diffuse and specular reflec- tions from VDT screens were discussed in the section "Reflection Characteristics." This section describes several types of filters that are currently available and discusses their effectiveness. Kinds of Filters Circular Polarizer with Antireflection Coating A circular polarizer filter with antireflection coating can be used to reduce both specular and diffuse reflections. It is the most expensive filter available and probably one of the most effective. The outside surface of this type of filter is coated with several layers of optically transparent materials to form what is called an antireflection coating. The effect of the coating is to signifi- cantly reduce specular reflections from the surface of the filter. The rest of the filter package consists of substrate material (typically glass) sandwiched around the more delicate components, a linear polarizer and a quarter-wave plate. The linear polarizer and the quarter-wave plate together form what is commonly known as a circular polarizer. The circular polarizer converts unpolarized incident light to circularly polarized light. The light is changed from right-handed circularly polarized light to left- handed circularly polarized light (or vice versa) on reflection from the VDT screen. Because of the optical physics of the circular polarizer, the light is blocked from getting back through the filter in much the same way that light is blocked by crossed linear polarizers. This type of filter reduces specular reflections in two ways: by reducing specular reflections from the filter itself through the use of the antireflection coating and by eliminating specular reflections from the underlying VDT screen through use of the circular polarizer. Diffuse reflections are reduced primarily by the light attenuation effects of the polarizer material, which allows only about 35 percent of the incident unpolarized light to

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105 pass through the filter to the phosphor surface of the VDT screen. The light is diffusely scattered by the phosphor surface, thus losing most of its polarization characteristics; and it is again reduced to about 35 percent as it passes back through the filter toward the user. This process results in an improvement of the display contrast since the ambient incident light (illumination) is attenuated twice by the filter (once as it arrives at the screen and again as it diffusely reflects through the filter toward the operator), while the VDT character luminance is attenuated only once as it passes through the filter to the operator. Neutral Density Filters A neutral density filter is probably the simplest of the contrast enhancement filters. It typically consists of a neutrally tinted plastic that allows the passage of some percentage (usually 15-25 percent) of the light that falls on it. This filter is most effective in reducing diffuse reflections. Light from ambient sources is attenuated twice as it passes through the filter to the VDT phosphor surface and is reflected from the phosphor surface through the filter toward the operator. Since the light from the MDT characters passes through the filter only once, the display contrast is improved. Specular reflections may not be reduced by this type of filter unless the surface of the filter is treated with an antireflection coating (as discussed above) or with a matte finish coating that blurs the specular reflections. A filter that is apparently not commonly available but that would appear to be both effective and inexpensive is a neutral density filter formed into a spherical concave shape. Because such a shape is opposite in direction to the curvature of the VDT screen, the edges of the filter would have to be located a short distance from the screen. If the radius of curvature of the screen were approximately equal to the operator's viewing distance, alla the screen were tilted somewhat below the operator's eye level, reflection sources would be limited to the operator's chest and abdominal areas. And if those areas were kept somewhat dark, for example by an operator's wearing dark clothing, specular reflec- tions should not be a problem. Diffuse reflections would be reduced as they are with any neutral density filter. A filter based on the physical curvature of the filter material is described in U.S. Patent 3,744,893 entitled "Viewing Device with Filter Means for Optimizing Image Quality" issued to Chandler (1973~. As described, the filter was intended for use with a film viewing device but could be adapted to VDTs.

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106 Notch or Color Filters Notch or color filters are designed to allow transmission of a high percentage of incident light of some specified wavelengths (typically in the green portion of the spectrum) and a high absorption at other wavelengths. The principle of this type of filter is essentially the same as that of a neutral density filter, but in notch or color filters the bandpass (color) is tuned to the VDT screen color. A green filter placed over a VDT with a green phosphor will allow most of the display luminance to pass through the filter to the operator, while ambient illumination, which is usually broadband white, is largely absorbed by the filter (except for the green portion). This process reduces the ambient light that causes diffuse reflections on the VDT screen, thus improving contrast. As is the case with neutral density filters, control of specular reflections with this type of filter depends on the surface treatment of the filter. Directional Filters Directional filters use geometric or optical means to prevent ambient light from reaching the VDT or to prevent reflections from reaching the user. One type of directional filter is com- posed of a thin sheet of material with tiny, opaque, imbedded slats that are perpendicular to the surface of the sheet. The slats act as a miniature Venetian blind, allowing light to travel only in certain directions. When the slats are oriented toward the oper- ator, light from the VDT can pass to the operator but light from overhead cannot reach the VDT screen. This process reduces contrast loss due to diffuse reflections. Specular reflections would have to be reduced by surface treatment of the filter, as described above. Evaluation of Filters General Comments Some general characteristics of filters should be noted. First, antireflection coatings tend to be somewhat delicate and will typically degrade with time, use, and cleaning. Second, plastics used in filters are softer than glass, and they also become scratched and degrade with time, thus reducing the effectiveness of the filter.

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107 TABLE 4.5 Effect of Several Filters on Contrast and Luminance of VDT Characters for a Smooth-Finish VDT Screen Dark Room Specular Reflectiona Diffuse Reflections Filter Contrast Luminance Contrast Luminance Contrast Luminance (% ~ (cd/ ~) (% ) (Cdl m2) (% ~ (cd/ m2) None 98.8 115.0 26.1 330.0 66.4 178.0 1 97.6 28.7 17.4 115.0 65.9 45.8 2 98.1 36.3 24.5 116.0 67.1 56.4 3 96.9 21.9 16.6 92.7 69.2 32.8 4 98.3 41.3 21.1 139.0 70.9 58.1 5 98.2 36.9 28.9 99.2 81.6 50.6 6 99.2 41.7 34.9 95.8 81.3 58.1 7 99.0 34.2 11.8 195.0 80.6 47.9 a Illumination at screen, 266 lux; luminance of specular reflection source, 2,950 cd/ m2 b Illumination at screen, 413 lux; luminance of specular reflection source, none. Third, matte-surface treatments are not very effective in dealing with specular reflections in terms of their effect on contrast, although they do reduce the sharpness of specular reflections. Unfortunately, matte finishes reduce the sharpness of the display characters as well, and this effect increases the farther from the VDT surface the filter is located. Some loss in character sharpness may be helpful, however, in reducing the dot structure of characters (see data on filters 1, 2, 3, and 4 in Tables 4.5 and 4.6~. Fourth, VDT screens are convex, curved surfaces and are therefore susceptible to specular reflections that are visible to the operator over a very wide range of angles (see Figure 4.24a). If a flat or concave filter is placed over the screen, the angles over which specular reflections may occur are drastically reduced and therefore more easily controlled (see Figure 4.24b), a subtle but signficant advantage for such filters. Effectiveness of Filters Because the effectiveness of a particular filter depends on many variables and combinations of variables, it is not possible to fully discuss the issue of effectiveness in this report. For a limited comparison of the effectiveness of several filters and filter types, we measured the effects of seven filters on two different types of CRT screens:

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108 TABLE 4.6 Effect of Several Filters on Contrast and Luminance of DOT Characters for a Matte-Finish VDT Screen Specular Reflectiona Diffuse Reflections Dark Room Plus Room Lights Plus Room Lights Filter Contrast Lurn~nance Contrast Lurn~nance Contrast Luminance (% ) (cd/ m2) (% ) (cd/ m2) (% ~ (cd/ m2) None 99.5 130.0 35.2 248.0 78.9 145.0 1 98.0 33.5 17.5 110.0 71.0 38.3 2 98.4 43.1 24.0 107.0 77.6 48.7 3 97.4 25.6 16.1 94.7 78.7 28.7 4 98.5 46.2 23.4 128.0 81.7 51.0 5 98.4 43.1 30.7 90.3 85.5 43.8 6 98.6 46.9 36.5 88.9 87.0 49.2 7 98.2 37.3 11.3 185.0 83.1 40.7 ~- a Illum~nation at screen, 293 lux; lurn~nance of specular reflection source, 2,950 cd/ m2 b illurn~nation at screen, 428 lux; luminance of specular reflection source, none. 1. Amber filter with matte finish (curved) 2. Gray filter with matte finish (curved) 3. Green filter with matte finish (curved) 4. Neutral filter with matte finish (curved) 5. Circular polarizer with antireflection coating (flat)-- manufacturer A 6. Circular polarizer with antireflection coating (flat,~- manufacturer B 7. Green filter with smooth finish (flat) The filters were measured under three conditions: in total darkness, in the presence of a specular source, and in the presence of a diffuse reflection source. The specular reflection source was a light box with a luminance of approximately 2,950 cd/m2 positioned approximately 1.5 m from the VDT screen and approxi- mately 17 off axis (see Figure 4.25~. Measurements under the specular reflection condition were taken with the room lights on, thus this condition was not one of a pure specular reflection. The diffuse reflection condition was achieved with a combination of normal room lights and a slide projector located off to one side to provide nonspecularly reflecting illumination (specular and diffuse reflections on a typical VDT screen are shown in Figure 4.10~. The illumination at the plane of the screen (which results in loss of contrast due to diffuse reflection) was measured under each of the three conditions. The measurements are shown in Tables 4.5 and 4~6.

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109 A B VDT ~- Screen < ~ "I Ad_ //: VDT I' / Screen -I N Flat Filter - - - - - ~ Observer Observer = Angle for which specular reflections occur for curved VDT screen = Angle for which specular reflections occur for flat filter NOTE: ~ < ~ FIGURE 4.24 Specular reflection angles for a curved VDT screen (a) and for a flat VDT filter (b). The contrast and the luminance of the VDT characters were measured without a filter on two different types of VDT screens. Table 4.5 shows the measurements made on a VDT screen with a smooth surface; Table 4.6 shows the measurements on a screen with a matte surface. There are several items worthy of special note in the data of Tables 4.5 and 4.6. Displays with both smooth and matte finishes have extremely high contrast in a dark room; it is the ambient

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110 VDT Screen_ l / Filter 't 17 ~ 17 - - Photometer - `~ ~ Light Box al FIGURE 4.25 Top view of geometry used to test filter effectiveness against specular reflections. environmental lighting that causes a loss of contrast. All filters reduce the luminance of the display characters. This means that when a filter is used, the VAT must be operated at a higher beam current to achieve the same character luminance as when no filter is used. The increased beam current causes the phosphor to age (become less efficient) more rapidly and reduces the lifetime of a CRT. For the smooth-finish screen (Table 4.5), the circular polarizer filters improved contrast under the specular reflection condition; the improvement, however, was only moderate. For the matte- finish screen (Table 4.6), under the specular reflection condition, none of the filters resulted in a significant improvement over the no-filter condition. For both the smooth-finish and matte-finish screens, several filters improved the contrast under the diffuse reflection condition compared with the no-filter condition, but again the improvement was only moderate. Filters 1, 2, and 3 not only did not improve contrast for either the smooth-finish or matte-finish screens, but resulted in poorer contrast under several conditions. Since the phosphor used in both VDTs was a P-4 white phosphor, these results indicate that color filters might not be expected to improve contrast unless the filter color is matched to that of the phosphor. Filter 7 (green, flat, smooth finish), however, performed well under the diffuse reflec- tion condition, but very poorly under the specular reflection condition. In general, filters are more effective in reducing diffuse reflections than in reducing specular reflections. This is unfortunate because specular reflections cause the greater loss of contrast and probably contribute more to problems encountered in viewing VDTs.