Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
CHAPTER V DEVICES F0R EYE PR0TECTI0N Rob Roy McGregor* An extensive research program was undertaken by agrencies of the Department of Defense and contractor personnel in attempting to develop eye protection against the flash of a nuclear weapon, and many of the results are applicable to designing an eye protective device against laser irradiation. Some countermeasure research has been performed by the Federal Government specifically for the protection of optical com- ponents and eyes against laser irradiation, but it is classified military security information. Devices constructed in the past have fallen into three categories: 1. Passive - Requires no activation or sensing of irradiation to function automatically. 2. Active-Passive - Requires sensing of irradiation and/or activa- tion of defensive mechanism. May employ an auxiliary passive system, such as dichroic mirror or neutral density filter. 3. Act ive - Requires sensing of irradiation and activation initially and/or during exposure to irradiation. All of the above devices should, if possible, have a very rapid re- sponse, withstand repeated attacks, be lightweight, economical, and revers- ible. In addition, an eye protective device should be transparent to vis- ible radiation and opaque between the region of ruby and neodymium laser wavelengths. Since no useful purpose is served in passing wavelengths that do not contribute to vision, no need exists for an eye protective device for transmitting "the infrared or ultraviolet spectra. Electronic vision systems utilizing these spectra can be electronically current- limited to protect both the system and the viewer's eyes, or protected by other means. A review of representative devices and their operating principles indicates two general classes of eye protective devices, i.e., dynamic devices that may or may not be activated, and static devices that reflect or filter a given wavelength more than others. Dynamic Devices This type of device is normally transparent to visible light and increases its optical density upon exposure to an intensive light source. Methods of achieving this effect include but are not limited to: â¢^Department of Physics, U. S. Army Tank Automotive Center, Detroit, Michigan 33
1. Phototropic, photochromic or thermotropic reactions to change optical density. 2. Magneto-optic or electro-optic effects to change optical density. 3. Placing a shutter in the field of view. k. Destroying the optical path upon exposure to intense light source. 5. Using an electronic image system for indirect viewing. Static Devices This type of device is normally partially transparent to ambient visible light, and of high optical density for special regions of the visible spectrum. Some methods of achieving this are: 1. Selectively fixed density filter, i.e., neutral density filter. 2. Partially reflecting, partially absorbing fixed filter. 3. Combination of the above. DISCUSSI0N 0F DEVICES Photoropic Reactions Theoretically, the photoropic spectral absorption mechanism is capable of nanosecond reaction time, and, consequently, a great deal of the re- search in countermeasures has been undertaken in this area. Some confusion has arisen in the use of various words, apparently in- terchangeably, to describe the phototropic reaction. Phototropism is a material property characterized by the material's ability to change its spectral absorption characteristics as a direct result of the absorption of quanta. Phototropism, photochromism, thermotropism, and thermochromium are used interchangeably to describe the effect. Since most of the re- actions are photochromic, usage has led to describing the absorption of visible light and consequent color change as either photochomic or photo- tropic. Thermotropism and thermochromism describe the spectral absorption change after the absorption of thermal energy by the material. Phtotropic reactions are reversible. The absorption of quanta by the molecules leads to vibrational or rotational twisting or photoionization, which changes the molecular configuration and changes the spectral ab- sorption characteristics. Moreover, phototropic reactions are energy dependent, and their reaction times are a function of the incident radi- ation intensity and wavelength. Consequently, some phototropic materials
react best to infrared, visible, or ultraviolet radiation. To date, closure times in the microsecond range have been achieved. In order to achieve the nanosecond closure times required for laser protection, however, it appears that external sensory and triggering circuits are not -feasible, and the directly activated system should be pursued. Magneto-optic or Electro-optic Reactions It is doubtful that these reactions can be completed in the nano- second time required for closure of a laser protective device. The Kerr-Electro-optic effect which causes a solid or fluid to become doubly refracting when subjected to a strong electric field has been success- fully used to provide closure times in the microsecond range. The magneto- optic effects (Faraday and Cotton-Mouton) consist of the double refractions of light in a transparent media caused by a magnetic field. The Faraday effect is small, even for fields of the order of 10,000 gauss, and the Cotton-Mouton effect is even smaller. The low order of magnitude of the magneto-optic effect makes it success as a laser countermeasure unlikely. Placing a Shutter in Field of View This approach appeared to be easiest in the early stages of research to provide protection against the nuclear flash of atomic weapons. Sev- eral interestingly different methods proved to be partially successful in that they could achieve closure in several hundred microseconds. These systems relied on a sensory system to sense the flash and a triggering mechanism to activate an explosive charge actuating the closure mechanism. Another type of system, i.e., the explosive light filter system, explos- ively fired a carbon colloid between two lens, resulting in a partial shutter. Systems of this type normally achieve an optical density of only four in fifty microseconds or more and are irreversible, necessitating replacement of filter element after each use. Although perhaps adequate for nuclear flash protection, their optical densities are too low and reaction times too slow for laser eye protection. Destroying 0ptical Path An evaporated thin film reflecting coating was used as one of the internal reflecting surfaces in an optical system. Upon sensing of an intense light source, the fi1m was-destroyed, interrupting the optical path. This approach was too slow, resulting in closure time in excess of 50 microseconds. Antother approach of exploding a film of oil con- taining carbon onto the face of a prism resulted in the matching of index of refraction and consequent absorption of incident light. This method proved to be capable of an optical density of four in fifty microseconds. The general method of destroying optical path by physical reaction to a light source appears to be possible for lasers only if the physical re- action requires no sensing or triggering mechanism. 85
Indirect Viewing by Electronic Image System This method is ideal for eye protection if the high cost, complexity, maintenance, and loss of resolution and depth of field can be tolerated. Since the intensity of the image cannot exceed a predetermined level, the only damage will be to electronic detector or phosphor and/or system com- ponents due to surge voltages or currents generated by the light pulse. Fixed Filter, i.e., Neutral Density, Absorbing and/or Reflecting Systems Several lightweight spectacle or goggle type protective devices have been developed which appear to offer excellent protection against pulsed laser sources at a relatively low cost and weight. These systems are partially transparent to the portion of the visible spectrum below the region of ruby lasers and enable the viewer to see while protecting against either or both ruby and neodymium lasers. The Jena colored glass type BG-18, made by Schott and Genossen in Germany, and distributed by Fish-Schurman Corporation of New York, is used in devices sold by American 0ptical Company, Southbridge, Mass. (Neodymium wavelength optical density of 11.2) and Fish- Schurman Corporation, 70 Portman Road, New Rochelle, New York (ruby wavelength optical density of 10). In addition, American 0ptical Company markets a mask goggle, #A0 488, with a combination of filters, A0 #585, that provides pro- tection against ruby and neogymium lasers. The effect of wearing these de- vices is similar to sunglasses, i.e., partial transmission in visible spec- trum and not unpleasant. Some change in preceived colors occurs, of course, as the red wavelengths are absorbed and per cent transmission of the visible spectrum is less than with good sunglasses. They are not effective against high power-levels and tend to craze with continued use. C0NCLUSI0NS Although a great deal of research has been accomplished to develop eye protection against nuclear flash with some success, the only commercially available methods of eye protection against lasers today are limited to fixed filter and/or reflection type systems. Phototropic absorption offers great promise, and research is continuing to develop this method of eye pro- tection. However, as high power lasers are developed in the visible spectrum, the vision protection problem becomes far more acute than at present. The fixed filters presently available would be inadequate unless narrow band cut-offs could be established without further reduction of visibility. 86
