U.S. Department of Justice Office of Justice Programs National Institute of Justice National Law Enforcement and Corrections Technology Center March 1996 National Institute of Justice Jeremy Travis, Director Scoping Out Night Vision In the United States today, there are more than 50 companies that either manufacture or distribute night vision equipment. Departments considering a purchase of this equipment will encounter a potentially confusing array of technical specifications and terminology, and mistakes can be costly. To help departments evaluating night vision systems to make informed decisions, this bulletin of the National Law Enforcement and Corrections Technology Center (NLECTC) presents basic information on night vision technology and terminology, factors to consider when evaluating night vision systems, and new developments in night vision. Background visible light that is available at night— contrast—when a pinpoint source of moonlight, starlight—and to invisible bright light, for example, a car’s head- The most widely used night vision aid in light from the near-infrared range of the lights or the flash of a cigarette lighter, law enforcement is image intensifier (I2) spectrum. The image intensifier tube enters the field of vision. Because such equipment. Developments in I2 technol- multiplies the light it takes in many bright, concentrated sources of light ogy are categorized in terms of “genera- thousands of times, essentially amplify- could induce image degradation/shut tions.” To date, there have been four ing the brightness of the image seen down and even tube damage, later generations of night vision devices, through the device. This process is generations of night vision devices numbered from zero through three. described in more detail below. In incorporated several features to protect Night vision technology was originally situations where there is almost no against bright light. Even with these developed to give the U.S. Armed Forces ambient light, such as on an overcast safety features, care should be taken to the tactical advantage of seeing at night night or inside an unlit building, an avoid such exposures and to keep the or under other conditions of low light infrared light source is required. Many objective (front) lens capped during the without using searchlights that would night vision systems have an integrated day and when the unit is not in use. reveal their position. The first night infrared light-emitting diode (IRLED) The development of the microchannel vision aids—“generation zero” (Gen that can be used in situations where plate (MCP) in the late 1960’s brought 0)—were sniper scopes that came into ambient illumination is low. An auxiliary the second generation (Gen 2) in night use during World War II and the Korean infrared light source, such as a high- vision. (See figure 2.) The MCP elimi- conflict. These were not true image intensity flashlight fitted with an infrared nated the need for three-stage light intensifiers, but rather image converters, filter, can also supply the light required amplification and thereby reduced the which required a source of invisible to operate a passive device. size, weight, and distortion of the image infrared (IR) light mounted on or near The “starlight scopes” developed during intensifier tube. With the MCP, night the device to illuminate the target area. the early 1960’s for use in Vietnam were vision goggles and hand-held viewers The objects in the target area reflected the first generation (Gen 1) of I2 devices. this IR light, which the device converted Some Gen 1 equipment is still in service. to visible light. Because they cannot In Gen 1 devices, three image intensifi- New NIJ Publications function without auxiliary IR light, Gen ers were connected in a series, which 0 devices are referred to as “active.” Available upon request is NLECTC achieved a high level of light amplifica- Bulletin, “Positional Asphyxia—Sud- The development of the image intensifier tion but also made the scopes longer and den Death,” dated June 1995. Cur- tube brought clearer, brighter image heavier than future night vision devices rently in production is a report on the quality to night vision. I2 devices are would be. (See figure 1.) Gen 1 devices Second Annual Law Enforcement Technology for the 21st Century con- called “passive” because they do not produce an image that is clear at the ference, held in June 1995 in Wash- center but distorted around the periphery require an auxiliary source of illumina- ington, D.C. Please contact NLECTC tion. These devices are capable of and also subject to “streaking” and for availability of these publications. responding both to the limited amount of “blooming”—temporary loss of became possible. The MCP also pro- Figure 1. First-generation (Gen 1) image intensifier vided much more robust operation when bright lights were in the field of view. Third-generation (Gen 3) image intensi- Intensifier Tube fiers were developed in the mid-1970’s Objective Lens Electron Optics Eyepiece and became available during the early 1980’s. Gen 3 introduced two major technological improvements: Available Focused Stage Stage Stage Intensified Light Light 1 2 3 Image n The gallium arsenide (GaAs) photo- Photocathode Photocathode Photocathode cathode, which increases the tube’s sensitivity to registering light from the near-infrared range of the spec- Field Iris Phosphor Phosphor Phosphor Screen Screen Screen trum, enables it to function at greater Fiberoptics detection distances and improves Plates system performance under low-light conditions. (Gen 1 and Gen 2 devices The tube is actually a sequence of three image intensifiers, each comprising a photocath- ode and phosphor screen. use an S–20 multialkali photocathode.) n Application of a metal oxide ion barrier to the MCP, which increases array. When an electron from the conditions will the equipment primarily the life of the image tube. The photocathode enters a given channel, be used? Mobility? Stationary surveil- operational life of Gen 3 image it strikes the channel wall, initiating lance? Hand-held discreet observation? intensifier tubes is in excess of 7,500 an avalanche process that results in Videotaping? Photography? Some night hours; that of Gen 2 tubes is about several hundred electrons exiting the vision equipment can be adapted to more 2,000 to 4,000 hours. other end of the channel. than one application. The best way to make an informed purchase is to begin n The electrons exiting the MCP How Image Intensifiers Work with an analysis of your department’s bombard a phosphor screen that needs. All I2 systems employ an objective converts their energy into a green (front) lens, an image intensification form of visible light. The process is This analysis should also take stock of tube, and an ocular (eyepiece). Available similar to the way images are pro- the equipment you presently have on light enters the objective lens and is duced on a television screen. hand. If you plan to use your night amplified as it passes through the image vision equipment for photography and n intensification tube. The process is as Because this light is in the same your department already has a 35-mm follows: pattern and proportion as the image camera and an assortment of lenses, look formed by the objective lens, the for a night vision device that can be n The light is focused on the photocath- image in the device’s eyepiece equipped with an adapter that will allow ode, the first component of the image corresponds precisely to the image you to use your existing lenses. Video intensifier tube. The photocathode is being viewed. cameras can also be coupled to certain an electron-emitting, photosensitive night vision devices with special relay screen located where the film in a Evaluating Image Intensifiers lenses that are adaptable to most camera would normally be. It absorbs 2 common lens mounts. High-intensity the light’s energy and converts it into Today I night vision equipment is flashlights can be converted to invisible electrons. available in the form of hand-held viewers for use with one eye (monocu- searchlights for use with night vision n Next, these electrons are accelerated lar) or both eyes (biocular or binocular); systems by attaching infrared filters. and multiplied. In Gen 1 tubes, this goggles that allow the user to walk, run, Where will your equipment be used? Gen was accomplished with three stages of or pilot a vehicle; binocular observation 2 equipment is generally a good choice for photocathode amplification. Gen 2 devices that provide magnification; and use in urban environments, where there is and Gen 3 tubes use a single photo- weapon sights. The first question to more ambient light. In rural environments, cathode and a microchannel plate address when considering an investment or in situations where the ambient illumi- comprising more than 2 million in night vision equipment is application: nation is extremely low, Gen 3 equipment microscopic hollow-glass conducting For what purpose and under what is preferable. channels fused into a disc-shaped 2 Performance Factors Signal-to-noise ratio Gain The best means of evaluating whether The signal-to-noise ratio (SNR) is a This is a measure of brightness. The gain the equipment you propose to purchase measurement of the amount of light of a night vision device is the number of will serve your needs is an onsite reaching the eye divided by the amount of times it amplifies the light entering the demonstration under your typical “noise,” or static disturbance, perceived by device. Tube gain, measured in the tens operating conditions. In comparing the the eye, and is considered to be the best of thousands, represents the actual performance specifications of different single indicator of a night vision device’s degree of amplification generated in the night vision systems, the following are low-light performance.