Death in the dark: night vision devices: how they work, history, personal experiences and latest developments.
Several nights before, a broadcast from Radio Sandi-no in Nicaragua had identified our camp's location and purpose, and me by name. They also had announced that the camp and communications center would be destroyed by the People's Revolutionary Forces. Intercepted radio traffic indicated the Gs (Guerillas) probably would attack our position that very night. Their force was estimated at between 500 and 1,000 in number. Along with the commander of the "A" camp, I had a feeling they would most likely assemble on the hill adjacent to our position and then move across the connecting saddle to assault our perimeter.
While the mortar crew began to plot on-call fire for the "60 mike-mike" onto the hill across the saddle, I placed the M60 and about 1,500 rounds of ammo, linked four ball to one tracer, on the edge of my firing pit. I had an excellent view of my primary fire sector, which faced the saddle.
Using a broken antenna pole, I set a limit stake to the left to avoid firing on the Ma Deuce crew, which we had positioned for better cover and concealment about 100 meters to my front. From the half-case of M67 hand grenades in the bottom of the firing pit, I put three, with pins straightened, on the top ledge of the pit along with bandoleers of HEDP M433 (M550 fuse) 40mm rounds for my M79.
I secured a secondary sector of fire 20 meters behind my pit at the edge of the hill and stashed some of the belted ammo for the M60 behind a large tree several meters away. Then I settled in to scan the sector in front of my gun. I was told that the Gs would most likely move on us between 2400 and 0400 hours. In anticipation of this, I attached an AN/PVS-2 passive night vision device (NVD) to the M60 GPMG.
At about 0100 hours, dogs began bark in the small village at the base of the hill. Someone was moving about in the area below our camp. The NVD revealed lights and movement on the south slope of the hill creeping toward my secondary fire sector. Rather than disclosing our positions by opening up with small arms fire, we heaved grenades down the hill.
Whoever was probing our position failed to make their play. Although the anxiety level had approached one million, the evening's pucker factor had been considerably reduced, at least for me, by my ability to cut through the night's sinister blackness with the AN/PVS-2 and observe the hill across the saddle. It provided me with the all-important edge that all combatants pray for when they grope about in high-tension darkness.
My first exposure to night vision equipment had come 30 years previously during basic training at Fort Carson, Colo. It was there that I fired the .30 M3 Carbine, which was nothing more than the selective-fire .30 M2 Carbine equipped with the so-called "Sniperscope." It exhibited all of the salient features and defects of infrared (IR) night vision equipment.
Dating back to World War II and Korea, these systems relied on a source of infrared light to illuminate the target, which was then viewed through the scope and appeared as a bright green image on a fluorescent screen. Infrared rays occupy that portion of the electromagnetic spectrum with a frequency less than that of visible light and are thermal, or heat, radiation. Infrared means "below the red," i.e., beyond the red, or low frequency (long wavelength), end of the visible spectrum.
The IR projector was attached to the scope by a bracket and powered by a substantial battery. Weight of the complete system, including the weapon, scope and battery, was 28 pounds. The unwieldy top-mounted IR projector upset the weapon's center of gravity.
The IR filter over the projector lamp absorbed most of the light output, and the maximum range was 300 meters--at most. Worst of all, the IR beam was easily visible to another viewer equipped with an IR observation scope or weapon as it was to the user.
Still, the Sniperscope wasn't too shabby for the time frame during which it was fielded. Before World War II, the only alternative was to pop illumination flares, and this was all too often of equal benefit to the enemy. However, this type of "active" NVD is by now completely obsolescent.
Research in this area plodded along with little progress until the United States became involved in Vietnam. The battlefield environment in Vietnam was begging for some innovative means to see in the dark. This time also coincided with the development of minuscule electronic circuitry, reduced-envelope batteries and several other scientific breakthroughs--all of which led to undetectable "passive" night vision equipment in the form of the image intensifying sight. It required no auxiliary light source, infrared or otherwise.
The technique eventually developed was to focus the visible image--however faint--onto a photosensitive screen. This screen would then emit electrons on its inner face in proportion to the amount of light from the objective lens falling on its outer face.
These electrons were then amplified and projected onto the outer face of a second screen that, in turn, would emit photons (light) from its inner face in response to the electrons impacting upon it. The further to increase the amplification, the photon stream emitted from this second screen was again amplified and projected onto a third screen. Finally, the electrons emitted from the inner face of the third screen were amplified and projected onto the optical eyepiece, whereupon the operator saw an image resembling a small, contrasty, green television picture.
Because of its three stages of amplification, the system was referred to as a "cascade tube." The total amplification (often referred to as "gain") was approximately 64,000 times with larger units. Thus, a very murky star--or moon-illuminated scene became almost as clear and distinct as if viewed under the noonday sun.
The AN/PVS-2 Starlight Scope I employed on the hill in El Salvador was a device of just this type. Total length of the AN/PVS-2 is about 18.3 inches. It weighs just less than 4 pounds, and is powered by a 6.75-volt disposable battery with a continuous service life of 72 hours. The useful range in a moonlit environment is approximately 400 meters. With only starlight to illuminate the scene, the range drops to about 300 meters. Passive night vision equipment of this general design is called "first generation" (aka "Gen 1"). The Gen 1 image is clear at the center, but may be distorted around the edges.
Not bad and quite an improvement over the Sniper-scope, but still, an 18-inch, 4-pound scope on top of an M14, M16 or even the M240 Bravo GPMG is somewhat clumsy. A more serious objection to military employment of first generation night vision equipment is its inability to cope adequately with magnesium flares or tracer ammunition passing across the field of view. A single round of 5.56 x 45mm or 7.62 x 51mm tracer may no more than cause a momentary streak across the screen. But, a continuous stream of bright tracer from a .50 cal. M2HB Browning machine gun may result in "blooming" (a bright saturated spot) and literally erase the image on the screen.
An experimental cal. 7.62 x 51mm NATO "dim-trace" cartridge was developed during the late 1960s for use with the Starlight scope. Some have stated it was for covert operations and could be seen only through the Starlight scope. Known as XM276, three types were manufactured. The third and final type, color-coded with a green and pink tip and most commonly headstamped either "FA 68" (Frankford Arsenal, 1968) or "LC 69" (Lake City, 1969), both with the NATO cross-in-circle, proved to be most satisfactory. However, it was never type-classified, as advances in the field of fiber optics permitted development of a second generation of image intensifying night vision equipment. Their several improvements included less sensitivity to tracer ammunition.
In both second and third generation systems, the objective lens collects light that cannot be seen with the naked eye and focuses it on an image intensifier. A photo cathode inside the image intensifier absorbs this light energy and converts it into electrons. Passing first through a micro channel plate (MCP) that multiplies them thousands of times, these electrons are drawn toward a phosphor screen.
When this highly intensified electron image strikes the phosphor screen, it causes the screen to emit visible light. Since the phosphor screen emits this light in precisely, the same pattern and intensity that the light was collected by the objective lens, the bright image seen in the ocular corresponds exactly to the scene being viewed.
The micro channel plate electron multiplier described above prompted second generation night vision development in the 1970s. The "gain" provided by the MCP eliminated the need for back-to-back tubes and thus improved both size and image quality. The MCP enabled the development of hand-held night vision units and helmet-mounted goggles.
Two major developments were responsible for the development of third generation night vision devices in the early 1980s: the gallium arsenide (GaAs) photocathode and the ion-barrier film on the MCP. Thus, third generation night vision equipment uses gallium-arsenide for the photo cathode and the micro channel plate is coated with an ion barrier film to increase tube life.
The very best third generation equipment provides very good to excellent low-light-level performance and long tube life. In addition, recent MilSpec-quality tubes feature no perceptible distortion.
The image intensifier resolution of state-of-the-art night vision is often measured by a unit known as "line pairs per millimeter" (lp/mm). This is usually determined from a 1951 Air Force Resolving Power Test Target. All the horizontal and vertical lines and the spaces between them must be distinguishable to qualify for a particular pattern. The higher the number of line pairs per millimeter, the better the unit's ability to provide a sharp image.
Most Russian-made night vision will provide no more than 20 - 30 1p/mm. The very best American-made night vision features resolution of 45 - 64 + 1p/mm.
There are several other parameters that influence the quality of night vision equipment. One is the signal-to-noise ratio (SNR), which is a measure of the light signal reaching the eye divided by the perceived noise as seen by the eye. A tube's SNR determines the low-light resolution of the image tube. Thus, the higher the SNR value, the better the tube's ability to resolve objects with good contrast under low-light conditions.
Another important factor is photosensitivity, or the ability of the photo cathode to produce an electrical response when subjected to light waves (photons). The higher the value, the better the ability to produce a visible image under darker conditions.
An additional important concept is the system gain. Gain is the number of times a night vision device amplifies light input. System gain is measured as the light output divided by the light input and is what the user actually sees. This value is usually in the thousands and U.S. military night vision equipment operates in the 3,000 + range. The higher the value for system gain the better, up to a point. Russian night vision devices frequently increase the tube gain far too high in order to achieve a brighter (but never clearer) image at the expense of tube life.
A relatively recent development that shows considerable promise is so-called "digital night vision." The highly sensitive image sensors of these units offer at least as good performance as second generation NVD and approaching that of third generation NVD even out to about 600-feet. There are both monochromatic and color versions. These digital units are unaffected by even strong sunlight and thus may be used in any illumination environment.
A further advantage is the ability of digital night vision equipment to be connected to a video camera or VCR for recording whatever the viewer sees. There are obviously numerous potential security applications for digital NVD and it is priced considerably less than MilSpec third generation NVD.
State-of-the-art night vision technology currently resides in the United States, as it has since the very inception of equipment in this area. Innovations, both evolutionary and innovative, will continue at an ever increasing pace.
Text and photos by Peter G.Kokalis
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|Author:||Kokalis, Peter G.|
|Date:||Mar 1, 2010|
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