Night vision is a technology that every police department should put near the top of its list for acquisition, but not only are there several night-vision technologies available, the advantages and disadvantages of each apparently aren’t widely known. Moreover, some manufacturers’ claims of the capabilities of their night-vision equipment is, shall we say, “optimistic.” Given the availability of a wide variety of night-vision equipment over a range of prices, it is difficult to select the equipment that will provide the required performance at a price that matches the performance level. We will explain available night-vision technologies and provide a few guidelines to help the user select the most useful and cost-effective equipment for their department’s requirements.
There are three currently available night-vision technologies—Image Intensifying (I²), thermal and digital. Let’s begin with digital because it is easiest to discuss. Despite some manufacturers’ advertising claims that their digital night vision is superior to any other night-vision technology, that simply isn’t true and digital night vision is still several years from being on a par with the best I² devices and digital doesn’t work in total darkness like thermal imagers. Digital in its current state is probably better than nothing, but I know of no digital night-vision device that can also be used as a weapon sight, nor of any that approach the latest Gen 3 I² devices in terms of overall performance. About the only thing that digital night vision presently has going for it is low price. With that out of the way, let’s explore night-vision technologies more appropriate to law enforcement use.
We’ll begin with the image intensifier (I²), since it is the most common technology. The intensifier tube is the heart of the I² night-vision device. A state-of-the-art intensifier will deliver crisp, bright images without noticeable grain unless the light level is very low. Most intensifier tubes are made by ITT, Litton or Northrop Grumman and manufacturers of devices like Optical Systems Technology’s AN/PVS-22 and AN/PVS-27 military night-vision sights (NVS) incorporate them into the products they manufacture, but how manufacturers incorporate the tubes into their products is also critical. Not all I² devices are equal. For example, only some have shock mitigated intensifier tubes. This is critical for devices that are to be used as weapon sights in addition to being used as a hand held optic. If you mount your night-vision optic on a rifle with heavy recoil such as a .338 Lapua Magnum or a .50 BMG, you will soon find out whether or not your optic is shock mitigated. On rifles with heavy recoil the absence of shock mitigated intensifier tubes will result in the recoil knocking the device out for the count rather quickly.
Three parameters determine the basic quality of the image intensifier; the sharpness (measured in line pairs per millimeter) and sensitivity to light (measured in microamps per lumen) and the noise level as measured by the Signal to Noise Ratio (SNR). In all cases, a higher number is better. All three parameters are important to performance, so look for the unit with the highest numbers in all three. When purchasing an I² device, make sure that the image intensifier performance is included in the specification or manual.
There are three generations of I² devices. The following guidelines will help in generating an I² specification. Refer to published manufacturers specifications for the most current data. The original, 1st -Generation (Gen 1) I², has low gain, good image sharpness, but is susceptible to “blooming” from bright lights and muzzle flashes. A Gen 1 intensifier does not have much light-amplifying power, though, so unless you “stack” two or three 1st-Generation intensifiers in a row, it isn’t much better than the dark-adapted eye. Low-cost night-vision devices frequently use 1st-Generation intensifiers, especially those made in Russia. Gen 1 intensifiers should generally be avoided for law enforcement night-vision applications.
The 2nd Generation (Gen 2) intensifiers use a multi-alkali detector surface and a microchannel plate image amplifier in a device that is much more compact than its 1st -Generation equivalent. Gen 2 intensifiers can exhibit high SNR and resolution, but have less sensitivity to light than the latest 3rd-Generation (Gen 3) I² devices. Western Europe produces some very good 2nd Generation intensifiers, the best of which are almost as good as some of the basic Gen 3 in sensitivity.
Some systems that use or claim to use 2nd-Generation intensifiers do not use the high-grade Western European intensifiers, or use Gen 1 intensifiers. Always check the specifications. A good Gen 2 image intensifier will exhibit the following performance characteristics:
Basic Gen 2 Grade: Resolution 45 lp mm minimum, SNR 18 minimum, and Photoresponse 500 µa/lumen minimum.
Gen 2 High-Performance Grade: Resolution 57 lp/mm minimum, SNR 20 minimum, and Photoresponse 600 µa/lumen minimum.
Best Gen 2 Grade: Resolution 64 lp/mm minimum, SNR 25 minimum, and Photoresponse 700 µa/lumen minimum.
Avoid Gen 2 intensifiers with specifications lower than the basic grade noted above. Using Gen 2 intensifiers generally does not reduce the cost of a unit. High quality Gen 2 intensifiers can be more expensive than better-performing Gen 3 intensifiers, so the potential user should carefully check the performance specifications of each night-vision device under consideration for purchase.
The 3rd Generation intensifier is the best currently available. This intensifier is distinguished by its use of a Gallium Arsenide (GaAs) detector surface. US manufacturers produce the best Gen 3 units. “Gen 3” image intensifiers manufactured outside of the US are currently not worth considering either as an alternative to the US Gen 3 intensifier or the best Western European Gen 2 image intensifier. The export of US made Gen 3 intensifiers and devices containing them is restricted based upon the product of the image intensifier SNR and resolution in lp/mm, and are therefore categorized in the following general performance ranges:
1250 Gen 3 FOM (typical specification): Resolution 64 lp/mm minimum, SNR 18 minimum, and Photoresponse 1350 µa/lumen minimum.
1600 Gen 3 FOM (typical specification): Resolution 64 lp/mm minimum, SNR 21 minimum, and Photoresponse 1800 µa/lumen minimum.
Full Performance Gen 3: Resolution 64 lp/mm minimum, SNR 22 minimum, and Photoresponse 1800 µa/lumen minimum.
Premium Performance Gen 3: Resolution 64 lp/mm minimum, SNR 26 minimum, and Photoresponse 2000 µa/lumen minimum.
Note that the photoresponse of even the lowest-grade Gen 3 intensifier is better than Gen 2 image intensifiers. Gen 3 intensifiers are very rugged and generally maintain performance for a much longer time than their Gen 2 equivalents. Therefore, when it is possible, choose a Gen 3 image intensifier. State-of-the-art premium military Gen 3 intensifiers include the AN/PVS-14, AN/PVS-22 and AN/PVS-27. Other Gen 3 devices like the AN/PVS-17 are too bulky and hand or helmet mounted night vision cannot be used as night-vision weapon sights. The AN/PVS-14 can be used as a weapon sight, but requires an adapter that affects eye relief, plus an infrared (IR) reticle. We thus recommend that law enforcement users purchase a dual-use night-vision optic like the AN/PVS-22 or AN/PVS-27, which mounts ahead of the day optic and requires no IR or illuminated reticle. Both are full military specification and are dual use in that they can be used as either hand held or mounted as weapon sight.
These devices have the additional advantage of mounting ahead of the weapon’s day optic, so no adapters are necessary, nor is an IR reticle required. These devices use the day optic’s reticle without any kind of illumination and removing them has less than one minute of angle (MOA) affect on zero. The one disadvantage of these state-of-the-art NVS is that short-barreled entry-gun-type carbines may have MIL-STD-1913 handguard rails that are too short to allow the NVS to fit between the optic and front sight. In cases such as this, there are smaller premium Gen 3 NVS, such as Optical Systems Technology’s Tactical Advanced NightVision System (TANS), also marketed under the same name by Trijicon. We have personally used this NVS on M4 type entry guns and it fits everything we have tried, as long as the M4 has standard length gas tube. The TaNS, while smaller than the PVS-22, gives up little in overall performance. Some night-vision companies advertise that their night-vision devices are Gen 4. There is no true Generation 4 I². Gen 4 is a marketing gimmick that was apparently invented to highlight a specific style of Gen 3 I².
Night vision is a technology that every police department should put near the top…
by Stanton Wormely, Jr. / Jun 21, 2008