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Selecting Image Intensifiers, Thermal Imagers and Night Sights
Written by Charlie Cutshaw
Night vision capability is one of those technologies that every police department should put near the top of its list for acquisition. However several night vision technologies are available, and the advantages and disadvantages of each aren’t widely known. Given the availability of a wide variety of night vision equipment over a range of prices, it is difficult to specify and select the equipment that will provide the required performance at a price that matches the performance level.
High price doesn’t necessarily equate to a high level of performance. Sometimes, low price can get useable performance. This article will provide an overview of available night vision technologies and provide a few guidelines to help the user purchase the most useful and cost-effective equipment for his department’s requirements.
Three basic night vision technologies are currently available. These are image intensifying (I²), thermal and digital. Let’s begin with digital because it is easiest to dispense with. Despite some manufacturers’ advertising claims that their digital night vision is the most superior such product available, that simply isn’t true, and digital night vision is still a few years from being on a par with the best I² devices. Digital doesn’t work in total darkness like thermal imagers. Digital 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 approaches the latest “premium” Gen 3 I² devices.
The image intensifier is the most common night vision 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 either ITT, Litton or Northrop Grumman. Different manufacturers of devices incorporate these tubes into their products, but how they do that is 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 handheld device.
Three parameters determine the basic quality of the image intensifier—the sharpness (measured in line pairs per millimeter); 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. There are three generations of I² devices. The following guidelines will help in generating an image intensifier specification. Refer to published manufacturers specifications for the most current data.
Gen 1 Intensifiers
The original, first generation (Gen 1) image intensifier 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 first generation intensifiers in a row, it isn’t much better than the dark-adapted eye. Low-cost night vision often uses first generation intensifiers, especially Russian optics. Gen 1 intensifiers should generally be avoided for law enforcement night vision applications.
Second 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 first generation equivalent. Gen 2 intensifiers can exhibit high SNR and resolution but have less sensitivity to light than the latest third generation (Gen 3) I² devices. Western Europe produces some very good second generation intensifiers, the best of which are almost as good as basic Gen 3 in sensitivity.
Some systems that use or claim to use second generation intensifiers do not use the high-grade Western European intensifiers, or they use Gen 1 intensifiers. Always check the specifications.
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.
The third generation intensifier is the best type available. This intensifier is distinguished by its use of a Gallium Arsenide (GaAs) detector surface. U.S. manufacturers produce the best Gen 3 units. Gen 3 image intensifiers manufactured outside of the U.S. are currently not worth considering either as an alternative to the U.S. Gen 3 intensifier or the best Western European Gen 2 image intensifier.
The photo response of even the lowest grade Gen 3 intensifier is much 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.
Some night vision companies advertise that their night vision devices are Gen 4. There is no true Generation 4 image intensifier. Gen 4 is a marketing gimmick that was apparently invented to highlight a specific style of Gen 3 image intensifier.
We recommend that law enforcement users purchase a dual-use night vision optic that mounts ahead of the day optic and requires no IR, or illuminated reticle. “Dual use” means they can be used as either handheld or mounted as weapon sight. These devices may 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 night vision sights (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.
Several criteria should be kept in mind when purchasing these devices. The potential NVS should be dual use, i.e., capable of being used both as a handheld observation device and as a weapon-mounted sight. Purchase from a manufacturer with a track record of manufacturing and delivering dual purpose, clip-on NVS. Choose a manufacturer that can provide references to reputable users (DOJ, DOD, DOE).
Watch out for knock-offs. Many unscrupulous manufacturers will make their units look like and their names sound like the real devices when they are not. Don’t select a device whose objective lens and output lens are at different heights. This will result in a parallax error that must be mentally compensated for by the user. Select a device whose output lens size is larger than, or as close as possible to, the size of the input lens of the day sight. Smaller lenses will reduce brightness at higher day sight magnifications.
Insist on specifications for boresight accuracy (sight zero shift) over the life of the device, not just as delivered from the factory; weapon platforms upon which the sight is guaranteed (the unit should be able to be used on weapons ranging from an M-4-type carbine to a bolt-action .50 caliber rifle); an image intensifier guarantee (be sure it is in writing and applies to all weapons platforms including .50 caliber weapons); the f/number (select a system that is as close to f/1.0 as possible); and the submersibility (10 meters or better is preferred).
The objective lens is the lens that forms the image on the front of a detector, such as an image intensifier. The lens on the front of an SLR camera is an example of an objective lens.
For I² systems, the lower the f number, the better. This is because the intensifier amplifies the existing image. If the existing image is dark, the amplified image will also be dark. The brightness of an image formed by a lens is inversely proportional to the f/number squared.
This means that a lens with an f/number of f/1 will produce an image 4 times as bright as a lens with an f/number of 2. The more light that is delivered to an intensifier, the sharper and brighter the image that comes out of it. Conversely, even the best intensifier in the world will operate poorly with a high f/number objective lens. In very dark conditions, the f/number of a lens might make the difference between seeing or not. In general, an f/number of the objective lens should be between f/0.9 and f/1.4. Night vision lenses also gain much of their sensitivity to light by detecting and responding to infrared light as well as the visible light detected by commercial cameras. Starlight, moonlight, and incandescent lighting are rich in infrared light.
For longer range weapons, use a unit with a larger objective lens. The operating range of an inline sight is directly proportional to the amount of light gathered by the objective lens. If a larger sight is selected, choose one with dual locking levers to prevent the device from rocking back and forth (and possibly hitting the rail) when it is used on a high-power weapon. Check the accessibility of the controls. Remember that this device is situated in front of the day sight, so the user may have to get out of shooting position to reach controls that are not located near the rear of the unit.
Make sure that a device to be purchased is specified using testable specifications. It is not uncommon for units to be advertised in terms that cannot be verified. For example, an intensified camera system might be specified as “Operates in starlight using a Canon 50 mm f/2.8 lens.” That sounds good, but what does it mean? How can it be tested? How can operational range be tested? On a clear night, even the poorest scopes can see stars that are many light years away.
Specify items such as objective lens f/number and focal length; objective lens operating wavelength range; submersibility (depth and duration); operating temperature range; image intensifier type and performance parameters; accuracy (for weapon sights); system limiting resolution; and system resolution at a low-light level (typically starlight).
The potential NVS should be manufactured to full military specifications, if possible.
State-of-the-art, dual-use devices like those above aren’t cheap, but our local SWAT team recently purchased four of these, one for each team sniper, using a Homeland Security grant. If your local department doesn’t have enough cash to buy state-of-the-art night vision, federal grant money is a possibility.
Image intensifiers are subject to “blooming” or “halo” when exposed to bright light, such as street lamps or muzzle flashes. State-of-the-art night vision NVS greatly reduce this effect using a technology called “autogating.” Autogating is a method used to control the light gain of an image intensifier under high light levels. In the old days, when an image intensifier was subject to high light levels, it became blurry and washed out as a result of its adjustment to the high light level.
Intensifiers that incorporate autogating will stay crisp and clear even at high light levels. This is a useful feature for devices that are used in areas where the user must view very dark scenes and scenes that are illuminated by man-made lighting. Autogating can affect the exportability and the cost of the unit, regardless of whether it is Gen 2 or Gen 3. We consider autogating to be highly desirable, if not mandatory.
In sum, the third generation intensifier is clearly superior to all other I² devices. It should be chosen unless cost, availability, magnification, or exportability dictates otherwise. Western European Gen 2 intensifiers are the next choice when an American-made Gen 3 is not an option.
Infrared (IR) illuminators are useful in areas where supplemental illumination is necessary, such as in the woods or in abandoned buildings. Illuminators act as a flashlight for the user, yet they are essentially invisible to the unaided eye. A good illuminator should be powerful (20 milliwatts or more) for CQB type weapons because the user can use the IR for clearing in the dark. The downside to the IR / white light combination is that the IR illuminator can serve as a beacon to bad guys who possess even the most primitive night-vision equipment. The IR illuminator should have some sort of mechanical lockout mechanism that prevents it from being accidentally energized.
Thermal imaging is a type of IR imaging that detects radiation in the IR range of the electromagnetic spectrum and produces images of that radiation, essentially based on the heat of the object in question. Since infrared radiation is emitted by all objects based on their temperatures, thermal imaging makes it possible to “see” one’s environment without illumination. The amount of radiation emitted by an object increases with its temperature, therefore thermal imaging allows one to see variations in temperature, hence the name.
When viewed by thermal imagers, warm objects stand out against cooler backgrounds; humans and other warm-blooded animals become easily visible against the environment, day or night. The warmer the object, the brighter it appears through the imager. Most thermal imagers show black and white or shades of gray, depending on the object’s IR output, although “false color” thermal imagers exist. These show images in various color hues.
Modern thermal imagers are similar to the latest I² devices in that they can be mounted ahead of the day optic, and while thermal imagers can “see” in total darkness, they are expensive, typically costing about nearly three times as much as state-of-the-art I² devices. There are no “generations” of thermal imagers as is the case with I² devices. While thermal imagers are capable of operating without ambient light, they also have several limitations that the potential user should be aware of prior to purchasing.
Thermal imagers have limited capability for facial recognition and details of objects viewed because they see only the “heat” emitted by the object. Thermal imagers cannot “see” through clear glass or thick foliage because both are insulators that block heat transfer. As is the case with I² devices, thermal imagers should be only purchased from reputable manufacturers with a proven track record of performance with the same customers mentioned under our discussion of I² devices. Beware of manufacturers who make exorbitant claims about their products’ capabilities.
Finally, the potential user should be aware of a recently introduced night vision technology that combines thermal and Gen 3 image intensifier technology in a single unit. These are only now coming onto the law enforcement market and are very expensive, but for the organization that wants to combine the best of both worlds into one, these new NVS are current state of the art and are dual-use devices that mount ahead of the weapon’s day optic.
Charlie Cutshaw is a small arms, ammunition and infantry weapons editor for Jane’s Defense Information. He served as an Army infantry, ammunition and intelligence officer before retiring in 1996. His military assignments included a tour of duty in Vietnam as an adviser. He currently lives in Alabama, where he is a full-time writer and reserve officer. He can be reached at CQCutshaw@aol.com.
Published in Tactical Response, Jul/Aug 2008
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