Most people prefer to work and live during the day, but cops know the good stuff happens after dark. Operating in that environment means that you have to be able to see to do any good, so we've been developing and buying flashlights that are bigger, brighter, smaller, lighter, and more efficient (pick any two). Flashlights have an inherent weakness in that they tell the other guy where you are, even if you can't see them.

Night vision and thermal imaging devices allow you to see the other guy without this drawback, and sometimes even allow you to see him when you wouldn't be able to do so in bright daylight.

The category of devices called "night vision" and "thermal imaging" are used in similar settings and often for similar purposes. Even so, they are distinctly different in both the way they operate and in their capabilities.

Image Intensification (I²)

The technology usually called night vision is actually a form of image intensification. Very low levels of light are amplified, or intensified, to produce an image our eyes can view.

Light energy is composed of photons, elementary physical particles that are way too small to describe. The more light there is, the more photons are present. Night vision devices use the photoelectric effect to eject electrons from a metal plate when the plate is struck by photons, then shoot the electrons at a phosphor screen, converting the electrons back into photons and making portions of the screen glow. The glow pattern is the image seen by the user.

The technology has evolved through (so far) four generations. Generation Zero or "Gen 0" focused the electrons through a funnel-like anode, accelerating them toward the cathode phosphor screen. The resulting image was highly distorted, so that you knew something was there, but might not be able to tell what it was. The concentrated stream of electrons was also hard on the phosphor screen, so the image tubes didn't last long.

Gen 1 used more efficient designs to produce electrons from photons, so the devices worked at lower light levels with slightly less image distortion. But they retained the same problems with burning out the image tubes as the Gen 0 models. Gen 1 technology produced the "starlight scopes" used by the military during the Vietnam era.

Gen 2 night vision technology incorporated a microchannel disk into the scope design, placing the disk between the photon capture plate and the cathode image tube. The microchannel disk is perforated with millions of tubes (channels), passing the electrons through without compressing the stream through the funnel of the Gen 0 and Gen 1 scopes, decreasing distortion. The electrons are multiplied thousands of times as they pass through the channels, producing a clearer image from less light.

Some night vision devices of foreign manufacture, especially those offered as military surplus, claim to use Gen 2 technology. Many of these lack the microchannel plate, so they are really Gen 1, with the inherent problems with short image tube service life.

Gen 3 is the state of the art. The microchannel disk has more and smaller holes and the electrons that pass through them are multiplied even more than before. This improves the resolution of the image, and works with even less light. Gallium-arsenide photocathode image tubes convert the electrons flowing from the microchannel plate more efficiently. The images are still monochrome (no color), but the difference is dramatic, especially with telescopic images.

Night vision devices use a standard of line-pairs-per-millimeter (lp/mm) to measure resolution of the image intensifiers. Larger numbers indicate superior resolution, or clarity of image. Some advertisements for Gen 2 night vision devices (NVDs) claim 52 lp/mm resolution, but that may be snake oil. Representatives from ITT Night Vision, one of the most advanced manufacturers of Gen 3 gear, say their best equipment renders about 51 lp/mm, and standard Gen 3 units produce about 45 lp/mm.

Older I² tubes were prone to blooms when a light source was aimed into the scope. The bloom or flash overloaded the tube, washing out all detail, required some cool-down time to recover, and sometimes destroyed the operator's accommodation to low light for several minutes. Newer technology includes overload circuits that prevent this.[PAGEBREAK]

Thermal Imaging

As the name suggests, thermal imaging makes use of the difference in temperature between an object and its surroundings. Thermal radiation occurs in wavelengths of about 900 to 1,400 nanometers, above that of visible light. This band is called the infrared (pronounced infra-RED, or IR) spectrum. There are many applications for thermal imaging outside of law enforcement and military operations, such as finding leaky areas in the insulation of a house or detecting overheating and failure-prone connections on high voltage power lines. Thermal imagers are also called thermographic cameras, bolometers, or microbolometers.

It's easy to confuse thermal imaging with IR cameras and photography, as they share similar terminology. Some consumer still and movie cameras come equipped with IR filters and illuminators for low-light photography, and surveillance cameras use IR light to monitor dimly lit areas. This type of infrared technology uses a shorter wavelength band below the thermal zone, called near-infrared. Near-infrared light occurs around 700 nanometers, just above visible red light.

IR cameras don't "see" temperature differences-they see IR light reflected back to the lens. In order for IR imaging to work, there has to be a source of IR light. Most IR cameras have IR emitters arranged around the lens to produce this light. People can usually see active emitters in the dark if they look directly at them. Seen with bare eyes, the emitters show a dull red glow. If you happen to be wearing IR goggles, they'll show up like car headlights. For this reason, IR night vision has limited effectiveness. It's great if you're the only one using it, but when the other guys have it too, you might as well turn on the overhead lights and sirens.

Thermal imaging used in law enforcement applications is passive, in that there is no IR source at the viewing end. A man in dark clothing hiding in a tree at night may be all but invisible to the naked eye, and even difficult to spot if a flashlight is turned on him. To a thermal imager, the body heat coming off his skin and through his clothing is as bright as a road flare in a cave.

High-end thermal imagers have color displays showing temperature gradients across the entire observed area, and are very expensive. Law enforcement is usually more concerned with "warm" vs. "not warm," so the equipment is simpler and not nearly as costly.

Traditionally, thermal imaging gear has been large, heavy, and expensive. The most sensitive equipment required mechanically cooled housings with Stirling engines or liquid nitrogen to keep their sensors chilled, and that added to weight, bulk, and reliability. The newest equipment doesn't require cooling and brings the size and weight down comparable to a handheld telescope or binocular.

FLIR H-Series

At last year's IACP show, FLIR debuted its H-Series Compact Tactical Thermal Night Vision Camera. The handheld monocular device, about the size of a soda can, runs on rechargeable AA batteries that provide about four hours of use. An included AC charger restores the batteries in two hours. The standard Patrol model has a 24-degree field of view, and an optional 2X extender provides extra magnification. There is also a long-range model with a 7-degree field of view. Resolution on all models is 320x240 pixels-about one-quarter of a standard TV display.

The upgraded Command model incorporates still and video image recording capability. Data is saved to a standard SD card, with all images encoded with the date and time captured. Incorporated into the Command model is a radio transceiver that will send the camera output to a receiver or monitor up to 30 feet away.

List price for the H-Series Patrol model is $6,248, with a street price around $5,000.[PAGEBREAK]

Submersible Night Vision

L-3 EOS (Electro-Optical Systems) offers a monocular I² night vision system based on the military AN/PVS-18 model, called the M983. Cut a pair of binoculars in half lengthwise, and you've got an idea of the size and shape of the device. The M983 was produced for use by U.S. Navy SEALs, and will function underwater up to 20 meters for two hours. It runs on a single AA battery, and weighs 350 grams-about three-quarters of a pound.

This is a highly versatile piece of gear. It incorporates an IR illuminator for situations when there just isn't enough ambient light to provide the desired level of detail. It has a field of view of 40 degrees with mounts available for use as a handheld pocket scope, left or right mounting on a helmet, as a gun sight, or for photographic use when mated to an SLR camera.

L-3 EOS supplies five grades of I² tubes, and any one of them can be housed in the M983. The tubes are designated Alpha, Gamma, Delta, Omega, and Infinity, with specs of 45 to 64 lp/mm.

Best of Both Worlds

ITT Night Vision recently rolled out its Dual Sensor Night Vision Goggle (DSNVG). The DSNVG combines an I² tube and IR microbolometer into one monocular device. It uses a separate battery pack and can be either mounted on a helmet or handheld.

There is a clear advantage to a device that combines I² and IR technologies. Where I² is useless in total darkness, IR finds warm spots regardless of light conditions. In an environment that has too many warm targets to discriminate the one you're interested in, the I² scope allows you to identify him visually. And best of all, the thermal image can be overlaid onto the I² view, so you see your target and his surroundings. The entire package weighs less than two pounds.

I² or IR?

The resolution of thermal imagers lags considerably behind image intensifiers and visible light optics. Where an I² image would permit recognition of facial features and reading of large print, a thermal image from the same range is going to look more like a blob, or maybe several blobs. Still, the I² gear requires some ambient light, while the thermal imager needs none at all. If you combine the two technologies, which is now possible, you'll own the night and anyone wandering around in it. 

Tim Dees is a retired police officer and the former editor of two major law enforcement Websites who writes and consults on technology applications in criminal justice. He can be reached via editor@policemag.com.

Night Vision and Thermal Imaging Manufacturers:

American Technologies Network

Autoliv NOPTIC

FLIR Systems

ITT Night Vision

Insight Tech-Gear

L-3 Electro-Optical Systems

MoroVision

N-Vision Optics

Night Optics USA

Nivisys

Xenonics

Zistos