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.