Night vision goggles and scopes make images from visible light, just like your eyes. So, their familiar green-cast images provide excellent situational awareness at night. Unfortunately, magnifying the amount of light in an image is only part of the solution to true night vision.
A night vision device (NVD) is an optical instrument that allows images to be produced in levels of light approaching total darkness. They are most often used by the military and law enforcement agencies, but are available to civilian users. The term usually refers to a complete unit, including an image intensifier tube, a protective and generally water-resistant housing, and some type of mounting system. Many NVDs also include sacrificial lenses, IR illuminators, and telescopic lenses.
Night vision devices were first used in World War II, and came into wide use during the Vietnam War. The technology has evolved greatly since their introduction, leading to several “generations” of night vision equipment with performance increasing and price decreasing. Another term is “night optical device” or NOD.
The critical element they don’t address is visible image contrast. That’s what you really need to be able to see at night. When a target is brighter than its surroundings it has good visual contrast, and is easy to see.
When a target doesn’t have good visual contrast you can’t distinguish it from its surroundings. No night vision scope can reveal contrast that’s just not there.
Thermal cameras see heat, not light. The more heat an object gives off the more thermal contrast it generates, and the easier it is to see.
Atmospheric conditions, like smoke, rain, or fog, reflect light, making night vision goggles and scopes even less effective. But thermal cameras see through these obscurants and clearly.
Night vision goggles and scopes also need to have the right amount of light to work well. If light levels are too low, like in rural areas and when looking into deep shadows, they won’t be able to see anything.
If there’s too much light, from street lights, security lights, or car lights, they become over-saturated. Thermal cameras have none of these limitations. That’s why airborne law enforcement units and special operations forces around the world choose FLIR when they need to see hard to find targets at night.