Night vision binoculars help you look around at night. These are particularly useful in situations where light cannot be used. Examples are night hunting, wild life photography and nocturnal military operations. One important thing to note is that night vision binoculars are different from night vision goggles. Night goggles help you see in the dark but the field view remains the same while you can look at a long distance with a night binocular.
There are two types of night vision binoculars viz. thermal and image enhancement. Image enhancement binoculars detect tiny visible light radiations and amplify them for the viewer to see. The light radiation coming from the source goes through a polarizer and is then cast on to a photo tube. This photo tube then amplifies the signal up to 15000 times and then it is emitted from the other end through a lens onto a phosphorus screen.
The incident radiation causes the screen to glow and emit visible green light. The image thus created can be viewed by the user. These binoculars are cheaper and easier to use as compared to thermal imagers. One big disadvantage is that it needs at least slight amount of visible light to be present.
Thermal night vision uses a completely different technology. Instead of visible night, the thermal binoculars detect infrared radiation coming from the source and show a thermal map to the user. Every object (even the apparently cold earth) emits infrared rays which are directly proportional to the heat content of the object. Objects like animals and human beings are considerably hotter compared to background objects like walls and trees and hence can be easily identified. If you want to know what a thermal image looks like, watch the predator movie. In that movie the aliens are shown to be using thermal detection technology.
Thermal binoculars are much more efficient than image enhancement binoculars and do not need any visible light at all. However they are much more complex to use and cost a fortune. Hence, these are mostly used for military and medical operations.
For home users, an image enhancement night vision binocular should suffice. One should buy only well known brands as there is a lot of cheap stuff in the market. These will either create a poor quality images or stop working after some time. If you want to save on costs you go for a night vision monocular but they are not much fun.