Night vision glasses usually refer to any number of glasses, binoculars, goggles, or telescopes that use special means to increase vision at night. Some use thermal vision, some use an image intensifier, and some use active infrared. They can range in cost from simple consumer products that cost less than $100 US Dollars (USD), to high-end goggles that can cost many thousands of dollars.
There are two main categories of night vision: active and passive systems. Active systems use a device to bathe an area in non-visible light, usually in the infrared range of between 700nm-1000nm, and then have a special camera meant to see that range of light. Active night vision glasses are used by consumers and for basic security such as CCTV cameras and local law-enforcement, but aren’t particularly useful for military applications, since the light being let out can be viewed by an enemy using a night vision device as well. Passive night vision glasses collect ambient light, usually given off by the stars, and intensify it such that the viewer can make out details.
There are four main generations of night vision glasses, as designated by the US Army Night Vision and Electronic Sensors Directorate. In addition to these four, there is a fifth category of Omnibus-VII night vision glasses, which are considerably more advanced than the most recent generation, but have not yet been classified as the actual next generation. The generations range from Generation 0 to Generation 3 Omnibus-VII.
The earliest night vision glasses, known as Generation 0, were used during World War II as a sighting device for snipers. Generation 0 glasses were active, casting infrared light, and it is this active nature that has led to them being designated Generation 0, as they are not considered to be true military-grade devices by many people. These glasses continued to be used during the Korean War, but were phased out in the lead up to the Vietnam War.
Generation 1 night vision glasses first saw field use in the Vietnam War, and they were the first passive devices used. They used an S-20 photocathode, and were extremely bulky, while providing only about 100X light amplification, making them require fairly bright moonlight to function well. Generation 2 devices use a micro-channel plate to get substantially more light amplification than Generation 1 devices, and some modern Generation 2 night vision glasses have up to 20,000X amplification, along with markedly increased resolution.
Generation 3 devices build off of Generation 2 devices, by continuing to use a micro-channel plate. Instead of the simpler early photocathodes used, however, these devices use a new type of photocathode, created with gallium arsenide. This gives them substantially better resolution than earlier generations, and increases the amplification to as much as 50,000X. Omnibus-VII devices, while technically still Generation 3, are sometimes referred to by consumers as Generation 4 night vision glasses. This level of device uses a technology called autogating to be able to change quickly as light conditions change, so that visibility is retained even in the midst of bright explosions or flares.