An image intensifier is a device that amplifies visible light from an image so that a dimly lit scene can be viewed by a camera or by eye. Unlike an infrared camera, an image intensifier does not work in total darkness, when there is no light to amplify. It does, however, create a more realistic image, because the intensities it shows are related to true optical intensity and not to temperature. This realism makes it more suitable for use by untrained operators and can be used to view objects not visible by a difference in temperature alone. Image intensifiers are also much less expensive than thermal imaging.
Image intensifiers work by having a vacuum tube with a photocathode at one end that converts incoming photons to electrons. From there, a charge differential in the tube accellerates the electron until it hits a phosphor screen at the other end, converting it back to a photon again. The phosphor is usually green, as the human eye can see more shades of green than any other color.
Image intensifiers were invented by Vladimir Zworykin, an employee of RCA during World War II. His work and creation of the first generation 0 device became the basis for the sniperscope and snooperscope. Parallel development in Germany occurred by AEG in 1936, producing a prototype for the Pak anti-tank gun in 1939, which were later mounted on panzer tanks, and the "Vampir" man-portable system for infantry with MP44 rifles.
The first application for night vision was for snipers in World War II. Nicknamed the "sniperscope" and "snooperscope", they were designated the M1 and M3 infrared night sighting devices. They are simple devices that do not produce a net amplification of light, but rather allow a user to see near-infrared light. Along with beam filters, this allowed snipers to illuminate their target without their target being aware of it. However, night vision became employed by both sides, and as a result the "active" IR beams began to betray the sniper's position.
Generation 0 devices took a lot of power to use, for both the tube and the IR illuminator, had a very distorted picture due to a cone-shaped design, and a short tube life due to the high electrical charge. Generation 0 featured a photocathode made of a mixture of alkalis called S-0 which provided approximately 60 mA/lm sensitivity to light.
Generation 1 devices are also called "Starlight scopes", and were a tremendous improvement upon generation 0. They are much more power efficient, amplify light better, and produced a superior image. These devices were initially used in the Vietnam War, but were unable to function well without moonlight until heavy and bulky 3-stage tubes were deployed. Generation 1 also used a different photocathode, S-20, which provided about three times the photo sensitivity of Generation 0.
However, generation 1 devices still have a relatively short tube life, and do not amplify light much better than a dark-adjusted eye unless multiple stages are used. They still carry the benefit of being able to use a somewhat "invisible" IR illuminator, though.
Generation 1 remains one of the most popular types of night vision today. Despite its poor performance, its low cost entices people who are looking to pick up night vision as a toy.
Generation 2 was a major technological breakthrough. Although the photocathode material, S-25, wasn't much of an improvement over Generation 1's S-20, generation 2 devices introduced the microchannel plate. This plate is situated behind the photocathode and amplifies the number of electrons that pass through it. For every one electron that passes through the plate, another approximately 10,000 electrons are added to it. This allows there to be less charge present in the tube as acceleration is not the principle source of light amplification, increasing battery life, tube life, and reducing distortion noticeably. As good as generation 2 was though, it was soon to be overshadowed by a new photocathode material.
Generation 3 is the latest "generation" and is in use by the US military and others. It is essentially generation 2 technology with a new photocathode material—gallium arsenide and a better MCP. Gallium arsenide provides far better response to near-infrared light. This is very important as the majority of starlight is in the IR spectrum. However, this comes at a cost of not being able to see blue light very well at all.
Generation 3 tubes provide significantly better resolution and sensitivity and less noise, and have better overall light amplification than generation 2. Because it can be operated entirely passively outdoors, this allows soldiers to see at great distance at night without betraying their position as generations 0 and 1 technology did with IR illuminators.
There have been recent developments in image intensification tubes, modifying generation 3 devices to be "gated and filmless". This allows for even better resolution and sensitivity, and less "blooming" in urban environments. Some manufacturers have begun calling this "generation 4", but this term has not been officially adopted by the US military.
One common misconception about night vision is that the battery life is extremely short. While this may have been true with generation 0 devices, modern generation 2 and 3 tubes can run for 40 hours or more off a single AA battery, which is far superior to a flashlight.