Image intensifier
From Wikipedia, the free encyclopedia
An image intensifier is a device that amplifies visible and near-infrared light from an image so that a dimly lit scene can be viewed by a camera or by eye. Unlike a thermographic camera, an image intensifier does not work in the total absence of visible (or near infra-red) light. 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 were invented by Vladimir Zworykin, an employee of U.S. company 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.
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[edit] How it works
Image intensifiers work by having an objective lens focusing an image into a vacuum tube with a photocathode at one end that releases electrons by the photoelectric effect on the incidence of incoming photons. From there, the photoelectron is accelerated through around 5000 volts into a tilted microchannel plate. The high energy electron releases some microchannel plate electrons, which further release other electrons, in a process called secondary cascaded emission. The MCP is tilted to encourage more electron collisions, thus increasing the rate of emission of secondary electrons.
The electrons all move together due to the potential difference across the tube, and where one or two electrons entered, thousands may emerge. A separate (lower) charge differential in the tube accelerates the secondary electrons until they hit a phosphor screen at the other end, releasing a photon for every electron. Provided the electric field is uniform, the electrons have a linear path, so correspond to the exact incident image. The image is focused by conventional optics using an ocular lens. The only multiplicative stage is in the secondary cascaded emission. The phosphor is usually green, as the human eye is more sensitive to green than other colors (hence the soldiers' nickname 'green TV' for image intensification devices).
[edit] Generation 0
The first application for night vision was for American 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 electrode design, and a short tube life due to the high electrical voltage. Generation 0 featured a photocathode made of a mixture of silver, caesium, and oxygen called S-1 which provided approximately 60 mA/lm sensitivity to light.
[edit] Generation 1
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.
[edit] Generation 2
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 (MCP, see: Micro-channel plate). The microchannel plate consists of a bundle of thousands of tiny glass fibres fused together in parallel, sliced transversely, and polished on both faces. Electrons impinging on one side of the plate tend to travel along the fibres, perpendicular to the plate's faces, thus preserving a coherent image. 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 principal 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.
[edit] Generation 3
Generation 3 is the latest "generation" and is in use by the U.S. 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 decreased sensitivity to blue light.
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 U.S. 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 using a single AA battery, using less power than a flashlight.
[edit] Later Generations
There are newer night vision systems available. Generation 3 Ultra and Generation 4 tubes exist. Most civilian equipment remains based on Generation 3 equipment and lower, however. Generation 3 Ultra and 4 still use MCP technology, but are designed to offer greater range, and higher resolution.
