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An Image Intensifier Tube is a vacuum tube device for increasing the intensity of available light in an optical system to allow use under low light conditions such as as night, to facilitate visual imaging of low-light processes such as fluorescence of materials to X-rays or Gamma rays, or for conversion of non-visible light sources such as near-infrared or short wave infrared to visible.
Contents
1 Introduction
2 History
2.1 Pioneering Work
2.2 Generation 0 - Early Infrared Electro-optical Image Converters
2.3 Solar Blind
2.4 Generation 1 - Significant Amplification
2.5 Cascade (Passive) Image Intensifier Tubes
2.6 Generation 2 - The Micro-Channel Plate
2.7 Generation 3 - High Sensitivity and improved frequency response
2.8 Generation 4
3 Terminology
3.1 Sensitivity
3.2 Resolution
3.3 Gain
3.4 MTTF (Mean Time To Failure)
3.5 MTF (Modulation Transfer Function)
4 See also
5 References
6 External links
//
Introduction
Image Intensifier Tubes (IITs) are an electro-optical device that allows many devices such as night vision devices and medical imaging devices to function. They convert low levels of light from various wavelengths into visible quantities of light at a single wavelength.
History
Development of Image Intensifier Tubes began during the 20th Century and has led to continuous development since inception.
Pioneering Work
The idea of an image tube was first proposed by G Holst and H De Boer (Netherlands) in 1928[1] but early attempts to create one were not successful. It was not until 1934 that Holst, working for Philips, created the first successful Infrared Converter tube. This tube consisted of a photocathode in close proximity to a fluorescent screen. Using a simple lens, an image was focused on the photocathode and a potential difference of several thousand volts was maintained across the tube, causing electrons dislodged from the photocathode by photons to strike the fluorescent screen. This caused the screen to light up with the image of the object focused onto the screen, however the image was non-inverting. With this image converter type tube, it was possible to view infrared light in real time, for the first time.
Generation 0 - Early Infrared Electro-optical Image Converters
Development continued in the US as well during the 1930s and mid-1930, the first Inverting Image Intensifier was developed at RCA. This tube used an electrostatic inverter to focus an image from a spherical cathode onto a spherical screen (The choice of spheres was to reduce off-axial aberrations). Subsequent development of this technology led directly to the first Generation 0 Image Intensifiers which were used by the military during World War Two to allow vision at night with infrared lighting for both shooting and personal night vision. Early night vision devices based on these technologies were used by both sides and used to great effect in Okinawa, to target Japanese soldiers coming out of caves during the night. However the downside of active night vision (When infrared light is used) is that it is quite obvious to anyone else using the technology.
Unlike later technologies, early Generation 0 night vision devices were unable to significantly amplify the available ambient light and so required the infra-red source to be useful. These devices used a S1 photocathode or "Silver-Oxygen-Caesium" photocathode, discovered in 1930 which had a sensitivity of around 60 ?A/lm and a quantum efficiency of around 1% in the ultraviolet region and around 0.5% in the infrared region. Of note, the S1 photocathode had sensitivity peaks in both the infrared and ultraviolet Spectrum and with sensitivity over 950nm was the only photocathode material that could be used to view infrared light above 950nm.
Solar Blind
Solar Blind Photocathodes were not of direct military use and are not covered by "Generations". Discovered in 1953 by Taft and Apker[2], they were originally made from Caesium telluride. The characteristics of "Solar Blind" type photocathodes are a response below 280nm in the ultraviolet spectrum, which is below the wavelength of light that the atmosphere passes through from the sun.
Generation 1 - Significant Amplification
With the discovery of more effective photocathode materials, which increased in both sensitivity and quantum efficiency, it became possible to achieve significant levels of gain over Gen0 devices. In 1936, the S-11 cathode (Caesium-Antimony) was discovered by Gorlich, which provided sensitivity of approximately 80 ?A/lm with a quantum efficiency of around 20%, however this only included sensitivity in the visible region with a threshold wavelength of approximately 650nm.
But it was not until the development of the Bialkali Antimonide photocathodes...(and so on)
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