Editing Image intensifier (section)

==History==
The development of image intensifier tubes began during the 20th century, with continuous development since inception.

===Pioneering work===
The idea of an image tube was first proposed by G. Holst and H. De Boer in 1928, in the [[Netherlands]] [http://spie.org/samples/PM165.pdf], 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 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 II]] to allow vision at night with infrared lighting for both shooting and personal night vision. The first military [[night vision device]] was introduced by the German army{{citation needed|date=April 2019}} as early as 1939, developed since 1935. Early night vision devices based on these technologies were used by both sides in World War II. 

Unlike later technologies, early Generation 0 night vision devices were unable to significantly amplify the available ambient light and so, to be useful, required an infrared source. These devices used an S1 photocathode or "[[silver]]-[[oxygen]]-[[caesium]]" photocathode, discovered in 1930, which had a sensitivity of around 60 μA/lm (Microampere per Lumen) 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 950&nbsp;nm was the only photocathode material that could be used to view infrared light above 950&nbsp;nm.

===Solar blind converters===
Solar blind converters, also known as solar blind photocathodes, are specialized devices that detect ultraviolet (UV) light below 280 nanometers (nm) in wavelength. This UV range is termed "solar blind" because it is shorter than the wavelengths of sunlight that typically penetrate the Earth's atmosphere. Discovered in 1953 by Taft and Apker [http://www.opticsinfobase.org/abstract.cfm?URI=josa-43-2-81], solar blind photocathodes were initially developed using [[cesium telluride]]. Unlike night-vision technologies that are classified into "generations" based on their military applications, solar blind photocathodes do not fit into this categorization because their utility is not primarily military. Their ability to detect UV light in the solar blind range makes them useful for applications that require sensitivity to UV radiation without interference from visible sunlight.

===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 Generation 0 devices. In 1936, the S-11 cathode ([[cesium]]-[[antimony]]) was discovered by Gorlich, which provided sensitivity of approximately 80 μA/lm with a quantum efficiency of around 20%; this only included sensitivity in the visible region with a threshold wavelength of approximately 650&nbsp;nm.

It was not until the development of the bialkali antimonide photocathodes ([[potassium]]-[[cesium]]-antimony and [[sodium]]-potassium-antimony) discovered by A.H. Sommer and his later multialkali photocathode (sodium-potassium-antimony-cesium) S20 photocathode discovered in 1956 by accident, that the tubes had both suitable infrared sensitivity and visible spectrum amplification to be useful militarily. The S20 photocathode has a sensitivity of around 150 to 200 μA/lm. The additional sensitivity made these tubes usable with limited light, such as moonlight, while still being suitable for use with low-level infrared illumination.

===Cascade (passive) image intensifier tubes===
[[Image:Gen1C-2-Comparison.jpg|thumb|right| A photographic comparison between a first generation cascade tube and a second generation wafer tube, both using electrostatic inversion, a 25mm photocathode of the same material and the same F2.2 55mm lens. The first generation cascade tube exhibits pincushion distortion while the second generation tube is distortion corrected. All inverter type tubes, including third generation versions, suffer some distortion.]]

Although originally experimented with by the Germans in World War Two, it was not until the 1950s that the U.S. began conducting early experiments using multiple tubes in a "cascade", by coupling the output of an inverting tube to the input of another tube, which allowed for increased amplification of the object light being viewed. These experiments worked far better than expected and night vision devices based on these tubes were able to pick up faint starlight and produce a usable image. However, the size of these tubes, at 17 in (43&nbsp;cm) long and 3.5 in (8.9&nbsp;cm) in diameter, were too large to be suitable for military use. Known as "cascade" tubes, they provided the capability to produce the first truly passive night vision scopes. With the advent of fiber optic bundles in the 1960s, it was possible to connect smaller tubes together, which allowed for the first true [[night vision device#Generation 1 (GEN I)|Starlight scope]]s to be developed in 1964. Many of these tubes were used in the [[AN/PVS-2]] rifle scope, which saw use in Vietnam.

An alternative to the cascade tube explored in the mid 20th century involves [[optical feedback]], with the output of the tube fed back into the input. This scheme has not been used in rifle scopes, but it has been used successfully in lab applications where larger image intensifier assemblies are acceptable.<ref>[[Martin L. Perl]] and [[Lawrence W. Jones]], Optical Feedback Image Intensifying System, {{US patent|3,154,687}}, Oct. 27, 1964.</ref>

===Generation 2: micro-channel plate===
Second generation image intensifiers use the same multialkali photocathode that the first generation tubes used, however by using thicker layers of the same materials, the S25 photocathode was developed, which provides extended red response and reduced blue response, making it more suitable for military applications. It has a typical sensitivity of around 230 μA/lm and a higher quantum efficiency than S20 photocathode material. [[Redox|Oxidation]] of the cesium to cesium oxide in later versions improved the sensitivity in a similar way to third generation photocathodes. The same technology that produced the fiber optic bundles that allowed the creation of cascade tubes, allowed, with a slight change in manufacturing, the production of [[microchannel plate detector|micro-channel plate]]s, or MCPs. The micro-channel plate is a thin glass wafer with a [[Nichrome]] electrode on either side across which a large potential difference of up to 1,000 volts is applied.

The wafer is manufactured from many thousands of individual hollow glass fibers, aligned at a "bias" angle to the axis of the tube. The micro-channel plate fits between the photocathode and screen. Electrons that strike the side of the "micro-channel" as they pass through it elicit secondary electrons, which in turn elicit additional electrons as they too strike the walls, amplifying the signal. By using the MCP with a proximity focused tube, amplifications of up to 30,000 times with a single MCP layer were possible. By increasing the number of layers of MCP, additional amplification to well over 1,000,000 times could be achieved.

Inversion of Generation 2 devices was achieved through one of two different ways. The Inverter tube uses electrostatic inversion, in the same manner as the first generation tubes did, with a MCP included. Proximity focused second generation tubes could also be inverted by using a fiber bundle with a 180 degree twist in it.

===Generation 3: high sensitivity and improved frequency response===
[[Image:Gen3-Image-Tube.jpg|thumb|right| A third generation Image Intensifier tube with overlaid detail]]
While the third generation of tubes were fundamentally the same as the second generation, they possessed two significant differences. Firstly, they used a [[gallium(III) arsenide|GaAs]]—[[cesium oxide|CsO]]—[[aluminium gallium arsenide|AlGaAs]] photocathode, which is more sensitive in the 800&nbsp;nm-900&nbsp;nm range than second-generation photocathodes. Secondly, the photocathode exhibits negative [[electron affinity]] (NEA), which provides photoelectrons that are excited to the conduction [[band theory of solids|band]] a free ride to the vacuum band as the Cesium Oxide layer at the edge of the photocathode causes sufficient [[band theory of solids|band]]-bending. This makes the photocathode very efficient at creating photoelectrons from photons. The Achilles heel of third generation photocathodes, however, is that they are seriously degraded by positive ion poisoning. Due to the high electrostatic field stresses in the tube, and the operation of the MicroChannel Plate, this led to the failure of the photocathode within a short period - as little as 100 hours before photocathode sensitivity dropped below Gen2 levels. To protect the photocathode from positive ions and gases produced by the MCP, they introduced a thin film of [[sintered]] [[aluminium oxide]] attached to the MCP. The high sensitivity of this photocathode, greater than 900 μA/lm, allows more effective low light response, though this was offset by the thin film, which typically blocked up to 50% of electrons.

===Super second generation===
Although not formally recognized under the U.S. generation categories, Super Second Generation or SuperGen was developed in 1989 by Jacques Dupuy and Gerald Wolzak. This technology improved the tri-alkali photocathodes to more than double their sensitivity while also improving the microchannel plate by increasing the open-area ratio to 70% while reducing the noise level. This allowed second generation tubes, which are more economical to manufacture, to achieve comparable results to third generation image intensifier tubes. With sensitivities of the photocathodes approaching 700 μA/lm and extended frequency response to 950&nbsp;nm, this technology continued to be developed outside of the U.S., notably by Photonis and now forms the basis for most non-US manufactured high-end night vision equipment.

===Generation 4===
In 1998, the US company Litton developed the filmless image tube. These tubes were originally made for the Omni V contract and resulted in significant interest by the US military. However, the tubes suffered greatly from fragility during testing and, by 2002, the [[United States Army Communications-Electronics Research, Development and Engineering Center|NVESD]] revoked the fourth generation designation for filmless tubes, at which time they simply became known as Gen III Filmless. These tubes are still produced for specialist uses, such as aviation and special operations; however, they are not used for weapon-mounted purposes. To overcome the ion-poisoning problems, they improved scrubbing techniques during manufacture of the MCP ( the primary source of positive ions in a wafer tube ) and implemented autogating, discovering that a sufficient period of autogating would cause positive ions to be ejected from the photocathode before they could cause photocathode poisoning.

Generation III Filmless technology is still in production and use today, but officially, there is no Generation 4 of image intensifiers.

===Generation 3 thin film===
Also known as Generation 3 Omni VII and Generation 3+, following the issues experienced with generation IV technology, Thin Film technology became the standard for current image intensifier technology. In Thin Film image intensifiers, the thickness of the film is reduced from around 30 Angstrom (standard) to around 10 Angstrom and the photocathode voltage is lowered. This causes fewer electrons to be stopped than with third generation tubes, while providing the benefits of a filmed tube.

Generation 3 Thin Film technology is presently the standard for most image intensifiers used by the US military.

===4G===
In 2014, French image tube manufacturer PHOTONIS released the first global, open, performance specification; "4G". The specification had four main requirements that an image intensifier tube would have to meet.

* Spectral sensitivity from below 400&nbsp;nm to above 1000&nbsp;nm
* A minimum figure-of-merit of FOM1800
* High light resolution higher than 57 lp/mm
* Halo size of less than 0.7mm