Skiatron
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The skiatron is a type of cathode ray tube (CRT) that replaces the conventional
phosphor A phosphor is a substance that exhibits the phenomenon of luminescence; it emits light when exposed to some type of radiant energy. The term is used both for fluorescent or phosphorescent substances which glow on exposure to ultraviolet or ...
with some type of scotophor, typically
potassium chloride Potassium chloride (KCl, or potassium salt) is a metal halide salt composed of potassium and chlorine. It is odorless and has a white or colorless vitreous crystal appearance. The solid dissolves readily in water, and its solutions have a sa ...
. When hit by the
electron beam Since the mid-20th century, electron-beam technology has provided the basis for a variety of novel and specialized applications in semiconductor manufacturing, microelectromechanical systems, nanoelectromechanical systems, and microscopy. Mechani ...
from the back of the CRT, this normally white material turns a magenta color, producing a dark spot or line on the display. The pattern remains on the display until erased by heating the potassium chloride layer. Skiatrons were used as an early form of projection television display, particularly in
radar Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...
stations during
World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
. These tubes are also sometimes known as dark trace CRTs or dark trace tubes.


Description

During World War II, radar displays using potassium chloride evaporated on a mica plate as target material were actively developed in England, Germany, the United States, and the Soviet Union. Being naturally cathodochromic, potassium chloride did not require any special processing or treatment to become a CRT target material. When hit by the
electron beam Since the mid-20th century, electron-beam technology has provided the basis for a variety of novel and specialized applications in semiconductor manufacturing, microelectromechanical systems, nanoelectromechanical systems, and microscopy. Mechani ...
from the back of the CRT, this normally white material turns a magenta color, producing a dark spot or line on the display, which resulted in the term "''dark trace''" being applied to these devices. The pattern remains on the display until erased by heating the potassium chloride layer. This physical property is known as tenebrescence or reversible
photochromism Photochromism is the reversible change of color upon exposure to light. It is a transformation of a chemical species (photoswitch) between two forms through the absorption of electromagnetic radiation (photoisomerization), where each form has a d ...
. Skiatrons were used as an early form of projection television display, particularly in
radar Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...
stations during
World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
. The skiatron was mounted below a translucent plotting table surface and brightly lit with mercury arc stage lights. The image on the surface reflected onto the bottom of the plotting table, using a spherical mirror and a
Schmidt corrector plate A Schmidt camera, also referred to as the Schmidt telescope, is a catadioptric astrophotographic telescope designed to provide wide fields of view with limited aberrations. The design was invented by Bernhard Schmidt in 1930. Some notable ...
, in the same fashion as an
opaque projector The opaque projector, or episcope is a device which displays opaque materials by shining a bright lamp onto the object from above. The episcope must be distinguished from the diascope, which is a projector used for projecting images of transparent ...
, producing an image of the radar display at a much larger size. In RAF stations, the surface had a map on it, in
Royal Navy The Royal Navy (RN) is the naval warfare force of the United Kingdom. It is a component of His Majesty's Naval Service, and its officers hold their commissions from the King of the United Kingdom, King. Although warships were used by Kingdom ...
ships it was normally a series of radial lines. Operators viewing the surface would place markers on the projected traces, adding new markers as the traces moved. This produced trails of markers making the path of the targets clear. A variety of methods were used to erase the skiatrons. UK radars used fans to cool the tubes which were being heated by the
stage lighting Stage lighting is the craft of lighting as it applies to the production of theater, dance, opera, and other performance arts.
of the projectors. Simply turning off the fans made the tube begin to warm up, the erasure taking perhaps 10 to 20 seconds. German examples used a thin, transparent layer of
tungsten Tungsten (also called wolfram) is a chemical element; it has symbol W and atomic number 74. It is a metal found naturally on Earth almost exclusively in compounds with other elements. It was identified as a distinct element in 1781 and first ...
deposited on the front of the tube, which heated up when current was passed through it. This provided much faster erasing.


Post-war developments

After the war, skiatrons were also used for
storage oscilloscope An oscilloscope (formerly known as an oscillograph, informally scope or O-scope) is a type of electronic test instrument that graphically displays varying voltages of one or more signals as a function of time. Their main purpose is capturing ...
s, which were viewed directly instead of as a projection system. Some examples included separate areas on the screen covered with potassium chloride or phosphor, allowing the display to be set up on the phosphor section and then recorded on the skiatron section. There was some interest in the post-war era using skiatrons for large-format projection televisions, but no known commercial use can be found. Even the use in radar was not widespread; looking for an even larger format system with better properties, the RAF turned to the Photographic Display Unit, a system that took a photograph of the display, rapidly processed it, and then projected it through a modified
movie projector A movie projector (or film projector) is an optics, opto-mechanics, mechanical device for displaying Film, motion picture film by projecting it onto a movie screen, screen. Most of the optical and mechanical elements, except for the illuminat ...
. Even with this complexity, it was faster than the skiatrons, producing a new image every 15 seconds while the skiatron units were typically longer due to the erasure process.


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Bibliography

* * * * * * * * * * {{cite web , url=http://lampes-et-tubes.info/cr/16LM4G.pdf , publisher= Moscow Electric Lamp Plant (МЭЛЗ/MELZ) , title=16ЛМ4Г Скиатрон data sheet , language=ru Television technology Cathode ray tube Radar