Scotophor
Encyclopedia
A scotophor is a material showing reversible darkening and bleaching when subjected to certain types of radiation. The name means dark bearer, in contrast to phosphor
, which means light bearer. Scotophors show tenebrescence
(reversible photochromism
) and darken when subjected to an intense radiation such as sunlight
. Minerals
showing such behavior include hackmanite sodalite
, spodumene
and tugtupite
. Some pure alkali halide
s also show such behavior.
Scotophors can be sensitive to light
, particle radiation
(e.g. electron beam – see cathodochromism), X-ray
s, or other stimuli. The induced absorption band
s in the material, caused by F-center
s created by electron bombardment, can be reverted to their non-absorbing state, usually by light and/or heating.
Scotophors sensitive to electron beam radiation can be used instead of phosphor
s in cathode ray tube
s, for creating a light absorbing instead of light emitting image. Such displays are viewable in bright light and the image is persistent, until erased.
The image would be retained until erased by flooding the scotophor with a high-intensity infrared light or by electro-thermal heating. Using conventional deflection and raster
formation circuity, a bi-level image could be created on the membrane and retained even when power was removed from the CRT.
In Germany, scotophor tubes were developed by Telefunken
as blauschrift-röhre ("dark-trace tube"). The heating mechanism was a layer of mica
with transparent thin film
of tungsten
. When the image was to be erased, current was applied to the tungsten layer; even very dark images could be erased in 5–10 seconds.
Scotophors typically need higher intensity of electron beam to change color than phosphors need to emit light. Screens with layers of a scotophor and a phosphor are therefore possible, where the phosphor, flooded with a dedicated wide-beam low-intensity electron gun
, produces backlight
for the scotophor, and optionally highlights selected areas of the screen if bombarded with electrons with higher energy but still insufficient to penetrate the phosphor and change the scotophor state.
The main application of scotophors was in plan position indicator
s, specialized military radar
displays. The achievable brightness allowed projecting the image to a larger surface. The ability to quickly record a persistent trace found its use in some oscilloscope
s.
is used as a scotophor with designation P10 in dark-trace CRTs (also called dark trace tubes, color center tubes, cathodochromic displays or scotophor tubes), e.g. in the Skiatron. This CRT
replaced the conventional light-emitting phosphor
layer on the face of the tube screen with a scotophor such as potassium chloride
(KCl). KCl has the property that when a crystal is struck by an electron beam, that spot would change from translucent white to a dark magenta
color. By backlight
ing such a CRT with a white or green circular fluorescent lamp
, the resulting image would appear as black information against a green background or as magenta information against a white background. A benefit, aside from the semi-permanent storage of the displayed image, is that the brightness of the resultant display is only limited by the illumination source and optics. The F-centers hoever have tendency to aggregate, and the screen needs to be heated to fully erase the image.
The image on KCl can be formed by depositing a charge of over 0.3 microcoulomb per square centimeter, by an electron beam with energy typically at 8–10 keV. The erasure can be achieved in less than a second by heating the scotophor at 150 °C.
KCl was the most common scotophor used. Other halide
s show the same property; potassium bromide
absorbs in bluish end of the spectrum, resulting in a brown trace, sodium chloride
produces a trace that is colored more towards orange.
Another scotophor used in dark-trace CRTs is a modified sodalite
, fired in reducing atmosphere or having some chlorides substituted with sulfate ions. Its advantage against KCl is its higher writing speed, less fatigue, and the F-centers do not aggregate, therefore it is possible to substantially erase the screen with light only, without heating.
Phosphor
A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness , and fluorescent materials, where the emission decay takes place over tens of nanoseconds...
, which means light bearer. Scotophors show tenebrescence
Tenebrescence
Tenebrescence, also known as reversible photochromism, is the ability of minerals to change colour when exposed to sunlight. The effect can be repeated indefinitely, but is destroyed by heating....
(reversible photochromism
Photochromism
Photochromism is the reversible transformation of a chemical species between two forms by the absorption of electromagnetic radiation, where the two forms have different absorption spectra. Trivially, this can be described as a reversible change of colour upon exposure to light...
) and darken when subjected to an intense radiation such as sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
. Minerals
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
showing such behavior include hackmanite sodalite
Sodalite
Sodalite is a rich royal blue mineral widely enjoyed as an ornamental gemstone. Although massive sodalite samples are opaque, crystals are usually transparent to translucent...
, spodumene
Spodumene
Spodumene is a pyroxene mineral consisting of lithium aluminium inosilicate, LiAl2, and is a source of lithium. It occurs as colorless to yellowish, purplish, or lilac kunzite , yellowish-green or emerald-green hiddenite, prismatic crystals, often of great size...
and tugtupite
Tugtupite
Tugtupite is a rare beryllium aluminium tectosilicate. It also contains sodium and chlorine and has the formula Na4AlBeSi4O12Cl. Tugtupite is a member of the silica deficient feldspathoid mineral group...
. Some pure alkali halide
Alkali halide
The alkali halides are the family of ionic compounds with simple chemical formula X+Y- or XY, where X is an alkali metal and Y is a halogen. One of the most well known of these is sodium chloride or common table salt....
s also show such behavior.
Scotophors can be sensitive to light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
, particle radiation
Particle radiation
Particle radiation is the radiation of energy by means of fast-moving subatomic particles. Particle radiation is referred to as a particle beam if the particles are all moving in the same direction, similar to a light beam....
(e.g. electron beam – see cathodochromism), X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s, or other stimuli. The induced absorption band
Absorption band
An absorption band is a range of wavelengths, frequencies or energies in the electromagnetic spectrum which are able to excite a particular transition in a substance...
s in the material, caused by F-center
F-Center
An F-Center or Farbe center is a type of crystallographic defect in which an anionic vacancy in a crystal is filled by one or more electrons, depending on the charge of the missing ion in the crystal. Electrons in such a vacancy tend to absorb light in the visible spectrum such that a material...
s created by electron bombardment, can be reverted to their non-absorbing state, usually by light and/or heating.
Scotophors sensitive to electron beam radiation can be used instead of phosphor
Phosphor
A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness , and fluorescent materials, where the emission decay takes place over tens of nanoseconds...
s in cathode ray tube
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
s, for creating a light absorbing instead of light emitting image. Such displays are viewable in bright light and the image is persistent, until erased.
The image would be retained until erased by flooding the scotophor with a high-intensity infrared light or by electro-thermal heating. Using conventional deflection and raster
Raster scan
A raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television. By analogy, the term is used for raster graphics, the pattern of image storage and transmission used in most computer bitmap image systems...
formation circuity, a bi-level image could be created on the membrane and retained even when power was removed from the CRT.
In Germany, scotophor tubes were developed by Telefunken
Telefunken
Telefunken is a German radio and television apparatus company, founded in Berlin in 1903, as a joint venture of Siemens & Halske and the Allgemeine Elektricitäts-Gesellschaft...
as blauschrift-röhre ("dark-trace tube"). The heating mechanism was a layer of mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
with transparent thin film
Thin film
A thin film is a layer of material ranging from fractions of a nanometer to several micrometers in thickness. Electronic semiconductor devices and optical coatings are the main applications benefiting from thin film construction....
of tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
. When the image was to be erased, current was applied to the tungsten layer; even very dark images could be erased in 5–10 seconds.
Scotophors typically need higher intensity of electron beam to change color than phosphors need to emit light. Screens with layers of a scotophor and a phosphor are therefore possible, where the phosphor, flooded with a dedicated wide-beam low-intensity electron gun
Electron gun
An electron gun is an electrical component that produces an electron beam that has a precise kinetic energy and is most often used in television sets and computer displays which use cathode ray tube technology, as well as in other instruments, such as electron microscopes and particle...
, produces backlight
Backlight
A backlight is a form of illumination used in liquid crystal displays . As LCDs do not produce light themselves , they need illumination to produce a visible image...
for the scotophor, and optionally highlights selected areas of the screen if bombarded with electrons with higher energy but still insufficient to penetrate the phosphor and change the scotophor state.
The main application of scotophors was in plan position indicator
Plan position indicator
The plan position indicator , is the most common type of radar display. The radar antenna is usually represented in the center of the display, so the distance from it and height above ground can be drawn as concentric circles...
s, specialized military radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
displays. The achievable brightness allowed projecting the image to a larger surface. The ability to quickly record a persistent trace found its use in some oscilloscope
Oscilloscope
An oscilloscope is a type of electronic test instrument that allows observation of constantly varying signal voltages, usually as a two-dimensional graph of one or more electrical potential differences using the vertical or 'Y' axis, plotted as a function of time,...
s.
Materials
Potassium chloridePotassium chloride
The chemical compound potassium chloride is a metal halide salt composed of potassium and chlorine. In its pure state, it is odorless and has a white or colorless vitreous crystal appearance, with a crystal structure that cleaves easily in three directions. Potassium chloride crystals are...
is used as a scotophor with designation P10 in dark-trace CRTs (also called dark trace tubes, color center tubes, cathodochromic displays or scotophor tubes), e.g. in the Skiatron. This CRT
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
replaced the conventional light-emitting phosphor
Phosphor
A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness , and fluorescent materials, where the emission decay takes place over tens of nanoseconds...
layer on the face of the tube screen with a scotophor such as potassium chloride
Potassium chloride
The chemical compound potassium chloride is a metal halide salt composed of potassium and chlorine. In its pure state, it is odorless and has a white or colorless vitreous crystal appearance, with a crystal structure that cleaves easily in three directions. Potassium chloride crystals are...
(KCl). KCl has the property that when a crystal is struck by an electron beam, that spot would change from translucent white to a dark magenta
Magenta
Magenta is a color evoked by light stronger in blue and red wavelengths than in yellowish-green wavelengths . In light experiments, magenta can be produced by removing the lime-green wavelengths from white light...
color. By backlight
Backlight
A backlight is a form of illumination used in liquid crystal displays . As LCDs do not produce light themselves , they need illumination to produce a visible image...
ing such a CRT with a white or green circular fluorescent lamp
Fluorescent lamp
A fluorescent lamp or fluorescent tube is a gas-discharge lamp that uses electricity to excite mercury vapor. The excited mercury atoms produce short-wave ultraviolet light that then causes a phosphor to fluoresce, producing visible light. A fluorescent lamp converts electrical power into useful...
, the resulting image would appear as black information against a green background or as magenta information against a white background. A benefit, aside from the semi-permanent storage of the displayed image, is that the brightness of the resultant display is only limited by the illumination source and optics. The F-centers hoever have tendency to aggregate, and the screen needs to be heated to fully erase the image.
The image on KCl can be formed by depositing a charge of over 0.3 microcoulomb per square centimeter, by an electron beam with energy typically at 8–10 keV. The erasure can be achieved in less than a second by heating the scotophor at 150 °C.
KCl was the most common scotophor used. Other halide
Halide
A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...
s show the same property; potassium bromide
Potassium bromide
Potassium bromide is a salt, widely used as an anticonvulsant and a sedative in the late 19th and early 20th centuries, with over-the-counter use extending to 1975 in the United States. Its action is due to the bromide ion...
absorbs in bluish end of the spectrum, resulting in a brown trace, sodium chloride
Sodium chloride
Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...
produces a trace that is colored more towards orange.
Another scotophor used in dark-trace CRTs is a modified sodalite
Sodalite
Sodalite is a rich royal blue mineral widely enjoyed as an ornamental gemstone. Although massive sodalite samples are opaque, crystals are usually transparent to translucent...
, fired in reducing atmosphere or having some chlorides substituted with sulfate ions. Its advantage against KCl is its higher writing speed, less fatigue, and the F-centers do not aggregate, therefore it is possible to substantially erase the screen with light only, without heating.