Penetron
Encyclopedia
The penetron, short for penetration tube, is a type of limited-color television
used in some military applications. Unlike a conventional color television
, the penetron produces a limited color gamut
, typically two colors and their combination. Penetrons, and other military-only cathode ray tube
s (CRTs), have been replaced by LCDs in modern designs.
on the inside face. When excited by high-speed electron
s, the phosphor gives off light, typically white but other colors are also used in certain circumstances. An electron gun
at the back of the tube provides a beam of high-speed electrons, and a set of electromagnets arranged near the gun allow the beam to be moved about the display. The television signal is sent as a series of stripes, each one of which is displayed as a separate line on the display. The strength of the signal increases or decreases the current in the beam, producing bright or dark points on the display as the beam sweeps across the tube.
In a color display the uniform coating of white phosphor is replaced by dots or lines of three colored phosphors, producing red, green or blue light (RGB) when excited. When excited in the same fashion as a B&W tube, the three phosphors give off different amount of these primary color
s, which mix in the human eye to produce a single apparent color. In order to produce the same resolution as the B&W display, a color screen needs to have three times the number of pixels. This presents a problem for conventional electron guns, which cannot be focussed or positioned accurately enough to hit these much smaller individual patterns.
A number of companies were working on various solutions to this problem in the late 1940s. At the same time, the U.S. Federal Communications Commission
(FCC) had set aside a block of the UHF spectrum for new television channels, and later decided these new channels would be used to broadcast color signals. After extensive tests they found that the only usable system was one developed by CBS
that broadcast three B&W images in turn, a full frame of red, then green, then blue. This concept was known as "field-sequential" broadcast. CBS built sets that produced color by spinning a color-filter disk in front of a conventional B&W set, timed to follow the frames. In order to reduce flicker, the frame rate was increased from 60 to 144 frame/s. This made the CBS system incompatible with existing B&W broadcasts, but since it was being used only on the new UHF frequencies this was not considered to be a serious problem.
The disk, often called a color wheel, comprised three sectors, each a transparent color filter. As one would expect, the colors were red, green, and blue. Because a given sector had to cover the whole screen, image size was limited. Furthermore, any fast motion created primary-colored trailing edges of the objects that were moving. Nevertheless, some recent projection TV receivers have used color wheels.
(GE). It was initially developed as a novel way to build a single-gun color television with the simplicity of a conventional B&W set. Like the B&W tube, it used a uniform coating of phosphor on the display with a single electron gun at the rear. However, the phosphor coating is applied in layers of different colors, red on the inside closest to the gun, and blue on the outside closest to the front face of the tube. Colors were selected by increasing the power of the electron beam, which allowed the electrons to flow through any lower layers to reach the proper color.
In a conventional set, voltage is used to control the brightness of the image, not its color, something that the new design also had to achieve. To address these competing needs, the color selection was provided by an external mechanism. The gun was modulated as it would be in a B&W set, with increasing power producing a brighter spot on the screen. A set of fine wires placed behind the screen provided the extra energy needed to select a particular color layer. Since the phosphors were relatively opaque, the system required very high accelerating voltages, between 25 and 40 kV. An improved version was introduced that used transparent phosphor layers and thin insulating layers between them that reduced the required voltages. The dielectric ensured that stray electrons, either off-voltage from the guns or secondary emission
from the phosphors themselves, were stopped before they reached the screen.
The penetron was ideally suited for use with the CBS system. The same timing that caused the mechanical wheel of the CBS sets to turn was instead used to change the voltage of the color selection grid, to the same end. The low switching rate, 144 times a second, meant that the changing high-voltage was not a major source of high-frequency noise. Unlike the mechanical CBS system, the penetron had no moving parts, could be built at any size (which was difficult to do with the disk), and had no problems with flicker. It represented a major advance in display technology.
Tektronix, a major manufacturer of oscilloscopes, offered a limited gamut of color in some of its CRT 'scopes, using Penetron-type technology.
as the new color standard in 1953.
The major disadvantage was the difficultly in correctly focussing the beam on the correct color, a problem RCA solved with their shadow mask
system. The shadow mask is a thin metal foil with small holes photoetched into it, positioned so the holes lie directly above one triplet of colored phosphor dots. Indeed, the mask served to expose photosensitive material to deposit the phosphor dots, one color at a time. Three separate electron guns are individually focussed on the mask, sweeping the screen as normal. When the beams pass over one of the holes, they travel through it, and since the guns are separated by a small distance from each other at the back of the tube, each beam has a slight angle as it travels through the hole. The phosphor dots are arranged on the screen such that the beams hit only their correct phosphor. A variant, the Sony Trinitron, used an array of vertical metal strips; the electron guns were side by side. Some major parts of the guns' electron optics were common to all three beams, hence the name.
The new broadcast system presented a serious problem for the penetron. The signal required the color to be selected at high speeds "on the fly" as the beam was being drawn across the screen. This meant the high voltage color selection grid had to be rapidly cycled, which presented numerous problems, notably high-frequency noise that filled the interior of the tube and interfered with the receiver electronics. Another modification was introduced to address this issue, using three separate guns, each fed with a different base voltage tuned to hit one of the layers. In this version no switching was required, eliminating the high-frequency noise.
Producing such a system proved difficult in practice, and for home television use GE instead introduced their "Porta-Color
" system, a dramatic improvement on RCA's shadow mask system. Other developers continued working with the basic system attempting to find ways of solving the high frequency switching issues, but none of these entered commercial production.
In the avionics role the penetron had other advantages as well. Its use of phosphors in layers instead of stripes meant that it had higher resolution, three times that of the RCA system. This was very useful for radar
display and IFF
systems, where the images were often overlaid with textual cues that required high resolution to be easily readable. Additionally, since all of the signal reached the screen in a penetron, as opposed to 15% of it in a shadow mask tube, for any given amount of power the penetron was much brighter. This was a major advantage in the avionics role where power budgets were often quite limited, yet the displays were often hit with direct sunlight and needed to be very bright. The lack of the shadow mask also meant the penetron was much more robust mechanically, and didn't suffer from color shifting under g-loads.
Penetrons were used from the late 1960s to the mid-1980s, mostly for radar or IFF systems where two-color displays (green/red/yellow) were commonly used. Improvements in conventional shadow masks removed most of its advantages during this period. Better focusing allowed the size of the holes in the shadow mask to increase in proportion to the opaque area, which improved display brightness. Brightness was further improved with the introduction of newer phosphors. Problems with doming were addressed through the use of invar
shadow masks that were mechanically robust and attached to the tube using a strong metal frame.
In a display where the colors are either on or off and various brightness levels do not have to be created, the system can be further simplified by removing the color selection grid and modulating the voltage of the electron gun itself. However, this also causes problems because the electrons will reach the screen faster when accelerated with higher voltages, which means that the deflection system has to be increased in power as well to ensure the scanning creates the same screen size and line widths on both passes.
Several alternative arrangements of the penetron were experimented on to address this problem. One common attempt used an electron multiplier at the tube face instead of the selection grid. In this system a low-energy scanning beam was used, and magnets were set to cause the electrons to strike the sides of the multipliers. A shower of higher-energy electrons would then be released and travel to the layered phosphors of a normal penetron arrangement. It was later noticed that the beams emanating from the multipliers landed in rings, which allowed a new arrangement of phosphors in concentric rings instead of layers.
The main advantage to the penetron is that it lacks the mechanical focusing system of a shadow mask television, which means that all of the beam energy reaches the screen. For any given amount of power, the penetron will be much brighter, typically 85% brighter. This is a major advantage in an aircraft setting, where power supply is limited but the displays need to be bright enough to be easily read even when directly lit by sunlight. The system is guaranteed to produce the correct colors in spite of external interference or the g-force
s of maneuvering – a very important quality in aviation settings. The penetron also offered higher resolutions because the phosphor was continuous, as opposed to the small spots in a shadow mask system. Additionally, the lack of the shadow mask makes the penetron much more robust mechanically.
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
used in some military applications. Unlike a conventional color television
Color television
Color television is part of the history of television, the technology of television and practices associated with television's transmission of moving images in color video....
, the penetron produces a limited color gamut
Gamut
In color reproduction, including computer graphics and photography, the gamut, or color gamut , is a certain complete subset of colors. The most common usage refers to the subset of colors which can be accurately represented in a given circumstance, such as within a given color space or by a...
, typically two colors and their combination. Penetrons, and other military-only 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 (CRTs), have been replaced by LCDs in modern designs.
Basic television
A conventional black and white television (B&W) uses a tube that is uniformly coated with a phosphorPhosphor
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...
on the inside face. When excited by high-speed electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s, the phosphor gives off light, typically white but other colors are also used in certain circumstances. An 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...
at the back of the tube provides a beam of high-speed electrons, and a set of electromagnets arranged near the gun allow the beam to be moved about the display. The television signal is sent as a series of stripes, each one of which is displayed as a separate line on the display. The strength of the signal increases or decreases the current in the beam, producing bright or dark points on the display as the beam sweeps across the tube.
In a color display the uniform coating of white phosphor is replaced by dots or lines of three colored phosphors, producing red, green or blue light (RGB) when excited. When excited in the same fashion as a B&W tube, the three phosphors give off different amount of these primary color
Primary color
Primary colors are sets of colors that can be combined to make a useful range of colors. For human applications, three primary colors are usually used, since human color vision is trichromatic....
s, which mix in the human eye to produce a single apparent color. In order to produce the same resolution as the B&W display, a color screen needs to have three times the number of pixels. This presents a problem for conventional electron guns, which cannot be focussed or positioned accurately enough to hit these much smaller individual patterns.
A number of companies were working on various solutions to this problem in the late 1940s. At the same time, the U.S. Federal Communications Commission
Federal Communications Commission
The Federal Communications Commission is an independent agency of the United States government, created, Congressional statute , and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, the spectrum, the...
(FCC) had set aside a block of the UHF spectrum for new television channels, and later decided these new channels would be used to broadcast color signals. After extensive tests they found that the only usable system was one developed by CBS
CBS
CBS Broadcasting Inc. is a major US commercial broadcasting television network, which started as a radio network. The name is derived from the initials of the network's former name, Columbia Broadcasting System. The network is sometimes referred to as the "Eye Network" in reference to the shape of...
that broadcast three B&W images in turn, a full frame of red, then green, then blue. This concept was known as "field-sequential" broadcast. CBS built sets that produced color by spinning a color-filter disk in front of a conventional B&W set, timed to follow the frames. In order to reduce flicker, the frame rate was increased from 60 to 144 frame/s. This made the CBS system incompatible with existing B&W broadcasts, but since it was being used only on the new UHF frequencies this was not considered to be a serious problem.
The disk, often called a color wheel, comprised three sectors, each a transparent color filter. As one would expect, the colors were red, green, and blue. Because a given sector had to cover the whole screen, image size was limited. Furthermore, any fast motion created primary-colored trailing edges of the objects that were moving. Nevertheless, some recent projection TV receivers have used color wheels.
Penetron
The penetron was original designed by Koller and Williams while working at General ElectricGeneral Electric
General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...
(GE). It was initially developed as a novel way to build a single-gun color television with the simplicity of a conventional B&W set. Like the B&W tube, it used a uniform coating of phosphor on the display with a single electron gun at the rear. However, the phosphor coating is applied in layers of different colors, red on the inside closest to the gun, and blue on the outside closest to the front face of the tube. Colors were selected by increasing the power of the electron beam, which allowed the electrons to flow through any lower layers to reach the proper color.
In a conventional set, voltage is used to control the brightness of the image, not its color, something that the new design also had to achieve. To address these competing needs, the color selection was provided by an external mechanism. The gun was modulated as it would be in a B&W set, with increasing power producing a brighter spot on the screen. A set of fine wires placed behind the screen provided the extra energy needed to select a particular color layer. Since the phosphors were relatively opaque, the system required very high accelerating voltages, between 25 and 40 kV. An improved version was introduced that used transparent phosphor layers and thin insulating layers between them that reduced the required voltages. The dielectric ensured that stray electrons, either off-voltage from the guns or secondary emission
Secondary emission
Secondary emission in physics is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The primary particles are often charged particles like electrons or ions. If the secondary...
from the phosphors themselves, were stopped before they reached the screen.
The penetron was ideally suited for use with the CBS system. The same timing that caused the mechanical wheel of the CBS sets to turn was instead used to change the voltage of the color selection grid, to the same end. The low switching rate, 144 times a second, meant that the changing high-voltage was not a major source of high-frequency noise. Unlike the mechanical CBS system, the penetron had no moving parts, could be built at any size (which was difficult to do with the disk), and had no problems with flicker. It represented a major advance in display technology.
Tektronix, a major manufacturer of oscilloscopes, offered a limited gamut of color in some of its CRT 'scopes, using Penetron-type technology.
NTSC
It was not long after the introduction of CBS's system that a new system was introduced by RCA that eventually won out. Unlike CBS's field-sequential system, RCA directly encoded the color for every spot on the screen, a system known as "dot-sequential". The advantage to the RCA system was that the primary component of the signal was very similar to the B&W signal used on existing sets, which meant the millions of B&W televisions would be able to receive the new signal while newer colors sets could see these in either B&W or color if that additional signal was provided. This was a huge advantage over the CBS system, and a modified version was selected by the NTSCNTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
as the new color standard in 1953.
The major disadvantage was the difficultly in correctly focussing the beam on the correct color, a problem RCA solved with their shadow mask
Shadow mask
The shadow mask is one of two major technologies used to manufacture cathode ray tube televisions and computer displays that produce color images. The other approach is aperture grille, better known by its trade name, Trinitron. All early color televisions and the majority of CRT computer monitors...
system. The shadow mask is a thin metal foil with small holes photoetched into it, positioned so the holes lie directly above one triplet of colored phosphor dots. Indeed, the mask served to expose photosensitive material to deposit the phosphor dots, one color at a time. Three separate electron guns are individually focussed on the mask, sweeping the screen as normal. When the beams pass over one of the holes, they travel through it, and since the guns are separated by a small distance from each other at the back of the tube, each beam has a slight angle as it travels through the hole. The phosphor dots are arranged on the screen such that the beams hit only their correct phosphor. A variant, the Sony Trinitron, used an array of vertical metal strips; the electron guns were side by side. Some major parts of the guns' electron optics were common to all three beams, hence the name.
The new broadcast system presented a serious problem for the penetron. The signal required the color to be selected at high speeds "on the fly" as the beam was being drawn across the screen. This meant the high voltage color selection grid had to be rapidly cycled, which presented numerous problems, notably high-frequency noise that filled the interior of the tube and interfered with the receiver electronics. Another modification was introduced to address this issue, using three separate guns, each fed with a different base voltage tuned to hit one of the layers. In this version no switching was required, eliminating the high-frequency noise.
Producing such a system proved difficult in practice, and for home television use GE instead introduced their "Porta-Color
Porta-Color
General Electric's Porta-Color was the first "portable" color television introduced in the United States. The Porta-Color set introduced a new display system that improved image brightness, allowing the set to operate at lower power than competing systems based on the almost universal shadow mask...
" system, a dramatic improvement on RCA's shadow mask system. Other developers continued working with the basic system attempting to find ways of solving the high frequency switching issues, but none of these entered commercial production.
Use in avionics
For other uses, however, the advantages of the penetron remained. Although it was not well suited to the dot-sequential method of color broadcast, that was only important if one was receiving over-the-air broadcasts. For uses where the details of the signal were not important, like in computer displays, the penetron remained a serious contender. When a full color gamut was not needed, the complexity of the penetron was further reduced and it became very attractive. This lent it to custom applications like military avionics, where the nature of the input signal was not important and the developer was free to use any signaling style they wished.In the avionics role the penetron had other advantages as well. Its use of phosphors in layers instead of stripes meant that it had higher resolution, three times that of the RCA system. This was very useful for 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...
display and IFF
IFF
IFF, Iff or iff may refer to:Technology/Science:* Identification friend or foe, an electronic radio-based identification system using transponders...
systems, where the images were often overlaid with textual cues that required high resolution to be easily readable. Additionally, since all of the signal reached the screen in a penetron, as opposed to 15% of it in a shadow mask tube, for any given amount of power the penetron was much brighter. This was a major advantage in the avionics role where power budgets were often quite limited, yet the displays were often hit with direct sunlight and needed to be very bright. The lack of the shadow mask also meant the penetron was much more robust mechanically, and didn't suffer from color shifting under g-loads.
Penetrons were used from the late 1960s to the mid-1980s, mostly for radar or IFF systems where two-color displays (green/red/yellow) were commonly used. Improvements in conventional shadow masks removed most of its advantages during this period. Better focusing allowed the size of the holes in the shadow mask to increase in proportion to the opaque area, which improved display brightness. Brightness was further improved with the introduction of newer phosphors. Problems with doming were addressed through the use of invar
Invar
Invar, also known generically as FeNi36 , is a nickel steel alloy notable for its uniquely low coefficient of thermal expansion . The name, Invar, comes from the word invariable, referring to its lack of expansion or contraction with temperature changes.It was invented in 1896 by Swiss scientist...
shadow masks that were mechanically robust and attached to the tube using a strong metal frame.
Description
In most versions of the penetron the tube has an inner layer of red and outer layer of green, separated by a thin dielectric layer. A complete image is produced by scanning twice, once with the gun set to a lower power that is stopped in the red layer, and then again at a higher power that travels through the red layer and into the green. Yellow can be produced by hitting the same location on both sweeps.In a display where the colors are either on or off and various brightness levels do not have to be created, the system can be further simplified by removing the color selection grid and modulating the voltage of the electron gun itself. However, this also causes problems because the electrons will reach the screen faster when accelerated with higher voltages, which means that the deflection system has to be increased in power as well to ensure the scanning creates the same screen size and line widths on both passes.
Several alternative arrangements of the penetron were experimented on to address this problem. One common attempt used an electron multiplier at the tube face instead of the selection grid. In this system a low-energy scanning beam was used, and magnets were set to cause the electrons to strike the sides of the multipliers. A shower of higher-energy electrons would then be released and travel to the layered phosphors of a normal penetron arrangement. It was later noticed that the beams emanating from the multipliers landed in rings, which allowed a new arrangement of phosphors in concentric rings instead of layers.
The main advantage to the penetron is that it lacks the mechanical focusing system of a shadow mask television, which means that all of the beam energy reaches the screen. For any given amount of power, the penetron will be much brighter, typically 85% brighter. This is a major advantage in an aircraft setting, where power supply is limited but the displays need to be bright enough to be easily read even when directly lit by sunlight. The system is guaranteed to produce the correct colors in spite of external interference or the g-force
G-force
The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...
s of maneuvering – a very important quality in aviation settings. The penetron also offered higher resolutions because the phosphor was continuous, as opposed to the small spots in a shadow mask system. Additionally, the lack of the shadow mask makes the penetron much more robust mechanically.
Patents
- U.S. Patent 2,590,018, "Production of Colored Images", Louis Koller and Fred Williams/General Electric, filed 24 October 1950, issued 18 March 1952
- U.S. Patent 2,958,002, "Production of Colored Images", Dominic Cusano and Frank Studer/General Electric, filed 29 October 1954, issued 25 October 1960
- U.S. Patent 2,827,593, "High Purity Color Information Screen", Louis Koller/General Electric, filed 29 April 1955, issued 18 March 1958
- U.S. Patent 2,992,349, "Field Enhanced Luminescent System", Dominic Cusano/General Electric, filed 24 October 1957, issued 11 July 1961
- U.S. Patent 4,612,483, "Penetron color display tube with channel plate electron multiplier", Derek Washington/Philips Electronics, filed 22 September 1983, issued 16 September 1986