Geer tube
Encyclopedia
The Geer tube was an early single-tube color television
cathode ray tube
, developed by Willard Geer. The Geer tube used a pattern of small phosphor-covered three-sided pyramids on the inside of the CRT faceplate to mix together separate red, green and blue signals from three electron gun
s. The Geer tube had a number of disadvantages, and was never used commercially due to the much better images generated by RCA
's shadow mask
system. Nevertheless, Geer's patent was awarded first, and RCA purchased an option on it in case their own developments didn't pan out.
") that rotated in front of an otherwise conventional black and white television tube. Each frame encoded one color of the picture, and the wheel spun in sync with the signal so the correct gel was in front of the screen when that colored frame was being displayed. Because they broadcast separate signals for the different colors, all of these systems were incompatible with existing black and white sets. Another problem was that the mechanical filter made them flicker unless very high refresh rates were used.
RCA
worked along different lines entirely, using the luminance-chrominance system introduced by Georges Valensi
in 1938. This system did not directly encode or transmit the RGB signals; instead it combined these colors into one overall brightness figure, the "luminance
". Luminance closely matched the black and white signal of existing broadcasts, allowing it to be displayed on black and white televisions. This was a major advantage over the mechanical systems being proposed by other groups. Color information was then separately encoded and folded into the signal as a high-frequency modification to produce a composite video
signal – on a black and white television this extra information would be seen as a slight randomization of the image intensity, but the limited resolution of existing sets made this invisible in practice. On color sets the signal would be noticed, filtered out and added to the luminance to re-create the original RGB for display.
Although Valensi's system had enormous benefits, it had not been successfully developed because it was difficult to produce the display tubes. Black and white TVs used a continuous signal and the tube could be coated with an even deposit of phosphor. With Valensi's system, the color was changing continually along the line, which was far too fast for any sort of mechanical filter to follow. Instead, the phosphor had to be broken down into a discrete pattern of colored spots. Focusing the right signal on each of these tiny spots was beyond the capability of electron gun
s of the era.
, was lecturing on the mechanical methods of producing color television that were being experimented with in the 1940s, and stated that an electronically scanned system would be superior, if someone would only invent one. Mentioning it later to his wife, she replied "You'd better get busy and invent it yourself".
Geer solved the display problem with the novel application of optics. Instead of trying to focus the electron beams onto tiny spots, he instead focused them onto larger areas and used simple optics to re-combine each individual primary color at any given place on the screen into a single pixel
. The tube was arranged with three separate electron guns, one each for red, green and blue (RGB), arranged around the outside of the picture area. This make a Geer tube quite large; the "necks" of the tubes normally lie behind the display area and give the TV its depth, whereas in the Geer tube the necks projected around the outside of the display area, making it much larger.
The rear face of the screen was covered with a series of tiny triangular pyramids imprinted on an aluminum sheet, coated on the inside of each face with colored phosphor. Properly aligned, a given electron beam could only reach one face of the pyramids, striking it and traveling through the thin metal into the thicker phosphor layer inside. When all three guns hit their respective faces, the colored light was created in the interior of the pyramid where it mixed, producing a proper color display on the open base, which faced the user.
One enormous advantage of the Geer system is that it could be used with any of the proposed color television broadcasting systems. CBS
was promoting a "field sequential
" system at 144 frames per second that they intended to display with a mechanical color filter wheel. This same signal could be displayed on a Geer tube by sending each successive frame to a different gun, in turn. RCA
's "dot sequential" system could also be shown by de-multiplexing the signals and sending all three color signals to each of the appropriate guns at the same time. B&W signals could be displayed by sending the same signal, muted by 1/3, also to all three guns at the same time.
Getting the electron beam to hit the correct pyramid, and not surrounding ones, was a major problem for the design. The beam was roughly circular and aimed at a triangular target, making it difficult to capture the entire beam without some overscan. The problem was particularly difficult to solve because the angle between the beam and faces changed as the beams scanned the tube – pyramids near the gun would be hit at close to a right angle, but when hitting pyramids at the opposite side of the tube the angle was grazing, and the subtended angle that the pyramid represented was much smaller. Considering that each gun was offset from the CRT's main axis, it was necessary to make major geometrical corrections to the raster geometry.
purchased the patent rights and started development of prototype units in concert with the Stanford Research Institute, spending a reported $500,000 in 1950 (approx. equivalent, $4 million, in 2005) on development. The system was widely reported on at the time, including mentions in Time Magazine, Popular Science
, Popular Mechanics
, Radio Electronics
, and others.
Many other companies were also working on color television systems, most notably RCA
. They had filed a patent on their shadow mask
system only a few weeks after Geer. When Geer, and Technicolor, informed RCA of their patent, RCA took out licenses and adding further funding to the project as a "second iron in the fire" in case none of their in-house developments worked out.
In head-to-head testing against other color television systems for the NTSC
color standardization efforts that started in November 1949, Geer's tube did not fare particularly well. Overscan bled the colors into neighboring pixels and led to soft colors and poor color registration and contrast. This problem was by no means limited to the Geer tube; several different technologies were demonstrated at the show, and only the CBS mechanical system proved able to produce a picture that satisfied the judges. In 1950, the CBS system was adopted as the NTSC standard.
Geer continued to work on the overscan problems throughout the late 1940s and into the 1950s, filing additional patents on various corrections to improve the system. Other vendors were making similar strides with their own technologies, and in 1953 the NTSC reconvened a panel to consider the color issue. This time RCA
's shadow mask
system quickly demonstrated itself as superior to all the other systems, including Geer's, and the shadow mask has remained the primary method of building color televisions until the early 2000s. At the same time, RCA's version of color encoding into a signal that was compatible with existing B&W sets was also adopted, with modifications, and remained the primary U.S. television standard until 2009, when analog television was shut down.
It does not appear that this device was ever constructed, and the arrangement of aiming elements suggests focusing the image would be a serious problem. Two other inventors had been working on this problem was well, Dennis Gabor
in England (better known for the development of holograms) and William Aiken in the US. Both of their patents were filed before Geer's, and the Aiken tube
was successfully built in small numbers. More recently, similar concepts were used, combined with computer controlled convergence systems, to produce "flatter" systems, typically for computer monitor use. Sony sold small-screen monochrome TVs using basically-similar nearly-flat CRTs; they were used for outside-broadcast monitors, as well. However these were quickly displaced by LCD-based systems.
In 1960 he filed for a patent on a three-dimensional television system that used two color tubes and a 2-dimensional version of his pyramids. The vertical channels reflected the light in two directions, providing different images for each eye.
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....
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...
, developed by Willard Geer. The Geer tube used a pattern of small phosphor-covered three-sided pyramids on the inside of the CRT faceplate to mix together separate red, green and blue signals from three 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...
s. The Geer tube had a number of disadvantages, and was never used commercially due to the much better images generated by RCA
RCA
RCA Corporation, founded as the Radio Corporation of America, was an American electronics company in existence from 1919 to 1986. The RCA trademark is currently owned by the French conglomerate Technicolor SA through RCA Trademark Management S.A., a company owned by Technicolor...
's 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. Nevertheless, Geer's patent was awarded first, and RCA purchased an option on it in case their own developments didn't pan out.
Color television
Color television had been studied even before commercial broadcasting became common, but it was only in the late 1940s that the problem was seriously considered. At the time, a number of systems were being proposed that used separate red, green and blue signals (RGB), broadcast in succession. Most experimental systems broadcast entire frames in sequence, with a colored filter (or "gelColor gel
A color gel or color filter , also known as lighting gel or simply gel, is a transparent colored material that is used in theatre, event production, photography, videography and cinematography to color light and for color correction...
") that rotated in front of an otherwise conventional black and white television tube. Each frame encoded one color of the picture, and the wheel spun in sync with the signal so the correct gel was in front of the screen when that colored frame was being displayed. Because they broadcast separate signals for the different colors, all of these systems were incompatible with existing black and white sets. Another problem was that the mechanical filter made them flicker unless very high refresh rates were used.
RCA
RCA
RCA Corporation, founded as the Radio Corporation of America, was an American electronics company in existence from 1919 to 1986. The RCA trademark is currently owned by the French conglomerate Technicolor SA through RCA Trademark Management S.A., a company owned by Technicolor...
worked along different lines entirely, using the luminance-chrominance system introduced by Georges Valensi
Georges Valensi
Georges Valensi was a French telecommunications engineer who, in 1938, invented and patented a method of transmitting color images so that they could be received on both color and black & white television sets....
in 1938. This system did not directly encode or transmit the RGB signals; instead it combined these colors into one overall brightness figure, the "luminance
Luminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...
". Luminance closely matched the black and white signal of existing broadcasts, allowing it to be displayed on black and white televisions. This was a major advantage over the mechanical systems being proposed by other groups. Color information was then separately encoded and folded into the signal as a high-frequency modification to produce a composite video
Composite video
Composite video is the format of an analog television signal before it is combined with a sound signal and modulated onto an RF carrier. In contrast to component video it contains all required video information, including colors in a single line-level signal...
signal – on a black and white television this extra information would be seen as a slight randomization of the image intensity, but the limited resolution of existing sets made this invisible in practice. On color sets the signal would be noticed, filtered out and added to the luminance to re-create the original RGB for display.
Although Valensi's system had enormous benefits, it had not been successfully developed because it was difficult to produce the display tubes. Black and white TVs used a continuous signal and the tube could be coated with an even deposit of phosphor. With Valensi's system, the color was changing continually along the line, which was far too fast for any sort of mechanical filter to follow. Instead, the phosphor had to be broken down into a discrete pattern of colored spots. Focusing the right signal on each of these tiny spots was beyond the capability of 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...
s of the era.
Geer's solution
Charles Willard Geer, then an assistant professor University of Southern CaliforniaUniversity of Southern California
The University of Southern California is a private, not-for-profit, nonsectarian, research university located in Los Angeles, California, United States. USC was founded in 1880, making it California's oldest private research university...
, was lecturing on the mechanical methods of producing color television that were being experimented with in the 1940s, and stated that an electronically scanned system would be superior, if someone would only invent one. Mentioning it later to his wife, she replied "You'd better get busy and invent it yourself".
Geer solved the display problem with the novel application of optics. Instead of trying to focus the electron beams onto tiny spots, he instead focused them onto larger areas and used simple optics to re-combine each individual primary color at any given place on the screen into a single pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
. The tube was arranged with three separate electron guns, one each for red, green and blue (RGB), arranged around the outside of the picture area. This make a Geer tube quite large; the "necks" of the tubes normally lie behind the display area and give the TV its depth, whereas in the Geer tube the necks projected around the outside of the display area, making it much larger.
The rear face of the screen was covered with a series of tiny triangular pyramids imprinted on an aluminum sheet, coated on the inside of each face with colored phosphor. Properly aligned, a given electron beam could only reach one face of the pyramids, striking it and traveling through the thin metal into the thicker phosphor layer inside. When all three guns hit their respective faces, the colored light was created in the interior of the pyramid where it mixed, producing a proper color display on the open base, which faced the user.
One enormous advantage of the Geer system is that it could be used with any of the proposed color television broadcasting systems. 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...
was promoting a "field sequential
Field-sequential color system
A field-sequential color system is a color television system in which the primary color information is transmitted in successive images, and which relies on the human vision system to fuse the successive images into a color picture. One field-sequential system was developed by Dr. Peter Goldmark...
" system at 144 frames per second that they intended to display with a mechanical color filter wheel. This same signal could be displayed on a Geer tube by sending each successive frame to a different gun, in turn. RCA
RCA
RCA Corporation, founded as the Radio Corporation of America, was an American electronics company in existence from 1919 to 1986. The RCA trademark is currently owned by the French conglomerate Technicolor SA through RCA Trademark Management S.A., a company owned by Technicolor...
's "dot sequential" system could also be shown by de-multiplexing the signals and sending all three color signals to each of the appropriate guns at the same time. B&W signals could be displayed by sending the same signal, muted by 1/3, also to all three guns at the same time.
Getting the electron beam to hit the correct pyramid, and not surrounding ones, was a major problem for the design. The beam was roughly circular and aimed at a triangular target, making it difficult to capture the entire beam without some overscan. The problem was particularly difficult to solve because the angle between the beam and faces changed as the beams scanned the tube – pyramids near the gun would be hit at close to a right angle, but when hitting pyramids at the opposite side of the tube the angle was grazing, and the subtended angle that the pyramid represented was much smaller. Considering that each gun was offset from the CRT's main axis, it was necessary to make major geometrical corrections to the raster geometry.
Competing systems
Geer filed for a patent on his design on 11 July 1944. TechnicolorTechnicolor
Technicolor is a color motion picture process invented in 1916 and improved over several decades.It was the second major process, after Britain's Kinemacolor, and the most widely used color process in Hollywood from 1922 to 1952...
purchased the patent rights and started development of prototype units in concert with the Stanford Research Institute, spending a reported $500,000 in 1950 (approx. equivalent, $4 million, in 2005) on development. The system was widely reported on at the time, including mentions in Time Magazine, Popular Science
Popular Science
Popular Science is an American monthly magazine founded in 1872 carrying articles for the general reader on science and technology subjects. Popular Science has won over 58 awards, including the ASME awards for its journalistic excellence in both 2003 and 2004...
, Popular Mechanics
Popular Mechanics
Popular Mechanics is an American magazine first published January 11, 1902 by H. H. Windsor, and has been owned since 1958 by the Hearst Corporation...
, Radio Electronics
Radio electronics
*For the magazine, see Radio-ElectronicsRadio electronics is the sub-field of electrical engineering concerning itself with the class of electronic circuits which receive or transmit radio signals....
, and others.
Many other companies were also working on color television systems, most notably RCA
RCA
RCA Corporation, founded as the Radio Corporation of America, was an American electronics company in existence from 1919 to 1986. The RCA trademark is currently owned by the French conglomerate Technicolor SA through RCA Trademark Management S.A., a company owned by Technicolor...
. They had filed a patent on 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 only a few weeks after Geer. When Geer, and Technicolor, informed RCA of their patent, RCA took out licenses and adding further funding to the project as a "second iron in the fire" in case none of their in-house developments worked out.
In head-to-head testing against other color television systems for the NTSC
NTSC
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...
color standardization efforts that started in November 1949, Geer's tube did not fare particularly well. Overscan bled the colors into neighboring pixels and led to soft colors and poor color registration and contrast. This problem was by no means limited to the Geer tube; several different technologies were demonstrated at the show, and only the CBS mechanical system proved able to produce a picture that satisfied the judges. In 1950, the CBS system was adopted as the NTSC standard.
Geer continued to work on the overscan problems throughout the late 1940s and into the 1950s, filing additional patents on various corrections to improve the system. Other vendors were making similar strides with their own technologies, and in 1953 the NTSC reconvened a panel to consider the color issue. This time RCA
RCA
RCA Corporation, founded as the Radio Corporation of America, was an American electronics company in existence from 1919 to 1986. The RCA trademark is currently owned by the French conglomerate Technicolor SA through RCA Trademark Management S.A., a company owned by Technicolor...
's 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 quickly demonstrated itself as superior to all the other systems, including Geer's, and the shadow mask has remained the primary method of building color televisions until the early 2000s. At the same time, RCA's version of color encoding into a signal that was compatible with existing B&W sets was also adopted, with modifications, and remained the primary U.S. television standard until 2009, when analog television was shut down.
After NTSC
Geer continued to work on his basic concept for some time, as well as other television related concepts. In 1955 he filed a patent on a flat TV tube that used a gun arranged to lie beside the image area that fired upward towards the top. The beam was deflected through 90 degrees by a series of charged wires so the beam was now traveling horizontally across the back of the picture area. A second grid, located beside the first, then bent the beams through a small angle so they hit the back of the screen.It does not appear that this device was ever constructed, and the arrangement of aiming elements suggests focusing the image would be a serious problem. Two other inventors had been working on this problem was well, Dennis Gabor
Dennis Gabor
Dennis Gabor CBE, FRS was a Hungarian-British electrical engineer and inventor, most notable for inventing holography, for which he later received the 1971 Nobel Prize in Physics....
in England (better known for the development of holograms) and William Aiken in the US. Both of their patents were filed before Geer's, and the Aiken tube
Aiken tube
The Aiken tube was the first successful flat panel black and white television. Originally designed in the early 1950s, a small number of tubes were built in 1958 for military use in a collaboration with Kaiser Industries. An extended patent battle followed with a similar technology developed in the...
was successfully built in small numbers. More recently, similar concepts were used, combined with computer controlled convergence systems, to produce "flatter" systems, typically for computer monitor use. Sony sold small-screen monochrome TVs using basically-similar nearly-flat CRTs; they were used for outside-broadcast monitors, as well. However these were quickly displaced by LCD-based systems.
In 1960 he filed for a patent on a three-dimensional television system that used two color tubes and a 2-dimensional version of his pyramids. The vertical channels reflected the light in two directions, providing different images for each eye.
Patents
- U.S. Patent 2,480,848, "Color Television Device", Charles Willard Geer/Technicolor Motion Picture Corporation, filed 11 July 1944, issued 6 September 1949
- U.S. Patent 2,622,220, "Television Color Screen", Charles Willard Geer/Technicolor Motion Picture Corporation, filed 22 March 1949, issued 16 December 1952
- U.S. Patent 2,850,669, "Television Picture Tube or the Like", Charles Willard Geer, filed 26 April 1955, issued 2 September 1958
- U.S. Patent 3,184,630, "Three-Dimensional Display Apparatus", Charles Willard Geer, filed 12 July 1960, issued 18 May 1960
Further reading
- Mark Heyer and Al Pinsky, "Interview with Harold B. Law", IEEE History Center, 15 July 1975
See also
- ChromatronChromatronThe Chromatron is a color television cathode ray tube design invented by Nobel prize-winner Ernest Lawrence and developed commercially by Sony, Litton Industries and others. The Chromatron offered brighter images than conventional color television systems using a shadow mask, but a host of...
, another early color television CRT that is no longer used - Beam-index tubeBeam-index tubeThe beam-index tube is a color television cathode ray tube design, using phosphor stripes and active-feedback timing, rather than phosphor dots and a beam-shadowing mask as developed by RCA...
- Shadow maskShadow maskThe 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...
- Aperture grilleAperture grilleAn aperture grille is one of two major technologies used to manufacture color cathode ray tube televisions and computer displays; the other is shadow mask....