Shadow mask
Encyclopedia
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...
(CRT) television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
s and computer display
Computer display
A monitor or display is an electronic visual display for computers. The monitor comprises the display device, circuitry, and an enclosure...
s that produce color
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
images. The other approach is aperture grille
Aperture grille
An aperture grille is one of two major technologies used to manufacture color cathode ray tube televisions and computer displays; the other is shadow mask....
, better known by its trade name, Trinitron
Trinitron
Trinitron is Sony's brand name for its line of aperture grille based CRTs used in television sets and computer display monitors. One of the first truly new television systems to enter the market since the 1950s, the Trinitron was announced in 1966 to wide acclaim for its bright images, about 25%...
. All early color televisions and the majority of CRT computer monitors used shadow mask technology. Both of these technologies are largely obsolete, having been increasingly replaced since the 1990s by the Liquid Crystal Display
Liquid crystal display
A liquid crystal display is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals . LCs do not emit light directly....
(LCD).
A shadow mask is a metal plate punched with tiny holes that separate the colored 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 the layer behind the front glass of the screen. Three electron guns at the back of the screen sweep across the mask, with the beams only reaching the screen when they pass over the holes. As the guns are physically separated at the back of the tube, their beams approach the mask from three slightly different angles, so after passing through the holes they hit slightly different locations on the screen. The screen is patterned with dots of colored phosphor positioned so they can only be hit by the beam from only one of the guns passing through only one of the holes. For instance, a particular spot on the screen can only be hit by the beam from the "blue gun" passing through a particular hole in the mask. This arrangement allows the colored guns to address individual dots on the screen, even though their beams are much too large and too poorly aimed to do so without the mask in place.
The red
Red
Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of roughly 630–740 nm. Longer wavelengths than this are called infrared , and cannot be seen by the naked eye...
, green
Green
Green is a color, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 520–570 nanometres. In the subtractive color system, it is not a primary color, but is created out of a mixture of yellow and blue, or yellow and cyan; it is considered...
, and blue
Blue
Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...
phosphors for each 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....
are generally arranged in a triangular
Triangle
A triangle is one of the basic shapes of geometry: a polygon with three corners or vertices and three sides or edges which are line segments. A triangle with vertices A, B, and C is denoted ....
shape (sometimes called a "triad
Triad (computers)
In CRT or computer terminology, a triad is a group of three phosphor dots coloured red, green, and blue on the inside of the CRT display of a computer monitor or television set. By directing differing intensities of electron beams onto the three phosphor dots, the triad will display a colour by...
"). For television use, modern displays (starting in the late 1960s) use rectangular slots instead of circular holes, improving brightness.
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 first 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 the picture to be displayed on black and white televisions. 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 RCA'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 painting of phosphor. With RCA'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.
Numerous attempts
Through the 1940s and early 1950s a wide variety of efforts were made to address the color problem. A number of major companies continued to work with separate color "channels" with various ways to re-combine the image. RCA was included in this group; on 5 February 1940 they demonstrated a system using three conventional tubes combined to form a single image on a plate of glass, but the image was too dim to be useful.John Logie Baird
John Logie Baird
John Logie Baird FRSE was a Scottish engineer and inventor of the world's first practical, publicly demonstrated television system, and also the world's first fully electronic colour television tube...
. who made the first public color television broadcast using a semi-mechanical system on 4 February 1938, was already making progress on an all-electronic version. His design, the Telechrome, used three electron guns aimed at a phosphor covered plate in the center of the tube, the guns were aimed at different faces of the patterned phosphor. However, development had not progressed far when Baird died in 1946. A similar project was the Geer tube
Geer tube
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 guns...
, which used a similar arrangement of guns aimed at the faces of small three-sided phosphor covered pyramids.
However, all of these projects had problems with colors bleeding from one pixel to another. In spite of their best efforts, the wide electron beams simply could not focus tightly enough to hit the individual dots, at least over the entirety of the screen. Moreover, most of these devices were unwieldily; the arrangement of the electron guns around the outside of the screen resulted in a very large display with considerable "dead space".
Rear-gun efforts
A more practical system would use a single gun at the back of the tube, firing at a single multi-color screen on the front. Through the early 1950s, several major electronics companies started development of such systems.An outlier was General Electric
General Electric
General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...
's Penetron
Penetron
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...
, which used three stacked layers of phosphor and attempted to change the power of the electron beam to write to the correct one. More common were attempts to use a secondary focussing arrangement just behind the screen to produce the required accuracy. Paramount Pictures
Paramount Pictures
Paramount Pictures Corporation is an American film production and distribution company, located at 5555 Melrose Avenue in Hollywood. Founded in 1912 and currently owned by media conglomerate Viacom, it is America's oldest existing film studio; it is also the last major film studio still...
worked long and hard on the Chromatron
Chromatron
The 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...
, which used a set of wires just behind the screen as a secondary "gun", further focussing the beam and steering it towards the correct color. Philco
Philco
Philco, the Philadelphia Storage Battery Company , was a pioneer in early battery, radio, and television production as well as former employer of Philo Farnsworth, inventor of cathode ray tube television...
's "Apple" tube
Beam-index tube
The 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...
used additional stripes of phosphor that released a burst of electrons when the electron beam swept across them, by timing the bursts it could adjust the passage of the beam and hit the correct colors.
It would be years before any of these systems made their way into production. The Penetron never worked for color TV, but found niche roles in aerospace. Sony tried the Chromatron in the 1960s, but gave up and developed the Trinitron
Trinitron
Trinitron is Sony's brand name for its line of aperture grille based CRTs used in television sets and computer display monitors. One of the first truly new television systems to enter the market since the 1950s, the Trinitron was announced in 1966 to wide acclaim for its bright images, about 25%...
instead. The Apple tube re-emerged in the 1970s and had some success. But it was RCA's success with the shadow mask that dampened most of these efforts. Until 1968, every color television sold used the RCA shadow mask concept, in the spring of that year Sony
Sony
, commonly referred to as Sony, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan and the world's fifth largest media conglomerate measured by revenues....
introduced their first Trinitron sets.
Shadow mask
In 1938 German inventor Werner Flechsig first patented (received 1941, France) the seemingly simple concept of placing a sheet of metal just behind the front of the tube, and punching small holes in it. The holes would be used to focus the beam just before it hit the screen. Independently, Al Schroeder at RCA worked on a similar arrangement, but using three electron guns as well. When the lab leader explained the possibilities of the design to his superiors, he was promised unlimited manpower and funds to get it working. Over a period of only a few months, several prototype color televisions using the system were produced.The guns, arranged in a triangle at the back of the tube, were aimed to focus on the metal plate and scanned it as normal. For much of the time during the scan, the beams would hit the back of the plate. However, when the beams passed a hole they would briefly pass through it to the phosphor in front of the plate. In this way, the plate ensured that the beams were perfectly aligned with the colored phosphor dots. This still left the problem of focussing on the correct colored dot. Normally the beams from the three guns would each be large enough to light up all three colored dots on the screen. The mask helped by mechanically attenuating the beam to a small size just before it hit the screen.
But the real genius of the idea is that the beams approached the metal plate from different angles. After being cut off by the mask, the beams would continue forward at slightly different angles, hitting the screens at slightly different locations. The spread was a function of the distance between the guns at the back of the tube, and the distance between the mask plate and the screen. By painting the colored dots at the correct locations on the screen, and leaving some room between them to avoid crosstalk, the guns would be guaranteed to hit the right colored spot.
Although the system was simple, it had a number of serious practical problems.
As the beam swept the mask, the vast majority of its energy was deposited on the mask, not the screen in front of it. A typical mask of the era might have only 15% of its surface open. To produce an image as bright as the one on a traditional B&W television, the electron guns in this hypothetical shadow mask system would have to be five times more powerful. Additionally, the dots on the screen were deliberately separated in order to avoid being hit by the wrong gun, so much of the screen was black. This required even more power in order to light up the resulting image. And as the power was divided up among three of these much more powerful guns, the cost of implementation was much higher than a similar B&W set.
The amount of power deposited on the screen was so great that thermal loading was a serious problem. The energy the shadow mask absorbs from the electron gun in normal operation causes it to heat up and expand, which leads to blurred or discolored images (see doming
Doming (television)
Doming is a phenomenon found on some CRT televisions in which parts of the shadow mask become heated. In televisions that exhibit this behavior, it tends to occur in high-contrast scenes in which there is a largely dark scene with one or more localized bright spots. As the electron beam hits the...
). Signals that alternated between light and dark caused cycling that further increased the difficulty of keeping the mask from warping.
Furthermore, the geometry of the system required complex systems to keep the images in focus. If you consider the beam when it is sweeping across the middle area of the screen, the three beams from the individual guns are each traveling the same distance and meet the holes in the mask at equal angles. In the corners of the screen some beams have to travel further and all of them meet the hole at a different angle than at the middle of the screen. These issues required additional electronics and adjustments to keep the signal in focus.
Market introduction
During development, RCA was not sure that they could make the shadow mask system work. Although simple in concept, it was difficult to build in practice, especially at a reasonable price point. The company optioned several other technologies, including the Geer tube, in case the system didn't work out. When the first tubes were produced in 1950, these other lines were dropped.Wartime advances in electronics had opened up large swaths of high frequency to practical use, and in 1948 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) started a series of meetings on the use of what would become the UHF channels. At the time there were very few television sets in use in the United States, so the stakeholder groups quickly settled on the idea of using UHF for a new, incompatible, color format. These meetings eventually selected a competing semi-mechanical field-sequential color system
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...
being promoted by CBS. However, in the midst of the meetings, RCA announced their efforts on compatible color, but too late to influence the proceedings. CBS color was introduced in 1950.
However, the promise of the RCA system was so great that the National Television System Committee (NTSC) took up its cause. Between 1950 and 1953 they carried out a huge study on human color perception, and used that information to improve RCA's basic concept. RCA had, by this time, produced experimental shadow mask sets that were an enormous leap in quality over any competitors. The system was dim, complex, large, power hungry and expensive for all these reasons, but provided a usable color image, and critically, was compatible with existing B&W signals. This had not been an issue in 1948 when the first FCC meetings were held, but by 1953 the number of B&W sets had exploded; there was no longer any way they could simply be abandoned.
When the NTSC proposed that their new standard be ratified by the FCC, CBS dropped its interest in its own system. Everyone in the industry wanting to produce a set then licensed RCA's patents, and by the mid-1950s there were a number of sets commercially available. However, color sets were much more expensive than B&W sets of the same size, and required constant adjustment by field staff. By the early 1960s they still represented a small percentage of the television market in North America. The numbers exploded in the early 1960s, with 5,000 sets being produced a week in 1963.
Improvements, market acceptance
By the 1960s the first RCA patents were ending, while at the same time a number of technical improvements were being introduced. A number of these were worked into the GE Porta-ColorPorta-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...
set of 1966, which was an enormous success. By 1968 almost every company had a competing design, and color television moved from an expensive option to mainstream devices.
Doming problems due to thermal expansion of the shadow mask was solved in several ways. Some companies used a thermostat to measure the temperature, and adjust the scanning to match the expansion. Bi-metallic shadow masks, where differential expansion rates offset the issue, became common in the late 1960s. 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...
and similar low-expansion alloys were introduced in the 1980s This suffers from easy magnetization that can affect the colors, which was generally solved by including an automatic degaussing feature. The last solution to be introduced was the "stretched mask", where the mask was welded to a frame, typically glass, at high temperatures. The frame was then welded to the inside of the tube. When the assembly cooled the mask was under great tension, which no amount of heating from the guns would be able to remove.
Improving brightness was another major line of work in the 1960s. The use of rare earth
Rare earth element
As defined by IUPAC, rare earth elements or rare earth metals are a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides plus scandium and yttrium...
phosphors produced brighter colors and allowed the signal strength to be reduced slightly. Better focussing systems, especially automatic systems that meant the set spent more time closer to perfect focus, allowed the dots to grow larger on the screen. The Porta-Color used both of these advances and also re-arranged the guns to lie beside each other instead of in a triangle, allowing the dots to be extended vertically into slots that covered much more of the screen surface. This design, sometimes known as a "slot mask", became common in the 1970s.
Another change that was widely introduced in the early 1970s was the use of black masks on the inside of the phosphor patterns. This paint absorbed ambient light coming from the room, lowering the amount that was reflected back to the viewer. In order to make this work effectively, the phosphor dots were reduced in size, reducing their brightness. However, the improved contrast compared to ambient conditions allowed the faceplate to be made much more clear, allowing more light from the phosphor to reach the viewer, and actual brightness to increase. Grey-tinted faceplates dimmed the image, but provided better contrast, because ambient light was attenuated once before it reached the phosphors, and a second time as it returned to the viewer. Light from the phosphors was attenuated only once. This method changed over time, with TV tubes growing progressively more black over time.
In manufacturing color CRTs, shadow masks and aperture grilles exposed photoresist on the faceplate to ultraviolet light sources accurately positioned to simulate arriving electrons for one color at a time. This photoresist, when developed, permitted phosphor for only one color to be applied where required. The process was used a total of three times, once for each color. (The shadow mask or aperture grille had to be removable and accurately re-positionable for this process to succeed.)
Choice of technology
While many have long considered aperture grilleAperture grille
An aperture grille is one of two major technologies used to manufacture color cathode ray tube televisions and computer displays; the other is shadow mask....
technology to produce superior images, advances in shadow mask and hybrid technologies since the 1990s have made purchasing decisions more a matter of personal choice or specific application. The arrival of inexpensive liquid crystal display (LCD) monitors and other flat-screen designs has, for many applications, rendered CRTs obsolete.