Shadow mask
The shadow mask is one of the two technologies used in the manufacture of cathode-ray tube televisions and computer monitors which produce clear, focused color images. The other approach is the aperture grille, better known by its trade name, Trinitron. 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.
A shadow mask is a metal plate punched with tiny holes that separate the colored phosphors in the layer behind the front glass of the screen. Shadow masks are made by photochemical machining, a technique that allows for the drilling of small holes on metal sheets. Three electron guns at the back of the screen sweep across the mask, with the beams only reaching the screen if they pass through 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 that each can only be hit by one of the beams coming from the three electron guns. For instance, the blue phosphor dots are hit by the beam from the "blue gun" after passing through a particular hole in the mask. The other two guns do the same for the red and green dots. This arrangement allows the three guns to address the individual dot colors on the screen, even though their beams are much too large and too poorly aimed to do so without the mask in place.
A red, a green, and a blue phosphor are generally arranged in a triangular shape. For television use, modern displays use rectangular slots instead of circular holes, improving brightness. This variation is sometimes referred to as a slot mask.
Development
Color television
Color television had been studied even before commercial broadcasting became common, but it was not until 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, broadcast in succession. Most experimental systems broadcast entire frames in sequence, with a colored filter 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 first 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, called the "luminance". This closely matched the black and white signal of existing broadcasts, allowing the picture to be displayed on black and white televisions. The remaining color information was separately encoded into the signal as a high-frequency modulation 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 extra information would be detected, 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 guns 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, 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 two electron guns aimed at either side of a phosphor covered plate in the center of the tube. Development had not progressed far when Baird died in 1946. A similar project was the Geer tube, which used a similar arrangement of guns aimed at the back of a single plate covered with small three-sided phosphor covered pyramids.
However, all of these projects had problems with colors bleeding from one phosphor 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 unwieldy; 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.One contender was General Electric's Penetron, which used three layers of phosphor painted on top of each other on the back of the screen. Color was selected by changing the energy of the electrons in the beam so that they penetrated to different depths within the phosphor layers. Actually hitting the correct layer proved almost impossible, and GE eventually gave up on the technology for television use, although it went on to see some use in the avionics world where the color gamut could be reduced, often to three colors, which the system was able to achieve.
More common were attempts to use a secondary focussing arrangement just behind the screen to produce the required accuracy. Paramount Pictures worked long and hard on the Chromatron, which used a set of wires behind the screen as a secondary "gun", further focusing the beam and steering it towards the correct color. Philco's "Apple" tube 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. GE had given up on the Penetron by the early 1960s. Sony tried the Chromatron in the 1960s, but gave up and developed the Trinitron instead. The Apple tube re-emerged in the 1970s and had some success with a variety of vendors. 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 introduced their first Trinitron sets.
Shadow mask
In 1938 German inventor Werner Flechsig first patented 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 delta pattern 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 and be stopped. However, when the beams passed a hole they would continue 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 focusing 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 interactions, 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 for a similar B&W set.
The amount of power deposited on the color 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. Signals that alternated between light and dark caused cycling that further increased the difficulty of keeping the mask from warping.
Furthermore, the geometry required complex systems to keep the three beams properly positioned across the screen. If you consider the beam when it is sweeping across the middle area of the screen, the 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 farther 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 maintain correct beam positioning.