Shadow mask

The shadow mask is one of the two technologies used in the manufacture of cathode-ray tube (CRT) 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 (LCD). The shadow mask is one of the two technologies used in the manufacture of cathode-ray tube (CRT) 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 (LCD). 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. 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 (sometimes called a 'triad'). For television use, modern displays (starting in the late 1960s) use rectangular slots instead of circular holes, improving brightness. 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 (RGB), broadcast in succession. Most experimental systems broadcast entire frames in sequence, with a colored filter (or 'gel') 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. (This is conceptually similar to a DLP based projection display where a single DLP device is used for all three color channels.) 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. 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 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, which used a similar arrangement of guns aimed at the faces of small three-sided phosphor covered pyramids.