The beam-index tube is a color television cathode ray tube (CRT) design, using phosphor stripes and active-feedback timing, rather than phosphor dots and a beam-shadowing mask as developed by RCA. Beam indexing offered much brighter pictures than shadow-mask CRTs, reducing power consumption, and as they used a single electron gun rather than three, they were easier to build and required no alignment adjustments. Philco led the development of the beam-indexing concept in a series of experimental devices they called the Apple tube. In spite of lengthy development, they were never able to manufacture a cost-competitive indexing tube, and eventually abandoned the concept. The major problem was the cost of the indexing electronics, which in later models required an expensive photomultiplier tube. New detectors and transistor-based electronics led to the system being re-introduced as the Uniray in the 1970s. It was highly competitive in price terms, but competing against greatly improved shadow mask designs and the new Trinitron. Several Japanese companies used the Uniray for a variety of specialist purposes, the best-known being the Sony Indextron series. The system also saw some military use, due to its low sensitivity to magnetic interference, and in such use in the UK it was known as the Zebra tube.
History
Early color CRTs
In conventional black-and-white (B&W) televisions, the CRT screen has a uniform coating of phosphor that emits white light when struck by electrons. The beam from an electron gun at the back of the tube is deflected (most commonly) by the varying fields from magnetic coils so it may be directed at any point on the screen. Electronic circuits known as time base generators pull the beam across the tube and down, creating the scanning pattern used in television signals. An amplitude-modulated signal is used to control the beam current, controlling the brightness as it is pulled across the screen. Color televisions are based on using phosphors of the three additive primary colors (red, green and blue, RGB). In order to produce reasonable resolution similar to that of a black-and-white set, the phosphors have to be deposited in very small dots or stripes. An electron gun at the back of the tube cannot be focused tightly enough to hit only a single phosphor color if that phosphor is as small as desired. Some secondary system needs to be used to refocus the beam. RCA ultimately solved this problem with a shadow mask. In this system, three separate electron guns are each aimed from different directions at a spot just behind the screen. There, a metal plate with very small holes is used to refocus the beam. Because the beams hit the plate at different incoming angles, they separate again on the far side of the plate, hitting the individual dots of color phosphor. The downside to this approach is that the plate also cuts off the majority of the beam, as much as 85% of it, leading to low image brightness. It also required three electron guns, driving up the price of the tube, and keeping the guns in proper alignment with the mask was a constant problem. A number of solutions were attempted that used a single electron gun and some sort of electrical or magnetic field very close to the screen to provide the same result as the shadow mask. RCA worked on a system with charged wires that pulled the beams slightly toward them, with stripes of colored phosphors beyond them. The problem was that the wires had to be placed very close to each other to provide the required resolution, while also being powered with high voltages to provide enough deflection. This made it very difficult to keep the signals from leaking from wire to wire. Development was abandoned when the shadow mask proved successful. Ernest Lawrence developed a similar system known as Chromatron, which used a grid of fine wires behind the screen to electrically deflect the beam, but it suffered from the same basic problem as RCA's approach. In spite of years of development, no one was able to produce a commercially viable version. Sony's attempt to produce a practical Chromatron inspired the development of their Trinitron system.
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