Telechrome was the first all-electronic single-tube color television system. It was invented by well-known Scottish television engineer, John Logie Baird, who had previously made the first public television broadcast, as well as the first color broadcast using a pre-Telechrome system. Telechrome used two electron guns aimed at either side of a thin, semi-transparent mica sheet. One of the sides was covered in cyan phosphor and the other red-orange, producing a limited color gamut, but well suited to displaying skin tones. With minor modifications, the system could also be used to produce 3D images. Telechrome was selected as the basis for a UK-wide television standard by a committee in 1944, but the difficult task of converting the two-color system to three-color RGB was still under way when Baird died in 1946. The introduction of the shadow mask design by RCA produced a workable solution for color television, albeit one with considerably less image brightness. Interest in alternative systems like the Telechrome or Geer tube faded by the late 1950s. The only alternatives to see widespread use were General Electric's slot-mask, and Sony's Trinitron, both were modifications of the RCA concept. All CRT-based methods have since been almost completely replaced by LCD television, starting in the 1990s.
Background
Mechanical and hybrid color Baird performed one of the earliest public demonstrations of color television system on 3 July 1928 using an all-mechanical system with three Nipkow disk scanners synchronized with a single disk on the receiving end and three colored lights that were turned on and off in synchronicity with the broadcaster. The same basic system was used on 4 February 1938 to create the first color broadcast transmissions from The Crystal Palace to the Dominion Theatre in London. Baird was not the only one to experiment with mechanical color television, and a number of similar devices were demonstrated throughout this period, but Baird is recorded as the first to show a real over-the-air transmission in a public demonstration. In 1940 he introduced a much better solution using a system known today as hybrid color. This used a traditional black and white CRT with a rotating colored filter in front. Three frames, sent one after the other in a system known as sequential scan, were displayed on the CRT while the colored wheel was spun in synchronicity. This design was physically very long, leading to deep receiver chassis, but later versions folded the optical path using mirrors to produce a somewhat more practical system. Again, Baird was not the only one to produce such a system, with CBS displaying a very similar system at almost the same time. However, Baird was not happy with the design later stated that a fully electronic device would be better.
Fully electronic systems
The basic problem facing designers of color televisions was this: sending each frame of the moving image meant sending three complete images, one each for red, green and blue. Sequential systems, like Baird's earlier efforts, sent the three images one after another. In order for motion to appear smooth, images must change at least 16 times a second. To reduce flicker, over 40 frames per second (fps) is mandatory. For this reason, very high refresh (field) rates were necessary. CBS' system refreshed at 144 fps, 48 fps for each individual color. Peter Carl Goldmark's CBS team tried several field rates. Within the 6 MHz allowable channel bandwidth, the most acceptable rate was 144 fps. This rate made the picture signal incompatible with existing systems working at 50 or 60 Hz. A system sending all three signals at the same time at a conventional refresh rate would be greatly preferable. Transmitting such a signal could be accomplished by using three camera tubes, each with a color filter in front of them, using mirrors or prisms to aim at the same scene through a single lens. Each signal would then be separately broadcast using three conventional TV channels, and using the luminance concept, one of those could be received on a conventional black and white set. This would use a considerable amount of bandwidth, but this was a small cost in the era of only a few television channels. The problem, however, was how to combine the three separate signals back into a single display. The system used in the cameras, with three separate tubes combined together optically, was not practical due to the cost of a receiver set with three CRTs as well as the unwieldily chassis needed to contain them. One such example was the RCA Triniscope, which produced useful images but was extremely complex, required constant adjustment, and was the size of a contemporary refrigerator to produce a 10 inches (250 mm) display. A number of experiments were carried out using more conventional tubes and then filtering them, but the low output of the CRTs produced very dim images that were dismissed as impractical. Baird had previously worked on a high-intensity CRT system known as the "teapot tube" that saw some use in the UK and US as a projection system in theatres. These were normally built with two such CRTs side-by-side, with one acting as a hot backup in case the primary tube failed. In 1941 Baird converted a teapot projector to produce a two-color image by placing filters in front of the two tubes and projecting them onto a smaller screen to improve the effective intensity. He first showed this in 1941, and in 1942 the BBC described the resulting color image as "entirely natural". The image, of Paddy Naismith, is the first known image of color television to be published. A projection system with two CRTs was better than three, but still not practical for a home receiver. Baird continued to consider other solutions. One used a single conventional CRT with the two images displayed in a single frame, with the top half of the image containing the image for one color and the bottom the other. Lens systems focused on the display were positioned to see only the top or bottom image, passed them through filters, and then recombined them on a screen. There were drawings showing a similar system with three frames. Like many similar efforts from other experimenters, Baird abandoned this approach.
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