Zenith Electronics Corp, Glenview, Illinois is preparing a 1950s factory building in nearby Melrose Park that once turned out biscuits – cookies in the local argot – to manufacture a version of its flat tension mask display for use in television sets, the Associated Press reports. Key attractions of the flat tension mask – flat in this case means only that the tube is flat-fronted rather than slightly curved: the thing has the usual electron gun assembly at the back. The flat tension mask tube is covered by more than 100 patents, so Zenith doesn’t expect competitors. In a colour cathode ray tube, behind the face, which is coated with the red, blue and green phosphors, is a thin sheet of metal, the shadow mask, perforated with hundreds of thousands of holes, which directs the electron beams accurately to the phosphor dots. In a conventional tube – even the flatter, squarer ones so hyped these days, the shadow mask has must be curved to fit the tube’s face. Because of the curvature, it cannot be applied firmly to the screen, and instead is suspended by springs. But the electron beam causes the mask to heat up, so that it moves slightly out of alignment – which is the cause of the jazzing that occurs when someone is wearing a check jacket. But Zenith’s flat mask can be stretched taut, preventing it from moving; that means that a more powerful electron beam can be used, producing a brighter image with higher contrast. The company has been making them for monitors since 1987, but now it wants to do a 20 version for use in televisions – but going up in size from 14 has required a redesign of the production process. In tube manufacture, the phosphors are applied in a process that resembles photography. The light-sensitive phosphors are sprayed onto the inside surface of the tube, and the mask is put on top like a negative. Ultraviolet light is shined through, exposing only the dots, and the unexposed phosphor is washed away. In conventional manufacturing methods, no two shadow masks are identical so each tube has its own mask that follows it around the plant as the three colours of phosphor are applied. With the flat tubes, the masks must be held in tension in a heavy frame as they move around the plant, but 20 frames proved too unwieldy, so Zenith has had to invent a computerised device that stretches the masks for flat tubes to an exact tolerance so that one master mask can be used to apply the phosphors, eliminating the need to move the individual masks around the plant.
