Display units
The purpose of the display unit is to re-produce the original moving pictures in as faithful a way as possible. The faithfulness of the re-production depends on the type of unit used, its properties and mode of operation. The earliest display unit is the traditional CRT which remains the standard by which all other units (plasma, LCD, DLP, etc.) are compared.
The cathode ray tube
A great deal of research and development has gone into the technology of colour picture tubes over the years, and direct-view types are now avail- able with screen diagonals up to 89 cm. Improvements have been made in screen materials, gun technology and energy demand. The colour tube uses all the techniques of its simpler counterpart, and by extension and refinement of these is able to present a full colour picture on a single ‘inte- grated’ screen. These colour tubes work on the shadowmask principle originally brought to fruition by Dr A. N. Goldsmith and his research team in the American laboratories of RCA Ltd in 1950. The original device was rather clumsy and cumbersome by today’s standards, but it embodied all the fundamental features of the current generation of colour tubes, and was amenable to mass production with all the cost advantages that could bring. The practical realisation of a relatively compact and decidedly cheap domestic colour TV receiver was (and is) dependent on the direct- viewing shadowmask tube concept. Today a number of display devices are available including the LCD and Plasma screen. The CRT however, is standard display device with which all other devices are compared. It remains best the device in terms of picture quality.
In the CRT, high-speed electrons in the form of a beam current are emit- ted by an electron gun, focused and accelerated by an electron lens and then directed towards a screen which acts as a positively charged anode. The screen which is coated with phosphor gives a visible glow when hit by high-speed electrons. The colour of the emitted light is determined by the type of phosphor used. For monochrome display, only one type of phosphor coating is used. For a colour display, three types of phosphors are used in order to obtain the three primary colours.