ALTERNATIVE DISPLAY SYSTEMS
The picture-tubes considered so far combine a beam-scanning process with light generation and repetitive spot-intensity modulation which is fine for home viewing and desktop computer VDUs – at least it is a very good compromise between size, cost, complexity and power consumption. In other applications tubes are disadvantaged by their weight, power consumption, shape, limited operation life, light output limitations or such practical factors as the difficulty of making tiny high-definition colour types. These drawbacks come to the fore in the realms of camera and camcorder viewfinders; pocket- portable TVs; laptop computer monitors; stable and bright projection systems; TV and monitor screens in aircraft for leisure and operational roles; and so on. For these applications other display devices have been developed and have undergone intensive development. The most common of them is the LCD array.
LCD panel
An LCD (Liquid-Crystal Display) display panel consists of tens of thousands (for good, big displays, hundreds of thousands) of individual cells in a matrix on a flat transparent plate. The light coming from behind passes first through a polarising plate so that all the light entering each cell is (e.g.) vertically polarised. If it is allowed to travel thus through to the exit plate it is blocked by the horizontal polarising action of the latter: the cell passes no light. If, however, the light orientation is twisted through 90° in its passage it can get through the exit plate to make a white dot for that cell, see Fig. 5.10. Each cell is filled with liquid crystal, which has the property of twisting its molecular structure in proportion to applied voltage, and a light beam passing through it has its polarisation twisted likewise. Thus the transparency of each cell depends on applied voltage, and if many thousands of such cells are assembled into a matrix of lines and columns – and individually controlled – a picture can be produced from light passing through the plate, with each cell form- ing one picture element (pixel). Each cell is governed by its own thick- film driver transistor (TFT) formed on the surface of the panel, and addressed individually by the on-board drive circuit, a complex IC array. Fig. 5.11 gives an outline of the system in a 575 × 720 array containing 414 000 individual cells – and 414 000 separate TFTs. For colour displays it is necessary to put coloured filters over individual liquid crystal cells (Fig. 5.12) and route R, G and B signals to the corresponding TFTs; for the same picture definition there needs to be three times as many cells/pixels in acolour display as in a black- and-white one. Direct-view TV and monitor LCD panels have a back- light, generally made from one or more fluorescent tubes with a reflector and diffuser.
LCD projector
Liquid-crystal display panels provide an attractive alternative to electron tubes as a building block in a projection TV set. They do not wear out, are stable in operation and can be made to provide bright pictures with easy adjustment of image size on a screen which does not have to be ‘special’ in any way – any flat white surface is suitable. Fig. 5.13 illustrates the principle on which they work. A powerful metal-halide lamp with even spectral emission forms the backlight. The heat is taken out of the beam, then it is split into its R, G and B primary colours by a series of mirrors and dichroic (colour-discriminating) lenses. Thus each of the three LCD panels sees only one primary colour passing through it, and is fed with the corresponding primary-colour video signal from the colour decoder. The three separate R, G, B beams are recombined in a projector lens for transmission to the screen, which in ‘domestic’ models can be from (e.g.) 1.2 m to 5 m diagonal. Image size is adjusted by a simple optical ‘zoom’ lens: there is a trade-off between picture size and brightness, as with any projection system. In a typical home-cinema projector there are three 7.5 cm LCD panels each containing about
300 000 pixels illuminated by a 250 W metal halide lamp. The panels are force-cooled by a fan to keep them at 50°C or less, monitored by a sensor which cuts off the light at excessive temperatures.
The lamp is the most vulnerable component reliability-wise, though a life of several thousand hours may be expected. The bulb is initially fired by a 12 kV arc to vaporise a mercury blob and establish a conductive path between its two electrodes. Thereafter an a.c. cur- rent of about 2.5 A at 100 V is passed through the lamp, externally regulated for constant energy level and monitored by the system- control section.
Plasma display
A relatively recent development in flat-screen technology is the gas- plasma display system, in which a gas-filled (low pressure) cell forms each pixel. A discharge takes place when a potential of about 300 V is applied to the anode, and the resulting glow provides a point-light source. In the Sony Plasmatron this technology is combined with that of an LCD display: the plasma discharge acts as a switch to couple the signal data to the LCD cells in complete lines in sequence. This is called PALC, Phase-addressed Liquid-Crystal display. A typical panel of this type offers 256 000 colour variants and 770 × 450 pix- els in a 16:9 aspect-ratio configuration on a 63 cm diagonal screen which is less than 4 mm thick.