The Development of Data Projectors

The LCDs utilised in projection systems are usually small reflective or transmissive panels lit by a strong arc lamp source. A series of lenses magnifies the reflected or transmitted image then sends it onto a screen. With front-projection systems the LCD is situated on the same side of the screen as the viewer, but in rear-projection systems the screen is lit from behind. Projectors of greater expense and capability may be found with three discrete LCD panels, forming separate red, green, and blue images that come together to reflect a coloured picture on the screen.

The growth in desire for pictographic presentations has placed a special emphasis on the switching speed of liquid crystals. This has demanded the development of objects employing smectic liquid crystals, certain kinds of which have a speedier electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this point the most developed smectic device. Inside it the liquid crystal molecules are cast in layers perpendicular to the substrate planes, which are distanced by one or two micrometres, and in the layers the molecules are on a tilt, as displayed in the figure. The host liquid crystal holds optically active molecules, and a scarcely perceptible turn up of the optical activity and the slant of the molecules is the appearance of a permanent charge separation, or ferroelectric dipole, comparable to the ferromagnetic dipole of a magnet. The direction of this dipole is perpendicular to the tilt direction of the molecules and in the plane of the layers. Hence, there is a permanent charge separation through the liquid crystal layer in the SSFLC, and its sign is directly partnered to the tilt direction of the molecules. An applied voltage of the corresponding sign can reverse the direction of this dipole in tens of microseconds and hence reverse the tilt direction of the molecules. The corresponding change in optical properties can make a change from light to dark in the case that one or more polarizers are used.

SSFLC devices have been marketed for bigger passive-matrix displays, but their high cost and complex detail has stopped them from enjoying any particular movement on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have displayed some probability for use as parts in projection systems or as viewfinders in digital cameras. Their immediate responding allows them to be used in time-sequential colour systems, in which expensive colour filters are replaced by a coloured backlight that flashes red, green, and blue in quick succession (approximately 100 cycles in a second). For example, the liquid crystal might be switched to a transmissive state between the red and green periods and then to a nontransmissive state during the blue period, having the outcome that the eye sees an average of red and green light, or the colour yellow.

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