The LCDs utilised in projection systems are usually small reflective or transmissive panels illuminated by a bright arc lamp source. A series of lenses magnifies the reflected or transmitted image and casts it onto the screen. For front-projection systems the LCD is placed on the side of the screen as the viewer, however in rear-projection systems the screen is set off from behind. Projectors of higher cost and performance sometimes have three discrete LCD panels, creating separate red, green, and blue images that combine to create a coloured display on the screen.
The increase in demand for pictographic displays has put a particular emphasis on the switching speed of liquid crystals. This has led to the development of objects employing smectic liquid crystals, some types of which possess a speedier electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is in the current day the most progressive smectic device. With it the liquid crystal molecules are managed in layers perpendicular to the substrate planes, which are differentiated by one or two micrometres, and within the layers the molecules are on a tilt, as illustrated in the figure. The host liquid crystal possesses optically active molecules, and a scarcely perceptible outcome of the optical activity and the slant of the molecules is the presence of a permanent charge separation, or ferroelectric dipole, likeable to the ferromagnetic dipole of a magnet. The direction of this dipole is perpendicular to the tilt direction of the molecules and through the plane of the layers. Thus, there is a permanent charge separation across the liquid crystal layer in the SSFLC, and its sign is directly coupled 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 therefore reverse the tilt direction of the molecules. The resultant change in optical properties can make a change from light to dark in the case that one or more polarizers are utilised.
SSFLC devices have been produced for big passive-matrix presentations, but their expensiveness and complex detail has impeded them from enjoying any great progress on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have some promise for use as elements in projection systems or as viewfinders in digital cameras. Their speedy reaction allows them to be utilised in time-sequential colour systems, in which highly expensive colour filters are emulated with a coloured backlight that flashes red, green, and blue in quick speed (about 100 cycles every second). For example, the liquid crystal may be switched to a transmissive state during the red and green periods and then to a nontransmissive state during the blue period, with the result that the eye sees an average of red and green light, or the colour yellow.
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