The LCDs utilised for projection systems are generally small reflective or transmissive panels lit up by a forceful arc lamp source. A series of lenses expands the reflected or transmitted image and then casts it on the screen. For front-projection systems the LCD is located on the side of the screen as the viewer, although in rear-projection systems the screen is lit up from behind. Projectors of more expense and capability can be found with three distinct LCD panels, forming separate red, green, and blue images that mesh to form a coloured picture on the screen.
The growth in demand for pictographic displays has placed a particular emphasis on the switching speed of liquid crystals. This has led to the manufacture of objects build with smectic liquid crystals, certain ones 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 complex smectic device. With it the liquid crystal molecules are set out in layers that are perpendicular to the substrate planes, which are separated by one or two micrometres, and throughout the layers the molecules are slanted, as illustrated in the figure. The host liquid crystal contains optically active molecules, and a slight outcome of the optical activity and the angle of the molecules is the appearance of a permanent charge separation, or ferroelectric dipole, similar to the ferromagnetic dipole of a magnet. The direction of this dipole is perpendicular to the tilt direction of the molecules and throughout the plane of the layers. Thus, there is a permanent charge separation over the liquid crystal layer in the SSFLC, and its sign is directly attracted to the tilt direction of the molecules. An applied voltage of the correct sign can reverse the direction of this dipole in tens of microseconds and hence reverse the tilt direction of the molecules. The resultant change in optical properties can make a change from light to dark when one or more polarizers are employed.
SSFLC devices have been publicized for bigger passive-matrix presentations, but their expense and complex detail has impeded them from making any significant impact on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have shown some possibility for use as elements in projection systems or as viewfinders in digital cameras. Their quick response allows them to be employed in time-sequential colour systems, in which expensive colour filters are replaced by a coloured backlight that flashes red, green, and blue in fast succession (approx 100 cycles every second). For example, the liquid crystal may be switched to a transmissive state for the red and green periods and to a nontransmissive state during the blue period, creating the upshot that the eye sees an average of red and green light, or the colour yellow.
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