The LCDs put for projection systems are most often small reflective or transmissive panels lit up by a strong arc lamp source. A line of lenses expands the reflected or transmitted image and then displays it onto the screen. In front-projection systems the LCD is located on the same side of the screen as the viewer, while in rear-projection systems the screen is lit up from behind. Projectors of higher expense and capacity can use three discrete LCD panels, creating separate red, green, and blue images that blend to reflect a coloured display on the screen.
The increasing requirement for video presentations has placed a growing emphasis on the switching speed of liquid crystals. This has led to the development of items using smectic liquid crystals, certain ones of which possess a faster 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. Inside it the liquid crystal molecules are set out in layers perpendicular to the substrate planes, which are differentiated by one or two micrometres, and within the layers the molecules are slanted, as displayed in the figure. The host liquid crystal contains optically active molecules, and a scarcely perceptible consequence of the optical activity and the tilt of the molecules is the presence 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 within the plane of the layers. Therefore, 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 therefore reverse the tilt direction of the molecules. The consequential change in optical properties can create a change from light to dark in the case that one or more polarizers are employed.
SSFLC devices have been marketed for larger passive-matrix displays, but their expense and complex detail has stopped them from making any great effect on the market. Small transmissive and reflective active-matrix SSFLC displays, however, show some promise for use as elements in projection systems or as viewfinders in digital cameras. Their quick reacting allows them to be utilised in time-sequential colour systems, in which high cost colour filters are emulated by a coloured backlight that flashes red, green, and blue in quick pace (approximately 100 cycles a second). For example, the liquid crystal can be switched to a transmissive state in the red and green periods then to a nontransmissive state for the blue period, with the outcome that the eye sees an average of red and green light, or the colour yellow.
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