The LCDs utilised in projection systems are most often small reflective or transmissive panels illuminated by a bright arc lamp source. A series of lenses expands the reflected or transmitted image and sends it onto the screen. In front-projection systems the LCD is located on the same area of the screen as the viewer, however in rear-projection systems the screen is illuminated from behind. Projectors of higher expense and capacity may be found with three discrete LCD panels, casting separate red, green, and blue images that blend to reflect a coloured display on the screen.
The increase in need for film displays has granted a particular emphasis on the switching speed of liquid crystals. This has demanded the creation of devices employing smectic liquid crystals, particular kinds of which emit a quicker electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is currently the most progressive smectic device. Inside it the liquid crystal molecules are set out in layers that are perpendicular to the substrate planes, which are distanced by one or two micrometres, and throughout the layers the molecules are slanted, as displayed in the figure. The host liquid crystal contains optically active molecules, and a subtle 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. Therefore, there exists a permanent charge separation through the liquid crystal layer in the SSFLC, and its sign is directly paired up 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 consequential change in optical properties can cause a change from light to dark if one or more polarizers are used.
SSFLC devices have been publicized for large passive-matrix presentations, but their expense and complex nature has impeded them from having any great effect on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have displayed some promise for use as elements in projection systems or as viewfinders in digital cameras. Their immediate reacting allows them to be employed in time-sequential colour systems, in which dear colour filters are emulated with a coloured backlight that flashes red, green, and blue in rapid pulsing (approximately 100 cycles a second). For example, the liquid crystal can be switched to a transmissive state between the red and green periods but then to a nontransmissive state in 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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