The LCDs put for projection systems are usually small reflective or transmissive panels lit by a strong arc lamp source. A number of lenses expands the reflected or transmitted image and casts it onto a screen. With front-projection systems the LCD is placed on the same area of the screen as the viewer, but in rear-projection systems the screen is lit up from behind. Projectors of greater expense and capacity may have three discrete LCD panels, creating separate red, green, and blue images that come together to form a coloured display on the screen.
The growth in demand for video displays has granted a growth in emphasis on the switching speed of liquid crystals. This has demanded the creation of devices using smectic liquid crystals, some types of which give a speedier electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is currently the most developed smectic device. In 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 in the layers the molecules are slanted, as displayed in the figure. The host liquid crystal contains optically active molecules, and a scarcely perceptible turn up 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 in the plane of the layers. Thus, there has to be a permanent charge separation over the liquid crystal layer in the SSFLC, and its sign is directly paired 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 consequential change in optical properties can effect a change from light to dark if one or more polarizers are employed.
SSFLC devices have been marketed for big passive-matrix displays, but their cost and complex detail has hindered them from having any great effect on the market. Small transmissive and reflective active-matrix SSFLC displays, however, display some promise for use as aspects in projection systems or as viewfinders in digital cameras. Their fast responding allows them to be utilised in time-sequential colour systems, in which dear colour filters are replaced by a coloured backlight that flashes red, green, and blue in fast pace (about 100 cycles in a second). For example, the liquid crystal could be switched to a transmissive state during the red and green periods and then to a nontransmissive state during the blue period, with the upshot that the eye sees an average of red and green light, or the colour yellow.
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