The Development of Data Projectors
The LCDs used for projection systems are typically small reflective or transmissive panels illuminated by a forceful arc lamp source. A series of lenses enlarges the reflected or transmitted image and then casts it onto the screen. For front-projection systems the LCD is set on the same side of the screen as the viewer, however in rear-projection systems the screen is lit up from behind. Projectors of more expense and performance sometimes be found with three discrete LCD panels, creating separate red, green, and blue images that combine to reflect a coloured display on the screen.
The growing need for video presentations has placed a particular emphasis on the switching speed of liquid crystals. This has demanded the creation of objects using smectic liquid crystals, particular types of which possess a quicker 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. In it the liquid crystal molecules are cast in perpendicular layers to the substrate planes, which are differentiated by one or two micrometres, and inside the layers the molecules are on a slant, as demonstrated in the figure. The host liquid crystal contains optically active molecules, and a scarcely perceptible turn up 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 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 coupled to the tilt direction of the molecules. An applied voltage of the right sign can reverse the direction of this dipole in tens of microseconds and by doing so reverse the tilt direction of the molecules. The resultant change in optical properties can effect a change from light to dark when one or more polarizers are employed.
SSFLC devices have been produced for large passive-matrix displays, but their cost and detail has stopped them from enjoying any significant progress on the market. Small transmissive and reflective active-matrix SSFLC displays, however, show some possibility for use as elements in projection systems or as viewfinders in digital cameras. Their quick reacting allows them to be used in time-sequential colour systems, in which high cost colour filters are replaced by a coloured backlight that flashes red, green, and blue in fast pulsing (approximately 100 cycles a second). For example, the liquid crystal could be switched to a transmissive state in the red and green periods and then to a nontransmissive state during the blue period, with the end result that the eye sees an average of red and green light, or the colour yellow.
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