The Development of Data Projectors
The LCDs built in projection systems are most often small reflective or transmissive panels set off by a strong arc lamp source. A number of lenses expands the reflected or transmitted image and then casts it onto the screen. In front-projection systems the LCD is set on the same side of the screen as the viewer, while in rear-projection systems the screen is set off from behind. Projectors of greater cost and capacity can utilise three discrete LCD panels, creating separate red, green, and blue images that mesh to form a coloured display on the screen.
The growing need for visual displays has had a particular emphasis on the switching speed of liquid crystals. This has led to the manufacture of objects utilizing smectic liquid crystals, certain kinds of which have a faster electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this time the most progressive smectic device. Within it the liquid crystal molecules are set out in perpendicular layers to the substrate planes, which are separated by one or two micrometres, and within the layers the molecules are on a slant, as demonstrated in the figure. The host liquid crystal contains optically active molecules, and a subtle outcome of the optical activity and the angle 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 through the plane of the layers. Hence, there has to be a permanent charge separation across 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 corresponding 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 if or when one or more polarizers are utilised.
SSFLC devices have been produced for bigger passive-matrix presentations, but their high cost and intricacy has hindered them from making 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 quick responding allows them to be employed in time-sequential colour systems, in which costly colour filters are emulated by a coloured backlight that flashes red, green, and blue in fast pulsing (approximately 100 cycles every second). For example, the liquid crystal can be switched to a transmissive state in the red and green periods but to a nontransmissive state during the blue period, displaying the end result that the eye sees an average of red and green light, or the colour yellow.
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