The Development of Data Projectors
The LCDs used in projection systems are usually small reflective or transmissive panels set off by a strong arc lamp source. A series of lenses enlarges the reflected or transmitted image and casts it onto a screen. In 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 illuminated from behind. Projectors of greater expense and capacity might use three separated LCD panels, casting separate red, green, and blue images that combine to make a coloured image on the screen.
The increasing demand for pictographic displays has granted a growth in emphasis on the switching speed of liquid crystals. This has demanded the invention of devices utilizing smectic liquid crystals, particular 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 sophisticated smectic device. Within it the liquid crystal molecules are managed in layers perpendicular to the substrate planes, which are distanced by one or two micrometres, and within the layers the molecules are on a slant, as displayed in the figure. The host liquid crystal has optically active molecules, and a scarcely perceptible turn up of the optical activity and the shape 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. So, there exists a permanent charge separation through 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 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 corresponding change in optical properties can create a change from light to dark if or when one or more polarizers are utilised.
SSFLC devices have been commercialized for large passive-matrix presentations, but their cost and intricacy has prevented them from having any particular progress 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 speedy responding allows them to be made use of in time-sequential colour systems, in which expensive colour filters are removed for a coloured backlight that flashes red, green, and blue in fast pulsing (approx 100 cycles every second). For example, the liquid crystal may be switched to a transmissive state for the red and green periods but 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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