The Development of Data Projectors
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The LCDs built for projection systems are most often small reflective or transmissive panels lit by a powerful arc lamp source. A line of lenses magnifies the reflected or transmitted image and displays it on the screen. For front-projection systems the LCD is situated on the same area of the screen as the viewer, although in rear-projection systems the screen is set off from behind. Projectors of greater expense and capacity might utilise three separate LCD panels, forming separate red, green, and blue images that mesh to form a coloured image on the screen.
The increasing desire for video displays has put a special emphasis on the switching speed of liquid crystals. This has led to the invention of devices using smectic liquid crystals, particular types of which possess a faster electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is currently the most sophisticated smectic device. With it the liquid crystal molecules are set out in perpendicular layers to the substrate planes, which are separated by one or two micrometres, and inside the layers the molecules are slanted, as demonstrated in the figure. The host liquid crystal has optically active molecules, and a slight outcome of the optical activity and the tilt of the molecules is the presence 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 throughout the plane of the layers. So, there is a permanent charge separation over 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 hence reverse the tilt direction of the molecules. The respective change in optical properties can cause a change from light to dark if or when one or more polarizers are used.
SSFLC devices have been publicized for large passive-matrix displays, but their expensiveness and intricacy has impeded them from making any remarkable impact on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have shown some promise for use as aspects in projection systems or as viewfinders in digital cameras. Their fast reaction allows them to be made use of in time-sequential colour systems, in which high cost colour filters are replaced with a coloured backlight that flashes red, green, and blue in rapid succession (approximately 100 cycles every second). For example, the liquid crystal may be switched to a transmissive state in the red and green periods and to a nontransmissive state in the blue period, having the end result that the eye sees an average of red and green light, or the colour yellow.
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