The Development of Data Projectors
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The LCDs utilised for projection systems are generally small reflective or transmissive panels set off by a forceful arc lamp source. A series of lenses expands the reflected or transmitted image then sends it on the screen. For front-projection systems the LCD is located on the same side of the screen as the viewer, although in rear-projection systems the screen is lit up from behind. Projectors of more expense and performance sometimes have three discrete LCD panels, creating separate red, green, and blue images that blend to form a coloured display on the screen.
The increase in demand for visual presentations has put a special emphasis on the switching speed of liquid crystals. This has required the manufacture of devices employing smectic liquid crystals, some kinds of which have a faster electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is in the current day the most sophisticated smectic device. In 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 tilt, as displayed in the figure. The host liquid crystal holds optically active molecules, and a subtle result of the optical activity and the shape of the molecules is the appearance 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 within the plane of the layers. Thus, there is a permanent charge separation through the liquid crystal layer in the SSFLC, and its sign is directly partnered 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 if one or more polarizers are employed.
SSFLC devices have been marketed for large passive-matrix presentations, but their expense and complexity has hindered them from enjoying any remarkable impact on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have shown some possibility for use as elements in projection systems or as viewfinders in digital cameras. Their speedy responding allows them to be utilised in time-sequential colour systems, in which expensive colour filters are emulated with a coloured backlight that flashes red, green, and blue in fast pace (approximately 100 cycles a second). For example, the liquid crystal may be switched to a transmissive state in the red and green periods but to a nontransmissive state in the blue period, creating the result that the eye sees an average of red and green light, or the colour yellow.
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