The Development of Data Projectors
The LCDs put for projection systems are generally small reflective or transmissive panels lit up by a bright arc lamp source. A number of lenses expands the reflected or transmitted image and then sends it on the screen. In front-projection systems the LCD is placed on the same side of the screen as the viewer, but in rear-projection systems the screen is lit up from behind. Projectors of greater cost and capacity may use three separate LCD panels, reflecting separate red, green, and blue images that blend to create a coloured display on the screen.
The increasing desire for pictographic displays has placed a special emphasis on the switching speed of liquid crystals. This has necessitated the invention of devices utilizing smectic liquid crystals, certain ones of which possess a quicker 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 arranged in perpendicular layers to the substrate planes, which are differentiated by one or two micrometres, and in the layers the molecules are on a slant, as illustrated in the figure. The host liquid crystal contains optically active molecules, and a scarcely perceptible turn up of the optical activity and the tilt 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 through the plane of the layers. So, there must be a permanent charge separation through the liquid crystal layer in the SSFLC, and its sign is directly paired up 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 consequential change in optical properties can create a change from light to dark if one or more polarizers are employed.
SSFLC devices have been produced for bigger passive-matrix displays, but their cost and complex nature has impeded them from enjoying any significant movement on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have shown some probability for use as aspects in projection systems or as viewfinders in digital cameras. Their speedy responding allows them to be used in time-sequential colour systems, in which costly colour filters are emulated by a coloured backlight that flashes red, green, and blue in fast pace (approximately 100 cycles per second). For example, the liquid crystal may be switched to a transmissive state in the red and green periods but then to a nontransmissive state during 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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