The Development of Data Projectors
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The LCDs put for projection systems are generally small reflective or transmissive panels lit up by a bright arc lamp source. A line of lenses enlarges the reflected or transmitted image then displays it on the screen. In front-projection systems the LCD is situated on the same side of the screen as the viewer, however in rear-projection systems the screen is set off from behind. Projectors of greater expense and capability might be found with three distinct LCD panels, casting separate red, green, and blue images that come together to form a coloured display on the screen.
The increase in requirement for video displays has had a growth in emphasis on the switching speed of liquid crystals. This has demanded the invention of items using smectic liquid crystals, certain ones of which have a better 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 set out in perpendicular layers to the substrate planes, which are separated by one or two micrometres, and throughout the layers the molecules are tilted, as illustrated in the figure. The host liquid crystal holds optically active molecules, and a minor result of the optical activity and the angle of the molecules is the presence of a permanent charge separation, or ferroelectric dipole, analogous 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. Thus, there exists 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 corresponding sign can reverse the direction of this dipole in tens of microseconds and so reverse the tilt direction of the molecules. The corresponding change in optical properties can create a change from light to dark if one or more polarizers are utilised.
SSFLC devices have been commercialized for larger passive-matrix presentations, but their expense and intricacy has prevented them from enjoying any great movement on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have some promise for use as aspects in projection systems or as viewfinders in digital cameras. Their fast response allows them to be used in time-sequential colour systems, in which highly expensive colour filters are taken out for a coloured backlight that flashes red, green, and blue in rapid succession (approximately 100 cycles every second). For example, the liquid crystal can be switched to a transmissive state for the red and green periods but to a nontransmissive state for the blue period, creating the outcome that the eye sees an average of red and green light, or the colour yellow.
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