The Development of Data Projectors
Thursday, July 29th, 2010The LCDs built in projection systems are most often small reflective or transmissive panels lit up by a strong arc lamp source. A number of lenses magnifies the reflected or transmitted image and displays it onto the screen. For front-projection systems the LCD is set on the same side of the screen as the viewer, although in rear-projection systems the screen is illuminated from behind. Projectors of higher cost and capacity might use three separate LCD panels, reflecting separate red, green, and blue images that mesh to reflect a coloured picture on the screen.
The increasing need for pictographic presentations has granted a special emphasis on the switching speed of liquid crystals. This has led to the invention of items employing smectic liquid crystals, certain ones of which give a better electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this time the most progressive smectic device. Within it the liquid crystal molecules are set out in layers perpendicular to the substrate planes, which are distanced by one or two micrometres, and throughout the layers the molecules are tilted, as shown in the figure. The host liquid crystal has optically active molecules, and a subtle turn up of the optical activity and the angle of the molecules is the presence of a permanent charge separation, or ferroelectric dipole, likeable 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. Therefore, there has to be a permanent charge separation through 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 by doing so reverse the tilt direction of the molecules. The consequential change in optical properties can make a change from light to dark if or when one or more polarizers are used.
SSFLC devices have been publicized for large passive-matrix presentations, but their expense and complex detail has prevented them from enjoying any great impact on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have some possibility for use as parts in projection systems or as viewfinders in digital cameras. Their speedy response allows them to be made use of in time-sequential colour systems, in which costly colour filters are replaced with a coloured backlight that flashes red, green, and blue in fast speed (around 100 cycles a second). For example, the liquid crystal may be switched to a transmissive state during the red and green periods but to a nontransmissive state for the blue period, with the outcome that the eye sees an average of red and green light, or the colour yellow.
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