The Development of Data Projectors
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The LCDs put in projection systems are generally small reflective or transmissive panels illuminated by a strong arc lamp source. A line of lenses expands the reflected or transmitted image then sends it onto the screen. With front-projection systems the LCD is situated on the side of the screen as the viewer, however in rear-projection systems the screen is lit from behind. Projectors of greater cost and capability may have three discrete LCD panels, reflecting separate red, green, and blue images that mesh to make a coloured display on the screen.
The increase in requirement for pictographic displays has granted a growing emphasis on the switching speed of liquid crystals. This has demanded the manufacture of objects utilizing smectic liquid crystals, some types of which emit a quicker electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is in the current day the most progressive smectic device. In it the liquid crystal molecules are set out in layers perpendicular to the substrate planes, which are separated by one or two micrometres, and within the layers the molecules are on a slant, as illustrated in the figure. The host liquid crystal contains optically active molecules, and a subtle consequence of the optical activity and the angle 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 through the plane of the layers. So, there has to be 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 effect a change from light to dark if or when one or more polarizers are employed.
SSFLC devices have been commercialized for big passive-matrix displays, but their expensiveness and complexity has hindered them from enjoying any great effect on the market. Small transmissive and reflective active-matrix SSFLC displays, however, display some possibility for use as parts in projection systems or as viewfinders in digital cameras. Their quick responding allows them to be employed in time-sequential colour systems, in which dear colour filters are emulated by a coloured backlight that flashes red, green, and blue in quick pace (about 100 cycles per second). For example, the liquid crystal can be switched to a transmissive state during the red and green periods then to a nontransmissive state in the blue period, creating the upshot that the eye sees an average of red and green light, or the colour yellow.
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