The Development of Data Projectors
The LCDs used for projection systems are usually small reflective or transmissive panels illuminated by a bright arc lamp source. A number of lenses magnifies the reflected or transmitted image then sends it on the screen. In front-projection systems the LCD is located on the same side of the screen as the viewer, but in rear-projection systems the screen is set off from behind. Projectors of higher expense and capability may be found with three discrete LCD panels, forming separate red, green, and blue images that combine to form a coloured image on the screen.
The growing desire for visual displays has placed a growing emphasis on the switching speed of liquid crystals. This has necessitated the creation of items utilizing smectic liquid crystals, certain types of which have a better electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this point the most complex smectic device. In it the liquid crystal molecules are cast in perpendicular layers to the substrate planes, which are separated by one or two micrometres, and inside the layers the molecules are slanted, as displayed in the figure. The host liquid crystal has optically active molecules, and a subtle result 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 in the plane of the layers. So, there has to be a permanent charge separation throughout 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 correct sign can reverse the direction of this dipole in tens of microseconds and in so doing reverse the tilt direction of the molecules. The consequential change in optical properties can effect a change from light to dark in the case that one or more polarizers are used.
SSFLC devices have been publicized for bigger passive-matrix displays, but their expense and complex nature has impeded them from having any remarkable effect on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have displayed some promise for use as parts in projection systems or as viewfinders in digital cameras. Their immediate reacting allows them to be utilised in time-sequential colour systems, in which high cost colour filters are replaced by a coloured backlight that flashes red, green, and blue in fast speed (about 100 cycles every second). For example, the liquid crystal might be switched to a transmissive state between the red and green periods but to a nontransmissive state during the blue period, having the result that the eye sees an average of red and green light, or the colour yellow.
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