The Development of Data Projectors
The LCDs put in projection systems are usually small reflective or transmissive panels set off by a strong arc lamp source. A number of lenses magnifies the reflected or transmitted image then displays it on the screen. For front-projection systems the LCD is placed 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 capability might use three distinct LCD panels, creating separate red, green, and blue images that mesh to make a coloured picture on the screen.
The increase in requirement for film displays has put a particular emphasis on the switching speed of liquid crystals. This has led to the creation of items build with 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 at this point the most progressive smectic device. In it the liquid crystal molecules are managed in perpendicular layers to the substrate planes, which are distanced by one or two micrometres, and throughout the layers the molecules are on a tilt, as demonstrated in the figure. The host liquid crystal holds optically active molecules, and a minor result of the optical activity and the shape of the molecules is the presence 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 throughout 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 attracted 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 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 produced for large passive-matrix presentations, but their cost and complexity has impeded them from having any particular movement on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have displayed some promise for use as elements in projection systems or as viewfinders in digital cameras. Their fast response allows them to be employed in time-sequential colour systems, in which high cost colour filters are removed for a coloured backlight that flashes red, green, and blue in fast pace (approx 100 cycles a second). For example, the liquid crystal might be switched to a transmissive state during the red and green periods and then to a nontransmissive state in 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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