The Development of Data Projectors
The LCDs built in projection systems are usually small reflective or transmissive panels lit by a powerful arc lamp source. A number of lenses magnifies the reflected or transmitted image and sends it onto the screen. For front-projection systems the LCD is situated on the same area of the screen as the viewer, but in rear-projection systems the screen is illuminated from behind. Projectors of greater cost and performance can utilise three separate LCD panels, creating separate red, green, and blue images that come together to make a coloured image on the screen.
The increase in demand for pictographic displays has put a special emphasis on the switching speed of liquid crystals. This has led to the creation of items using smectic liquid crystals, some of which emit a better electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is currently the most complex smectic device. Within it the liquid crystal molecules are cast in layers that are perpendicular to the substrate planes, which are differentiated by one or two micrometres, and inside the layers the molecules are tilted, as shown in the figure. The host liquid crystal has optically active molecules, and a subtle 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 through the plane of the layers. Therefore, there exists a permanent charge separation throughout 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 right sign can reverse the direction of this dipole in tens of microseconds and hence reverse the tilt direction of the molecules. The respective change in optical properties can cause a change from light to dark if one or more polarizers are employed.
SSFLC devices have been publicized for big passive-matrix presentations, but their expense and complex detail has prevented them from enjoying any remarkable impact on the market. Small transmissive and reflective active-matrix SSFLC displays, however, show some promise for use as aspects in projection systems or as viewfinders in digital cameras. Their fast response allows them to be utilised in time-sequential colour systems, in which high cost colour filters are emulated by a coloured backlight that flashes red, green, and blue in rapid succession (around 100 cycles a second). For example, the liquid crystal may be switched to a transmissive state in the red and green periods but to a nontransmissive state for the blue period, displaying the outcome that the eye sees an average of red and green light, or the colour yellow.
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