30 Jun 10 The Development of Data Projectors
The LCDs built for projection systems are generally small reflective or transmissive panels lit by a strong arc lamp source. A line of lenses magnifies the reflected or transmitted image and sends it onto a screen. With front-projection systems the LCD is placed on the same area of the screen as the viewer, however in rear-projection systems the screen is illuminated from behind. Projectors of higher cost and capacity can utilise three separated LCD panels, reflecting separate red, green, and blue images that blend to make a coloured display on the screen.
The growing requirement for pictographic displays has put a special emphasis on the switching speed of liquid crystals. This has required the manufacture of items utilizing smectic liquid crystals, certain kinds of which give a speedier electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this point the most developed smectic device. With 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 in the layers the molecules are slanted, as demonstrated in the figure. The host liquid crystal holds optically active molecules, and a minor consequence of the optical activity and the shape 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 throughout the plane of the layers. Hence, there has to be a permanent charge separation across 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 right sign can reverse the direction of this dipole in tens of microseconds and hence reverse the tilt direction of the molecules. The corresponding change in optical properties can effect a change from light to dark in the case that one or more polarizers are employed.
SSFLC devices have been produced for larger passive-matrix presentations, but their cost and complex detail has prevented them from making any remarkable progress on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have displayed some probability for use as elements in projection systems or as viewfinders in digital cameras. Their quick response allows them to be employed in time-sequential colour systems, in which expensive colour filters are emulated by a coloured backlight that flashes red, green, and blue in rapid pace (about 100 cycles per second). For example, the liquid crystal may be switched to a transmissive state for the red and green periods and to a nontransmissive state for the blue period, having the outcome that the eye sees an average of red and green light, or the colour yellow.
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