The Development of Data Projectors
The LCDs built for projection systems are generally small reflective or transmissive panels lit up by a powerful arc lamp source. A number of lenses expands the reflected or transmitted image and casts it on the screen. In front-projection systems the LCD is located on the same area of the screen as the viewer, but in rear-projection systems the screen is lit from behind. Projectors of higher expense and capacity might be found with three distinct LCD panels, reflecting separate red, green, and blue images that combine to reflect a coloured image on the screen.
The growth in demand for film presentations has placed a particular emphasis on the switching speed of liquid crystals. This has led to the manufacture of devices employing smectic liquid crystals, certain types of which possess a quicker electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this time the most developed smectic device. In it the liquid crystal molecules are managed in layers perpendicular to the substrate planes, which are distanced by one or two micrometres, and in the layers the molecules are on a slant, as displayed 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 appearance of a permanent charge separation, or ferroelectric dipole, analogous 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 is a permanent charge separation through the liquid crystal layer in the SSFLC, and its sign is directly paired 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 therefore reverse the tilt direction of the molecules. The consequential change in optical properties can effect a change from light to dark when one or more polarizers are used.
SSFLC devices have been commercialized for bigger passive-matrix presentations, but their expensiveness and complexity has prevented them from making any significant progress on the market. Small transmissive and reflective active-matrix SSFLC displays, however, display some probability for use as parts in projection systems or as viewfinders in digital cameras. Their immediate response allows them to be employed in time-sequential colour systems, in which expensive colour filters are emulated with a coloured backlight that flashes red, green, and blue in quick pace (about 100 cycles a second). For example, the liquid crystal could be switched to a transmissive state between the red and green periods and to a nontransmissive state during the blue period, with the upshot that the eye sees an average of red and green light, or the colour yellow.
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