The Development of Data Projectors
The LCDs utilised for projection systems are most often small reflective or transmissive panels illuminated by a strong arc lamp source. A number of lenses expands the reflected or transmitted image and sends it on a screen. For front-projection systems the LCD is situated on the side of the screen as the viewer, while in rear-projection systems the screen is illuminated from behind. Projectors of greater cost and capacity can have three separate LCD panels, creating separate red, green, and blue images that blend to reflect a coloured display on the screen.
The increase in demand for visual displays has put a growing emphasis on the switching speed of liquid crystals. This has necessitated the manufacture of items employing 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 time the most progressive smectic device. With it the liquid crystal molecules are set out in layers perpendicular to the substrate planes, which are separated by one or two micrometres, and within the layers the molecules are on a slant, as shown in the figure. The host liquid crystal contains optically active molecules, and a scarcely perceptible outcome of the optical activity and the tilt of the molecules is the presence 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 through the plane of the layers. Therefore, 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 by doing so reverse the tilt direction of the molecules. The consequential change in optical properties can create a change from light to dark if one or more polarizers are utilised.
SSFLC devices have been produced for bigger passive-matrix displays, but their high cost and complex detail has prevented them from enjoying any remarkable impact on the market. Small transmissive and reflective active-matrix SSFLC displays, however, display some possibility for use as aspects in projection systems or as viewfinders in digital cameras. Their immediate reacting allows them to be utilised in time-sequential colour systems, in which dear colour filters are replaced by a coloured backlight that flashes red, green, and blue in quick speed (about 100 cycles every second). For example, the liquid crystal might be switched to a transmissive state for the red and green periods then to a nontransmissive state during 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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