The Development of Data Projectors
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The LCDs put in projection systems are generally small reflective or transmissive panels lit up by a forceful arc lamp source. A number of lenses enlarges the reflected or transmitted image then sends it on a screen. With front-projection systems the LCD is set on the side of the screen as the viewer, although in rear-projection systems the screen is illuminated from behind. Projectors of more expense and capability sometimes be found with three separate LCD panels, creating separate red, green, and blue images that combine to form a coloured display on the screen.
The growing need for video displays has granted a growing emphasis on the switching speed of liquid crystals. This has demanded the development of items build with smectic liquid crystals, some of which have a faster electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this point the most developed smectic device. Inside 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 within the layers the molecules are on a tilt, as shown in the figure. The host liquid crystal holds optically active molecules, and a scarcely perceptible 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 within the plane of the layers. Therefore, there has to be 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 so reverse the tilt direction of the molecules. The resultant change in optical properties can make a change from light to dark if or when one or more polarizers are utilised.
SSFLC devices have been produced for larger passive-matrix presentations, but their expense and intricacy has hindered them from having any significant impact on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have shown some possibility for use as parts in projection systems or as viewfinders in digital cameras. Their speedy responding allows them to be used in time-sequential colour systems, in which expensive colour filters are taken out for a coloured backlight that flashes red, green, and blue in quick pulsing (around 100 cycles per second). For example, the liquid crystal could be switched to a transmissive state for the red and green periods and to a nontransmissive state for the blue period, displaying the end result that the eye sees an average of red and green light, or the colour yellow.
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