The Development of Data Projectors
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The LCDs used for projection systems are usually small reflective or transmissive panels illuminated by a powerful arc lamp source. A number of lenses magnifies the reflected or transmitted image and displays it onto a screen. For front-projection systems the LCD is set on the same side of the screen as the viewer, however in rear-projection systems the screen is set off from behind. Projectors of more expense and performance sometimes have three separated LCD panels, reflecting separate red, green, and blue images that combine to create a coloured display on the screen.
The increasing need for film displays has granted a growing emphasis on the switching speed of liquid crystals. This has necessitated the creation of objects utilizing smectic liquid crystals, some types of which emit a faster electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this time the most complex smectic device. Within it the liquid crystal molecules are arranged in layers perpendicular to the substrate planes, which are separated by one or two micrometres, and throughout the layers the molecules are tilted, as displayed in the figure. The host liquid crystal possesses optically active molecules, and a slight result of the optical activity and the tilt 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 in the plane of the layers. Therefore, there has to be a permanent charge separation throughout the liquid crystal layer in the SSFLC, and its sign is directly paired up 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 hence reverse the tilt direction of the molecules. The resultant change in optical properties can cause a change from light to dark if or when one or more polarizers are used.
SSFLC devices have been marketed for large passive-matrix displays, but their high cost and intricacy has stopped them from creating any remarkable progress on the market. Small transmissive and reflective active-matrix SSFLC displays, however, show some probability for use as aspects in projection systems or as viewfinders in digital cameras. Their speedy reacting 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 quick pulsing (about 100 cycles a second). For example, the liquid crystal may be switched to a transmissive state during the red and green periods but then to a nontransmissive state during 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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