The Development of Data Projectors
The LCDs built in projection systems are generally small reflective or transmissive panels illuminated by a strong arc lamp source. A series of lenses expands the reflected or transmitted image and then sends it onto a screen. For front-projection systems the LCD is placed on the same side of the screen as the viewer, but in rear-projection systems the screen is set off from behind. Projectors of more expense and capacity sometimes use three separated LCD panels, casting separate red, green, and blue images that mesh to create a coloured image on the screen.
The increasing requirement for visual presentations has placed a growth in emphasis on the switching speed of liquid crystals. This has demanded the development of items build with smectic liquid crystals, certain kinds of which possess a speedier electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this point the most complex smectic device. Inside it the liquid crystal molecules are set out in layers that are perpendicular to the substrate planes, which are distanced by one or two micrometres, and within the layers the molecules are on a tilt, as illustrated in the figure. The host liquid crystal holds optically active molecules, and a minor outcome of the optical activity and the shape 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 in the plane of the layers. Thus, there has to be a permanent charge separation throughout the liquid crystal layer in the SSFLC, and its sign is directly partnered 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 therefore reverse the tilt direction of the molecules. The corresponding change in optical properties can make a change from light to dark if one or more polarizers are used.
SSFLC devices have been marketed for big passive-matrix presentations, but their expensiveness and complexity has prevented them from making any remarkable impact on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have shown some promise for use as elements in projection systems or as viewfinders in digital cameras. Their fast response allows them to be used in time-sequential colour systems, in which dear colour filters are taken out for a coloured backlight that flashes red, green, and blue in fast speed (around 100 cycles a second). For example, the liquid crystal could be switched to a transmissive state for the red and green periods then to a nontransmissive state during the blue period, creating the end result that the eye sees an average of red and green light, or the colour yellow.
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