The Development of Data Projectors
The LCDs built in projection systems are generally small reflective or transmissive panels illuminated by a forceful arc lamp source. A series of lenses enlarges the reflected or transmitted image and then sends it onto a screen. In front-projection systems the LCD is located on the side of the screen as the viewer, but in rear-projection systems the screen is set off from behind. Projectors of higher cost and performance can use three discrete LCD panels, creating separate red, green, and blue images that mesh to create a coloured picture on the screen.
The increasing demand for film presentations has had a growth in emphasis on the switching speed of liquid crystals. This has required the manufacture of items build with smectic liquid crystals, particular types of which have a better 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 managed in layers that are perpendicular to the substrate planes, which are distanced by one or two micrometres, and in the layers the molecules are on a tilt, as shown in the figure. The host liquid crystal contains optically active molecules, and a minor consequence of the optical activity and the slant 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 through 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 coupled to the tilt direction of the molecules. An applied voltage of the correct sign can reverse the direction of this dipole in tens of microseconds and hence reverse the tilt direction of the molecules. The corresponding change in optical properties can create a change from light to dark when one or more polarizers are used.
SSFLC devices have been publicized for larger passive-matrix presentations, but their expense and intricacy has stopped them from enjoying any particular movement 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 speedy responding allows them to be utilised in time-sequential colour systems, in which expensive colour filters are replaced by a coloured backlight that flashes red, green, and blue in fast speed (about 100 cycles in a second). For example, the liquid crystal may be switched to a transmissive state between the red and green periods then to a nontransmissive state in 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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