The LCDs put in projection systems are typically small reflective or transmissive panels illuminated by a strong arc lamp source. A number of lenses enlarges the reflected or transmitted image and sends it on a screen. In front-projection systems the LCD is set on the same side of the screen as the viewer, although in rear-projection systems the screen is illuminated from behind. Projectors of more expense and performance might use three separate LCD panels, creating separate red, green, and blue images that come together to form a coloured picture on the screen.
The increasing need for visual displays has put a growth in emphasis on the switching speed of liquid crystals. This has required the manufacture of items employing smectic liquid crystals, certain types of which have a speedier electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this point the most sophisticated smectic device. With it the liquid crystal molecules are arranged in layers perpendicular to the substrate planes, which are differentiated by one or two micrometres, and inside the layers the molecules are on a tilt, as demonstrated in the figure. The host liquid crystal has optically active molecules, and a subtle consequence of the optical activity and the tilt of the molecules is the appearance of a permanent charge separation, or ferroelectric dipole, likeable 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. Hence, there must be a permanent charge separation over 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 right sign can reverse the direction of this dipole in tens of microseconds and so reverse the tilt direction of the molecules. The respective change in optical properties can make a change from light to dark if or when one or more polarizers are used.
SSFLC devices have been produced for larger passive-matrix presentations, but their cost and complex nature has impeded them from making 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 quick reacting allows them to be employed in time-sequential colour systems, in which high cost colour filters are emulated with a coloured backlight that flashes red, green, and blue in fast pace (about 100 cycles per second). For example, the liquid crystal can be switched to a transmissive state in 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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