The LCDs used in projection systems are most often small reflective or transmissive panels set off by a forceful arc lamp source. A series of lenses magnifies the reflected or transmitted image and displays 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 set off from behind. Projectors of greater expense and capacity sometimes utilise three separate LCD panels, reflecting separate red, green, and blue images that come together to reflect a coloured display on the screen.
The growing need for visual displays has put a growth in emphasis on the switching speed of liquid crystals. This has necessitated the manufacture of objects using smectic liquid crystals, particular kinds 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 progressive smectic device. With it the liquid crystal molecules are managed in layers that are perpendicular to the substrate planes, which are differentiated by one or two micrometres, and in the layers the molecules are slanted, as demonstrated in the figure. The host liquid crystal contains optically active molecules, and a scarcely perceptible result 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. So, there has to be a permanent charge separation across 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 in so doing reverse the tilt direction of the molecules. The corresponding change in optical properties can effect a change from light to dark in the case that one or more polarizers are employed.
SSFLC devices have been produced for big passive-matrix displays, but their expensiveness and complex nature has stopped them from creating any particular movement on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have displayed some promise for use as aspects in projection systems or as viewfinders in digital cameras. Their fast reacting allows them to be made use of in time-sequential colour systems, in which costly colour filters are replaced with a coloured backlight that flashes red, green, and blue in quick pulsing (around 100 cycles in a second). For example, the liquid crystal might be switched to a transmissive state for the red and green periods and to a nontransmissive state in the blue period, displaying the result that the eye sees an average of red and green light, or the colour yellow.
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