The LCDs built for projection systems are typically small reflective or transmissive panels illuminated by a powerful arc lamp source. A series of lenses enlarges the reflected or transmitted image and then sends it onto a screen. With 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 set off from behind. Projectors of greater cost and performance sometimes utilise three separated LCD panels, casting separate red, green, and blue images that combine to reflect a coloured display on the screen.
The increase in desire for visual presentations has put a growth in emphasis on the switching speed of liquid crystals. This has necessitated the creation of items employing smectic liquid crystals, certain ones of which emit a speedier electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is in the current day the most developed smectic device. Within it the liquid crystal molecules are set out in perpendicular layers to the substrate planes, which are distanced by one or two micrometres, and within the layers the molecules are tilted, as shown in the figure. The host liquid crystal holds optically active molecules, and a slight outcome of the optical activity and the shape of the molecules is the appearance 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 throughout the plane of the layers. Therefore, there exists a permanent charge separation across 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 corresponding change in optical properties can cause a change from light to dark when one or more polarizers are employed.
SSFLC devices have been commercialized for big passive-matrix presentations, but their expense and complexity has prevented them from enjoying any remarkable impact on the market. Small transmissive and reflective active-matrix SSFLC displays, however, show some probability for use as elements in projection systems or as viewfinders in digital cameras. Their speedy reacting allows them to be used in time-sequential colour systems, in which costly colour filters are replaced by a coloured backlight that flashes red, green, and blue in quick succession (approximately 100 cycles in a second). For example, the liquid crystal could be switched to a transmissive state between the red and green periods but then to a nontransmissive state for the blue period, with the result that the eye sees an average of red and green light, or the colour yellow.
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