The LCDs put for projection systems are generally small reflective or transmissive panels illuminated by a powerful arc lamp source. A line of lenses enlarges the reflected or transmitted image and sends it onto a screen. With front-projection systems the LCD is set on the same side of the screen as the viewer, while in rear-projection systems the screen is illuminated from behind. Projectors of greater cost and performance might use three distinct LCD panels, casting separate red, green, and blue images that mesh to create a coloured display on the screen.
The growth in requirement for visual displays has granted a particular emphasis on the switching speed of liquid crystals. This has demanded the invention of items employing 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 currently the most sophisticated smectic device. Within it the liquid crystal molecules are arranged in perpendicular layers to the substrate planes, which are differentiated 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 appearance 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. Hence, there is a permanent charge separation across the liquid crystal layer in the SSFLC, and its sign is directly paired 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 in so doing reverse the tilt direction of the molecules. The resultant change in optical properties can cause a change from light to dark when one or more polarizers are employed.
SSFLC devices have been produced for bigger passive-matrix presentations, but their high cost and detail has impeded them from enjoying any remarkable progress on the market. Small transmissive and reflective active-matrix SSFLC displays, however, display some probability for use as parts in projection systems or as viewfinders in digital cameras. Their quick response allows them to be made use of in time-sequential colour systems, in which costly colour filters are emulated with a coloured backlight that flashes red, green, and blue in quick succession (approx 100 cycles in a second). For example, the liquid crystal can be switched to a transmissive state between the red and green periods but then to a nontransmissive state for the blue period, with the upshot that the eye sees an average of red and green light, or the colour yellow.
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