The LCDs utilised in projection systems are generally small reflective or transmissive panels set off by a forceful arc lamp source. A line of lenses enlarges the reflected or transmitted image and then displays it on the screen. With front-projection systems the LCD is situated on the same area of the screen as the viewer, although in rear-projection systems the screen is illuminated from behind. Projectors of higher cost and capacity can have three separate LCD panels, creating separate red, green, and blue images that mesh to reflect a coloured display on the screen.
The increase in requirement for film presentations has had a particular emphasis on the switching speed of liquid crystals. This has led to the development of devices using smectic liquid crystals, certain types of which emit a faster 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 managed in layers perpendicular to the substrate planes, which are differentiated by one or two micrometres, and inside the layers the molecules are tilted, as shown in the figure. The host liquid crystal holds optically active molecules, and a minor consequence of the optical activity and the shape of the molecules is the appearance of a permanent charge separation, or ferroelectric dipole, comparable 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. Thus, there is 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 correct sign can reverse the direction of this dipole in tens of microseconds and by doing so reverse the tilt direction of the molecules. The corresponding 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 large passive-matrix displays, but their expensiveness and intricacy has prevented them from creating any significant effect on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have shown some probability for use as aspects in projection systems or as viewfinders in digital cameras. Their immediate response allows them to be used in time-sequential colour systems, in which costly colour filters are replaced with a coloured backlight that flashes red, green, and blue in rapid 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 but 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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