The LCDs utilised in projection systems are generally small reflective or transmissive panels lit up by a bright arc lamp source. A line of lenses magnifies the reflected or transmitted image and then displays it onto a screen. For front-projection systems the LCD is placed on the side of the screen as the viewer, while in rear-projection systems the screen is lit up from behind. Projectors of greater cost and capacity might use three discrete LCD panels, casting separate red, green, and blue images that combine to create a coloured image on the screen.
The increase in desire for pictographic presentations has granted a growth in emphasis on the switching speed of liquid crystals. This has necessitated the creation of items employing smectic liquid crystals, particular types of which give a quicker electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is currently the most complex smectic device. Inside it the liquid crystal molecules are cast in perpendicular layers to the substrate planes, which are distanced by one or two micrometres, and in the layers the molecules are on a slant, as shown in the figure. The host liquid crystal contains optically active molecules, and a minor outcome of the optical activity and the tilt of the molecules is the presence 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 in the plane of the layers. So, there is a permanent charge separation throughout 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 therefore reverse the tilt direction of the molecules. The consequential change in optical properties can effect a change from light to dark if or when one or more polarizers are employed.
SSFLC devices have been publicized for bigger passive-matrix presentations, but their expensiveness and complex detail has stopped them from making any significant effect on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have displayed some possibility for use as elements in projection systems or as viewfinders in digital cameras. Their immediate response allows them to be used in time-sequential colour systems, in which highly expensive colour filters are removed for a coloured backlight that flashes red, green, and blue in quick succession (approx 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 in the blue period, displaying the end result that the eye sees an average of red and green light, or the colour yellow.
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