Roger Out

The LCDs used for projection systems are generally small reflective or transmissive panels illuminated by a forceful arc lamp source. A series of lenses expands the reflected or transmitted image and then sends it onto the 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 from behind. Projectors of higher expense and performance can have three discrete LCD panels, creating separate red, green, and blue images that mesh to make a coloured picture on the screen.

The growing demand for visual displays has granted a growth in emphasis on the switching speed of liquid crystals. This has required the creation of objects employing 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 at this time the most progressive smectic device. With it the liquid crystal molecules are arranged in layers that are perpendicular to the substrate planes, which are separated by one or two micrometres, and in the layers the molecules are tilted, as shown in the figure. The host liquid crystal holds optically active molecules, and a slight consequence of the optical activity and the slant 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 throughout the plane of the layers. Therefore, there must be a permanent charge separation over 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 resultant change in optical properties can make a change from light to dark when one or more polarizers are employed.

SSFLC devices have been marketed for bigger passive-matrix presentations, but their cost and complexity has hindered them from enjoying any significant progress on the market. Small transmissive and reflective active-matrix SSFLC displays, however, display some promise for use as elements in projection systems or as viewfinders in digital cameras. Their speedy reacting allows them to be employed in time-sequential colour systems, in which high cost colour filters are emulated with a coloured backlight that flashes red, green, and blue in quick speed (approximately 100 cycles every second). For example, the liquid crystal can be switched to a transmissive state during the red and green periods and to a nontransmissive state in the blue period, having the result that the eye sees an average of red and green light, or the colour yellow.

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