Trivia Galaxy

The LCDs put in projection systems are generally small reflective or transmissive panels illuminated by a bright arc lamp source. A series of lenses enlarges the reflected or transmitted image and displays it on a screen. For front-projection systems the LCD is located on the same side of the screen as the viewer, while in rear-projection systems the screen is lit from behind. Projectors of greater expense and capability can have three separated LCD panels, reflecting separate red, green, and blue images that blend to reflect a coloured image on the screen.

The increasing desire for visual displays has had a special emphasis on the switching speed of liquid crystals. This has led to the creation of devices using smectic liquid crystals, some 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 developed smectic device. Inside it the liquid crystal molecules are arranged in perpendicular layers 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 possesses 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 within 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 coupled 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 therefore 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 used.

SSFLC devices have been commercialized for large passive-matrix displays, but their high cost and complexity has prevented them from enjoying any great movement on the market. Small transmissive and reflective active-matrix SSFLC displays, however, display some promise for use as parts in projection systems or as viewfinders in digital cameras. Their speedy response allows them to be employed in time-sequential colour systems, in which costly colour filters are emulated with a coloured backlight that flashes red, green, and blue in fast succession (about 100 cycles a second). For example, the liquid crystal can be switched to a transmissive state in the red and green periods then to a nontransmissive state for 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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