The Development of Data Projectors
The LCDs used in projection systems are generally small reflective or transmissive panels lit by a forceful arc lamp source. A series of lenses enlarges the reflected or transmitted image and then sends it onto a screen. For front-projection systems the LCD is situated on the side of the screen as the viewer, however in rear-projection systems the screen is lit from behind. Projectors of higher expense and capacity may have three discrete LCD panels, forming separate red, green, and blue images that mesh to create a coloured image on the screen.
The increasing requirement for video presentations has placed a growth in emphasis on the switching speed of liquid crystals. This has required the manufacture of devices employing smectic liquid crystals, certain types of which emit a quicker electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is in the current day the most developed smectic device. In it the liquid crystal molecules are cast in perpendicular layers to the substrate planes, which are distanced by one or two micrometres, and within the layers the molecules are on a tilt, as illustrated in the figure. The host liquid crystal possesses optically active molecules, and a subtle outcome of the optical activity and the shape of the molecules is the appearance of a permanent charge separation, or ferroelectric dipole, likeable 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. Thus, there must be a permanent charge separation through 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 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 in the case that one or more polarizers are used.
SSFLC devices have been marketed for bigger passive-matrix presentations, but their expense and detail has impeded them from creating any significant effect 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 fast responding allows them to be utilised in time-sequential colour systems, in which highly expensive 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 could be switched to a transmissive state during the red and green periods and 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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