The Development of Data Projectors
The LCDs put for projection systems are generally small reflective or transmissive panels lit up by a powerful arc lamp source. A line of lenses expands the reflected or transmitted image and casts it onto the screen. With front-projection systems the LCD is located on the side of the screen as the viewer, while in rear-projection systems the screen is lit up from behind. Projectors of higher cost and capacity sometimes have three distinct LCD panels, reflecting separate red, green, and blue images that combine to reflect a coloured image on the screen.
The growing demand for video presentations has granted a growing emphasis on the switching speed of liquid crystals. This has necessitated the creation of devices using smectic liquid crystals, particular kinds of which give a faster electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is currently the most progressive smectic device. In it the liquid crystal molecules are cast in perpendicular layers to the substrate planes, which are differentiated by one or two micrometres, and within the layers the molecules are slanted, as displayed in the figure. The host liquid crystal has optically active molecules, and a slight consequence of the optical activity and the angle of the molecules is the appearance of a permanent charge separation, or ferroelectric dipole, analogous to the ferromagnetic dipole of a magnet. The direction of this dipole is perpendicular to the tilt direction of the molecules and through the plane of the layers. Hence, 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 correct sign can reverse the direction of this dipole in tens of microseconds and hence reverse the tilt direction of the molecules. The corresponding change in optical properties can create a change from light to dark if one or more polarizers are used.
SSFLC devices have been produced for big passive-matrix presentations, but their cost and complexity has impeded them from making any particular effect on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have shown some possibility for use as parts in projection systems or as viewfinders in digital cameras. Their quick response allows them to be used in time-sequential colour systems, in which expensive colour filters are replaced with a coloured backlight that flashes red, green, and blue in quick pulsing (approx 100 cycles a second). For example, the liquid crystal can be switched to a transmissive state for the red and green periods but to a nontransmissive state during 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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