The Development of Data Projectors
The LCDs put in projection systems are most often small reflective or transmissive panels set off by a bright arc lamp source. A series of lenses enlarges the reflected or transmitted image and then casts it on a screen. In front-projection systems the LCD is situated on the same area of the screen as the viewer, however in rear-projection systems the screen is set off from behind. Projectors of greater expense and capacity sometimes have three separate LCD panels, reflecting separate red, green, and blue images that combine to create a coloured display on the screen.
The increasing demand for visual displays has had a growing emphasis on the switching speed of liquid crystals. This has required the manufacture of items using smectic liquid crystals, certain types of which give a speedier electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this point the most developed smectic device. Within it the liquid crystal molecules are set out in layers that are perpendicular to the substrate planes, which are separated by one or two micrometres, and throughout the layers the molecules are on a tilt, as displayed in the figure. The host liquid crystal has optically active molecules, and a subtle turn up of the optical activity and the shape of the molecules is the presence 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 within the plane of the layers. Hence, there has to be a permanent charge separation across 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 respective change in optical properties can cause a change from light to dark if one or more polarizers are employed.
SSFLC devices have been produced for big passive-matrix presentations, but their high cost and detail has hindered them from creating any great progress on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have shown some promise for use as parts in projection systems or as viewfinders in digital cameras. Their fast reacting allows them to be made use of in time-sequential colour systems, in which high cost colour filters are replaced by a coloured backlight that flashes red, green, and blue in quick pulsing (around 100 cycles a second). For example, the liquid crystal could be switched to a transmissive state during the red and green periods but then to a nontransmissive state for the blue period, having the upshot that the eye sees an average of red and green light, or the colour yellow.
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