The Development of Data Projectors
The LCDs utilised in projection systems are most often small reflective or transmissive panels illuminated by a bright arc lamp source. A number of lenses expands the reflected or transmitted image and then sends it onto a screen. For front-projection systems the LCD is located on the same area of the screen as the viewer, although in rear-projection systems the screen is lit up from behind. Projectors of higher cost and performance might be found with three separate LCD panels, forming separate red, green, and blue images that combine to reflect a coloured picture on the screen.
The growing demand for pictographic displays has had a particular emphasis on the switching speed of liquid crystals. This has required the invention of objects utilizing smectic liquid crystals, some kinds of which possess a better 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 arranged in layers perpendicular to the substrate planes, which are differentiated by one or two micrometres, and inside the layers the molecules are slanted, as illustrated in the figure. The host liquid crystal contains optically active molecules, and a slight turn up of the optical activity and the shape of the molecules is the presence 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 throughout the plane of the layers. Therefore, there has to be a permanent charge separation over the liquid crystal layer in the SSFLC, and its sign is directly attracted to the tilt direction of the molecules. An applied voltage of the corresponding sign can reverse the direction of this dipole in tens of microseconds and hence reverse the tilt direction of the molecules. The consequential change in optical properties can create a change from light to dark if one or more polarizers are used.
SSFLC devices have been publicized for larger passive-matrix presentations, but their high cost and intricacy has prevented them from enjoying any particular progress on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have displayed some possibility for use as elements in projection systems or as viewfinders in digital cameras. Their immediate reaction allows them to be made use of in time-sequential colour systems, in which dear colour filters are emulated with a coloured backlight that flashes red, green, and blue in quick succession (approx 100 cycles a second). For example, the liquid crystal can be switched to a transmissive state in the red and green periods and then to a nontransmissive state during 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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