The Development of Data Projectors
The LCDs put for projection systems are most often small reflective or transmissive panels lit up by a powerful arc lamp source. A number of lenses enlarges the reflected or transmitted image and sends it onto a screen. In front-projection systems the LCD is located on the same side of the screen as the viewer, but in rear-projection systems the screen is set off from behind. Projectors of higher cost and capability might utilise three separate LCD panels, forming separate red, green, and blue images that blend to make a coloured image on the screen.
The increasing desire for visual displays has put a particular emphasis on the switching speed of liquid crystals. This has required the manufacture of devices employing smectic liquid crystals, certain kinds of which emit a faster 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. Inside it the liquid crystal molecules are set out in perpendicular layers to the substrate planes, which are differentiated by one or two micrometres, and within the layers the molecules are on a tilt, as shown in the figure. The host liquid crystal contains optically active molecules, and a minor consequence of the optical activity and the tilt 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. Thus, there exists a permanent charge separation over 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 correct sign can reverse the direction of this dipole in tens of microseconds and therefore reverse the tilt direction of the molecules. The corresponding change in optical properties can make a change from light to dark in the case that one or more polarizers are used.
SSFLC devices have been commercialized for larger passive-matrix presentations, but their high cost and complexity has impeded them from creating any significant progress on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have shown some probability for use as parts in projection systems or as viewfinders in digital cameras. Their fast reaction allows them to be used in time-sequential colour systems, in which dear colour filters are replaced by a coloured backlight that flashes red, green, and blue in quick succession (around 100 cycles a second). For example, the liquid crystal may be switched to a transmissive state in the red and green periods but to a nontransmissive state during the blue period, creating the result that the eye sees an average of red and green light, or the colour yellow.
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