The Development of Data Projectors
The LCDs used for projection systems are usually small reflective or transmissive panels set off by a forceful arc lamp source. A line of lenses enlarges the reflected or transmitted image then displays it on a screen. In front-projection systems the LCD is placed on the side of the screen as the viewer, while in rear-projection systems the screen is lit from behind. Projectors of higher expense and performance sometimes have three discrete LCD panels, casting separate red, green, and blue images that combine to reflect a coloured image on the screen.
The increase in demand for pictographic presentations has had a growth in emphasis on the switching speed of liquid crystals. This has demanded the development of devices build with smectic liquid crystals, particular kinds of which give a quicker electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this point 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 in the layers the molecules are slanted, as displayed in the figure. The host liquid crystal holds optically active molecules, and a slight result 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 through the plane of the layers. So, there exists a permanent charge separation through 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 correct sign can reverse the direction of this dipole in tens of microseconds and hence reverse the tilt direction of the molecules. The resultant change in optical properties can cause a change from light to dark if or when one or more polarizers are used.
SSFLC devices have been commercialized for bigger passive-matrix presentations, but their cost and complex detail has stopped them from enjoying any particular effect on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have some probability for use as parts in projection systems or as viewfinders in digital cameras. Their quick responding allows them to be utilised in time-sequential colour systems, in which highly expensive colour filters are emulated by a coloured backlight that flashes red, green, and blue in fast pace (about 100 cycles per second). For example, the liquid crystal may be switched to a transmissive state between the red and green periods but to a nontransmissive state for the blue period, with the result that the eye sees an average of red and green light, or the colour yellow.
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