The Development of Data Projectors
The LCDs used for projection systems are generally small reflective or transmissive panels lit by a bright arc lamp source. A line of lenses enlarges the reflected or transmitted image and then casts it onto the screen. In front-projection systems the LCD is set on the side of the screen as the viewer, but in rear-projection systems the screen is illuminated from behind. Projectors of more expense and capability can be found with three discrete LCD panels, casting separate red, green, and blue images that blend to form a coloured picture on the screen.
The increasing need for pictographic presentations has placed a growing emphasis on the switching speed of liquid crystals. This has demanded the manufacture of objects build with smectic liquid crystals, certain types of which have a better electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this time the most sophisticated smectic device. Within it the liquid crystal molecules are cast in layers that are perpendicular to the substrate planes, which are separated by one or two micrometres, and in the layers the molecules are tilted, as shown in the figure. The host liquid crystal has optically active molecules, and a scarcely perceptible outcome of the optical activity and the shape of the molecules is the presence 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 in the plane of the layers. Therefore, there exists 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 hence reverse the tilt direction of the molecules. The corresponding change in optical properties can effect a change from light to dark in the case that one or more polarizers are employed.
SSFLC devices have been marketed for big passive-matrix displays, but their expense and complexity has hindered them from having any significant effect on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have displayed some probability for use as elements in projection systems or as viewfinders in digital cameras. Their fast reaction allows them to be employed in time-sequential colour systems, in which dear colour filters are emulated with a coloured backlight that flashes red, green, and blue in rapid pulsing (approximately 100 cycles a second). For example, the liquid crystal may be switched to a transmissive state between the red and green periods and then to a nontransmissive state in 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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