The Development of Data Projectors
The LCDs utilised in projection systems are most often small reflective or transmissive panels lit up by a powerful arc lamp source. A line of lenses enlarges the reflected or transmitted image and displays it on the screen. With front-projection systems the LCD is placed on the side of the screen as the viewer, although in rear-projection systems the screen is lit from behind. Projectors of greater cost and capacity sometimes use three separate LCD panels, creating separate red, green, and blue images that blend to form a coloured picture on the screen.
The growth in demand for visual presentations has put a growing emphasis on the switching speed of liquid crystals. This has demanded the development of devices utilizing smectic liquid crystals, some types of which possess a quicker electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is in the current day the most progressive smectic device. Inside it the liquid crystal molecules are arranged in perpendicular layers 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 contains optically active molecules, and a subtle outcome of the optical activity and the angle 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 through the plane of the layers. So, there exists a permanent charge separation throughout the liquid crystal layer in the SSFLC, and its sign is directly partnered 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 so reverse the tilt direction of the molecules. The resultant change in optical properties can create a change from light to dark if or when one or more polarizers are utilised.
SSFLC devices have been marketed for bigger passive-matrix displays, but their expensiveness and intricacy has impeded them from creating any significant progress on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have shown some possibility for use as elements in projection systems or as viewfinders in digital cameras. Their speedy responding allows them to be used in time-sequential colour systems, in which expensive colour filters are emulated by a coloured backlight that flashes red, green, and blue in fast pace (about 100 cycles in a second). For example, the liquid crystal might be switched to a transmissive state for the red and green periods but then to a nontransmissive state during the blue period, creating the upshot that the eye sees an average of red and green light, or the colour yellow.
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