The Development of Data Projectors
The LCDs utilised for projection systems are generally small reflective or transmissive panels illuminated by a strong arc lamp source. A number of lenses magnifies the reflected or transmitted image then sends it onto a screen. In front-projection systems the LCD is situated on the side of the screen as the viewer, while in rear-projection systems the screen is set off from behind. Projectors of more expense and capacity sometimes have three separate LCD panels, creating separate red, green, and blue images that combine to create a coloured image on the screen.
The growing desire for pictographic presentations has had a particular emphasis on the switching speed of liquid crystals. This has led to the manufacture of objects using smectic liquid crystals, some of which emit a quicker electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this time the most progressive smectic device. In it the liquid crystal molecules are arranged in perpendicular layers to the substrate planes, which are distanced by one or two micrometres, and in the layers the molecules are on a tilt, as shown in the figure. The host liquid crystal holds optically active molecules, and a subtle result of the optical activity and the angle of the molecules is the appearance of a permanent charge separation, or ferroelectric dipole, comparable 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 must be a permanent charge separation throughout 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 by doing so reverse the tilt direction of the molecules. The respective change in optical properties can effect a change from light to dark in the case that one or more polarizers are utilised.
SSFLC devices have been publicized for big passive-matrix presentations, but their cost and complex detail has impeded them from creating any particular effect on the market. Small transmissive and reflective active-matrix SSFLC displays, however, show some promise for use as elements in projection systems or as viewfinders in digital cameras. Their speedy reacting allows them to be used in time-sequential colour systems, in which dear colour filters are emulated by a coloured backlight that flashes red, green, and blue in quick pulsing (approximately 100 cycles in a second). For example, the liquid crystal might be switched to a transmissive state during the red and green periods and to a nontransmissive state during the blue period, creating the end result that the eye sees an average of red and green light, or the colour yellow.
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