The Development of Data Projectors
The LCDs built in projection systems are usually small reflective or transmissive panels set off by a powerful arc lamp source. A series of lenses expands the reflected or transmitted image and sends it on the screen. In front-projection systems the LCD is set on the same side of the screen as the viewer, however in rear-projection systems the screen is illuminated from behind. Projectors of greater cost and capability sometimes use three separated LCD panels, reflecting separate red, green, and blue images that combine to create a coloured image on the screen.
The growing requirement for video presentations has had a special emphasis on the switching speed of liquid crystals. This has necessitated the creation of items build with smectic liquid crystals, some kinds of which give a better electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this time the most developed smectic device. In it the liquid crystal molecules are cast in layers perpendicular to the substrate planes, which are differentiated by one or two micrometres, and inside the layers the molecules are on a slant, as demonstrated in the figure. The host liquid crystal holds optically active molecules, and a subtle turn up of the optical activity and the shape of the molecules is the presence of a permanent charge separation, or ferroelectric dipole, analogous 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 across 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 hence reverse the tilt direction of the molecules. The resultant change in optical properties can create a change from light to dark when one or more polarizers are employed.
SSFLC devices have been produced for large passive-matrix presentations, but their cost and intricacy has impeded them from making any remarkable 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 speedy reaction allows them to be utilised in time-sequential colour systems, in which highly expensive colour filters are replaced with a coloured backlight that flashes red, green, and blue in quick pulsing (about 100 cycles a second). For example, the liquid crystal may be switched to a transmissive state in the red and green periods and to a nontransmissive state in the blue period, displaying the result that the eye sees an average of red and green light, or the colour yellow.
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