The Development of Data Projectors
The LCDs utilised for projection systems are usually small reflective or transmissive panels lit by a powerful arc lamp source. A number of lenses magnifies the reflected or transmitted image then casts it onto a screen. For front-projection systems the LCD is situated on the same 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 be found with three separate LCD panels, reflecting separate red, green, and blue images that blend to create a coloured picture on the screen.
The growth in requirement for pictographic displays has placed a growth in emphasis on the switching speed of liquid crystals. This has necessitated the creation of devices employing smectic liquid crystals, certain 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 progressive smectic device. With it the liquid crystal molecules are set out in layers that are perpendicular to the substrate planes, which are differentiated by one or two micrometres, and in the layers the molecules are slanted, as illustrated in the figure. The host liquid crystal holds optically active molecules, and a subtle result of the optical activity and the tilt 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 must be a permanent charge separation throughout the liquid crystal layer in the SSFLC, and its sign is directly paired to the tilt direction of the molecules. An applied voltage of the right sign can reverse the direction of this dipole in tens of microseconds and in so doing reverse the tilt direction of the molecules. The consequential change in optical properties can create a change from light to dark if or when one or more polarizers are used.
SSFLC devices have been produced for bigger passive-matrix displays, but their expensiveness and detail has impeded them from enjoying any particular impact on the market. Small transmissive and reflective active-matrix SSFLC displays, however, display some promise for use as aspects in projection systems or as viewfinders in digital cameras. Their fast reacting allows them to be employed in time-sequential colour systems, in which high cost colour filters are taken out for a coloured backlight that flashes red, green, and blue in quick pulsing (approximately 100 cycles in a second). For example, the liquid crystal could be switched to a transmissive state between the red and green periods then to a nontransmissive state in the blue period, displaying the upshot that the eye sees an average of red and green light, or the colour yellow.
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