The Development of Data Projectors
The LCDs built for projection systems are typically small reflective or transmissive panels illuminated by a powerful arc lamp source. A number of lenses expands the reflected or transmitted image and displays it on a screen. With front-projection systems the LCD is placed on the same side of the screen as the viewer, while in rear-projection systems the screen is lit up from behind. Projectors of higher cost and capability may utilise three discrete LCD panels, forming separate red, green, and blue images that come together to reflect a coloured image on the screen.
The increasing requirement for video displays has granted a growth in emphasis on the switching speed of liquid crystals. This has necessitated the creation of items utilizing smectic liquid crystals, certain ones of which have a speedier electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this point the most sophisticated smectic device. Inside it the liquid crystal molecules are cast in perpendicular layers to the substrate planes, which are differentiated by one or two micrometres, and inside the layers the molecules are slanted, as displayed in the figure. The host liquid crystal has optically active molecules, and a subtle turn up of the optical activity and the tilt of the molecules is the presence 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 throughout the plane of the layers. Thus, there exists a permanent charge separation through 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 corresponding 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 marketed for big passive-matrix presentations, but their cost and detail has stopped them from creating any significant movement on the market. Small transmissive and reflective active-matrix SSFLC displays, however, show some promise for use as parts in projection systems or as viewfinders in digital cameras. Their quick reaction allows them to be used in time-sequential colour systems, in which high cost colour filters are replaced with a coloured backlight that flashes red, green, and blue in fast succession (approximately 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 during the blue period, displaying the outcome that the eye sees an average of red and green light, or the colour yellow.
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