The Development of Data Projectors
The LCDs put in projection systems are typically small reflective or transmissive panels lit by a forceful arc lamp source. A series of lenses enlarges the reflected or transmitted image and then displays it on a screen. With front-projection systems the LCD is situated on the side of the screen as the viewer, while in rear-projection systems the screen is illuminated from behind. Projectors of higher cost and performance can use three separated LCD panels, reflecting separate red, green, and blue images that come together to form a coloured display on the screen.
The growing requirement for pictographic displays has granted a particular emphasis on the switching speed of liquid crystals. This has demanded the creation of objects employing smectic liquid crystals, some of which have a quicker electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this time the most complex smectic device. Within it the liquid crystal molecules are set out in perpendicular layers to the substrate planes, which are separated by one or two micrometres, and throughout the layers the molecules are tilted, as demonstrated in the figure. The host liquid crystal contains optically active molecules, and a minor outcome of the optical activity and the angle of the molecules is the presence of a permanent charge separation, or ferroelectric dipole, likeable 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. Therefore, 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 hence reverse the tilt direction of the molecules. The consequential change in optical properties can effect a change from light to dark if one or more polarizers are used.
SSFLC devices have been produced for larger passive-matrix presentations, but their expense and complexity has impeded them from creating any significant impact on the market. Small transmissive and reflective active-matrix SSFLC displays, however, display some possibility for use as aspects in projection systems or as viewfinders in digital cameras. Their fast reaction allows them to be utilised in time-sequential colour systems, in which high cost colour filters are replaced by a coloured backlight that flashes red, green, and blue in fast speed (about 100 cycles every second). For example, the liquid crystal can be switched to a transmissive state during the red and green periods and to a nontransmissive state for the blue period, displaying the end result that the eye sees an average of red and green light, or the colour yellow.
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