The Development of Data Projectors
The LCDs utilised for projection systems are generally small reflective or transmissive panels set off by a bright arc lamp source. A line of lenses enlarges the reflected or transmitted image then displays it onto the screen. For front-projection systems the LCD is situated on the side of the screen as the viewer, however in rear-projection systems the screen is illuminated from behind. Projectors of higher cost and performance might have three distinct LCD panels, forming separate red, green, and blue images that mesh to make a coloured image on the screen.
The growth in requirement for video displays has had a growing emphasis on the switching speed of liquid crystals. This has led to the invention of devices using smectic liquid crystals, some kinds of which have a faster electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this time the most developed smectic device. Within it the liquid crystal molecules are arranged in layers that are perpendicular to the substrate planes, which are separated by one or two micrometres, and throughout the layers the molecules are slanted, as displayed in the figure. The host liquid crystal has optically active molecules, and a slight outcome of the optical activity and the slant of the molecules is the appearance of a permanent charge separation, or ferroelectric dipole, similar 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. Thus, there has to be a permanent charge separation across 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 correct sign can reverse the direction of this dipole in tens of microseconds and hence reverse the tilt direction of the molecules. The respective change in optical properties can effect a change from light to dark if or when one or more polarizers are utilised.
SSFLC devices have been produced for large passive-matrix presentations, but their cost and complex detail has impeded them from having any remarkable impact on the market. Small transmissive and reflective active-matrix SSFLC displays, however, show some possibility for use as aspects in projection systems or as viewfinders in digital cameras. Their speedy response allows them to be utilised in time-sequential colour systems, in which high cost colour filters are removed for a coloured backlight that flashes red, green, and blue in fast succession (approximately 100 cycles a second). For example, the liquid crystal could be switched to a transmissive state for the red and green periods but to a nontransmissive state during the blue period, with the upshot that the eye sees an average of red and green light, or the colour yellow.
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