The Development of Data Projectors
June 30th, 2010The LCDs built for projection systems are typically small reflective or transmissive panels set off by a forceful arc lamp source. A line of lenses enlarges the reflected or transmitted image and casts it onto a screen. In front-projection systems the LCD is situated on the same area of the screen as the viewer, while in rear-projection systems the screen is illuminated from behind. Projectors of more expense and capacity might be found with three separated LCD panels, reflecting separate red, green, and blue images that mesh to create a coloured display on the screen.
The increase in desire for visual presentations has placed a special emphasis on the switching speed of liquid crystals. This has required the creation of items using smectic liquid crystals, particular types of which have a speedier electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this time the most complex smectic device. Inside 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 tilted, as demonstrated in the figure. The host liquid crystal possesses optically active molecules, and a subtle outcome 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 in the plane of the layers. Therefore, there is a permanent charge separation across the liquid crystal layer in the SSFLC, and its sign is directly partnered 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 resultant change in optical properties can make a change from light to dark in the case that one or more polarizers are employed.
SSFLC devices have been produced for larger passive-matrix presentations, but their cost and intricacy has prevented them from creating any significant movement on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have some promise for use as aspects in projection systems or as viewfinders in digital cameras. Their quick responding allows them to be used in time-sequential colour systems, in which highly expensive colour filters are emulated by a coloured backlight that flashes red, green, and blue in rapid speed (around 100 cycles per second). For example, the liquid crystal could be switched to a transmissive state in the red and green periods and to a nontransmissive state during the blue period, having the outcome that the eye sees an average of red and green light, or the colour yellow.
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