The Development of Data Projectors
The LCDs utilised for projection systems are most often small reflective or transmissive panels lit by a powerful arc lamp source. A line of lenses enlarges the reflected or transmitted image and then sends it onto the screen. For 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 illuminated from behind. Projectors of higher cost and performance may be found with three distinct LCD panels, creating separate red, green, and blue images that come together to create a coloured display on the screen.
The increasing desire for pictographic displays has had a growing emphasis on the switching speed of liquid crystals. This has demanded the invention of devices employing smectic liquid crystals, certain kinds of which give a speedier electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is currently the most progressive smectic device. With it the liquid crystal molecules are cast in perpendicular layers to the substrate planes, which are separated by one or two micrometres, and inside the layers the molecules are on a slant, as demonstrated in the figure. The host liquid crystal possesses optically active molecules, and a slight consequence of the optical activity and the shape of the molecules is the presence 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 through the liquid crystal layer in the SSFLC, and its sign is directly coupled 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 by doing so reverse the tilt direction of the molecules. The consequential change in optical properties can create a change from light to dark when one or more polarizers are utilised.
SSFLC devices have been marketed for larger passive-matrix presentations, but their expensiveness and complex nature has hindered them from having any remarkable progress 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 utilised in time-sequential colour systems, in which highly expensive colour filters are taken out for a coloured backlight that flashes red, green, and blue in quick pulsing (around 100 cycles in a second). For example, the liquid crystal could be switched to a transmissive state in the red and green periods and then to a nontransmissive state for the blue period, displaying the result that the eye sees an average of red and green light, or the colour yellow.
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