The LCDs put for projection systems are typically small reflective or transmissive panels lit up by a forceful arc lamp source. A number of lenses magnifies the reflected or transmitted image and then sends it onto a screen. For front-projection systems the LCD is set on the same side of the screen as the viewer, however in rear-projection systems the screen is illuminated from behind. Projectors of greater expense and capability sometimes have three separate LCD panels, creating separate red, green, and blue images that mesh to reflect a coloured image on the screen.
The increase in desire for video displays has put a growth in emphasis on the switching speed of liquid crystals. This has required the development of devices build with smectic liquid crystals, certain kinds of which have a better 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 set out in layers that are perpendicular to the substrate planes, which are separated by one or two micrometres, and within 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 slant 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 within the plane of the layers. Thus, there is a permanent charge separation across 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 correct 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 cause a change from light to dark if or when one or more polarizers are used.
SSFLC devices have been publicized for larger passive-matrix presentations, but their cost and detail has hindered them from making any particular movement 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 responding allows them to be employed 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 in a second). For example, the liquid crystal could be switched to a transmissive state in the red and green periods but to a nontransmissive state for the blue period, creating the end result that the eye sees an average of red and green light, or the colour yellow.
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June 30th, 2010UncategorizedRead More >No Comments
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