The Development of Data Projectors
The LCDs put for projection systems are generally small reflective or transmissive panels set off by a forceful arc lamp source. A number of lenses enlarges the reflected or transmitted image and displays it onto the screen. In front-projection systems the LCD is placed on the same area of the screen as the viewer, while in rear-projection systems the screen is lit from behind. Projectors of greater cost and capacity might have three separated LCD panels, casting separate red, green, and blue images that come together to form a coloured image on the screen.
The increasing need for pictographic displays has put a particular emphasis on the switching speed of liquid crystals. This has required the development of objects utilizing smectic liquid crystals, certain ones of which give a quicker electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is in the current day the most complex smectic device. Within it the liquid crystal molecules are set out in layers that are perpendicular to the substrate planes, which are distanced by one or two micrometres, and inside the layers the molecules are slanted, as displayed in the figure. The host liquid crystal contains optically active molecules, and a minor turn up of the optical activity and the slant 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 in the plane of the layers. So, there has to be 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 correct sign can reverse the direction of this dipole in tens of microseconds and hence reverse the tilt direction of the molecules. The corresponding 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 marketed for larger passive-matrix displays, but their high cost and complex nature has prevented them from enjoying any remarkable movement on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have shown some possibility for use as aspects in projection systems or as viewfinders in digital cameras. Their fast response allows them to be utilised in time-sequential colour systems, in which dear colour filters are replaced with a coloured backlight that flashes red, green, and blue in fast succession (about 100 cycles in a second). For example, the liquid crystal might be switched to a transmissive state for the red and green periods but to a nontransmissive state during the blue period, creating the outcome that the eye sees an average of red and green light, or the colour yellow.
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