The Development of Data Projectors
The LCDs utilised in projection systems are typically small reflective or transmissive panels set off by a forceful arc lamp source. A number of lenses enlarges the reflected or transmitted image and then casts it onto a screen. In front-projection systems the LCD is situated on the same side of the screen as the viewer, while in rear-projection systems the screen is illuminated from behind. Projectors of higher expense and capacity might have three distinct LCD panels, creating separate red, green, and blue images that come together to form a coloured image on the screen.
The growing requirement for video displays has granted a growing emphasis on the switching speed of liquid crystals. This has led to the manufacture of devices build with smectic liquid crystals, certain types of which emit a speedier electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this time the most developed 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 throughout the layers the molecules are slanted, as shown in the figure. The host liquid crystal possesses optically active molecules, and a scarcely perceptible outcome of the optical activity and the angle of the molecules is the appearance 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. So, there is 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 correct sign can reverse the direction of this dipole in tens of microseconds and so reverse the tilt direction of the molecules. The consequential change in optical properties can effect a change from light to dark if one or more polarizers are used.
SSFLC devices have been publicized for large passive-matrix displays, but their high cost and complex detail has stopped them from having any particular impact on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have displayed some probability for use as elements in projection systems or as viewfinders in digital cameras. Their quick reacting allows them to be made use of in time-sequential colour systems, in which expensive colour filters are taken out for a coloured backlight that flashes red, green, and blue in rapid pulsing (around 100 cycles in a second). For example, the liquid crystal can be switched to a transmissive state during the red and green periods and to a nontransmissive state in 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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