The LCDs built for projection systems are generally small reflective or transmissive panels lit by a forceful arc lamp source. A series of lenses enlarges the reflected or transmitted image and then displays it onto the screen. With front-projection systems the LCD is placed on the side of the screen as the viewer, but in [...]
The LCDs built for projection systems are generally small reflective or transmissive panels lit by a forceful arc lamp source. A series of lenses enlarges the reflected or transmitted image and then displays it onto the screen. With front-projection systems the LCD is placed on the side of the screen as the viewer, but in rear-projection systems the screen is set off from behind. Projectors of greater expense and capability might use three discrete LCD panels, casting separate red, green, and blue images that mesh to make a coloured display on the screen.
The growing desire for pictographic presentations has placed a special emphasis on the switching speed of liquid crystals. This has necessitated the creation of devices build with smectic liquid crystals, some types of which emit a speedier electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is in the current day the most developed smectic device. Inside it the liquid crystal molecules are set out in perpendicular layers to the substrate planes, which are separated by one or two micrometres, and in the layers the molecules are tilted, as shown in the figure. The host liquid crystal possesses optically active molecules, and a scarcely perceptible turn up 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 through the plane of the layers. Hence, there exists a permanent charge separation through the liquid crystal layer in the SSFLC, and its sign is directly paired 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 consequential change in optical properties can cause a change from light to dark in the case that one or more polarizers are employed.
SSFLC devices have been publicized for bigger passive-matrix presentations, but their expense and complex nature has stopped them from enjoying any remarkable impact on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have shown some probability for use as parts in projection systems or as viewfinders in digital cameras. Their immediate responding allows them to be utilised in time-sequential colour systems, in which expensive colour filters are removed for a coloured backlight that flashes red, green, and blue in rapid speed (about 100 cycles per second). For example, the liquid crystal could be switched to a transmissive state in the red and green periods then to a nontransmissive state during the blue period, having the upshot that the eye sees an average of red and green light, or the colour yellow.
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