Mack and Mark

The LCDs used for projection systems are usually small reflective or transmissive panels lit by a powerful arc lamp source. A line of lenses enlarges the reflected or transmitted image and sends it on a screen. For front-projection systems the LCD is placed on the side of the screen as the viewer, but in rear-projection systems the screen is lit up from behind. Projectors of more expense and performance sometimes use three distinct LCD panels, creating separate red, green, and blue images that combine to make a coloured picture on the screen.

The growth in requirement for film displays has granted a growth in emphasis on the switching speed of liquid crystals. This has led to the creation of devices employing smectic liquid crystals, certain kinds of which emit a quicker electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is in the current day the most sophisticated smectic device. With it the liquid crystal molecules are cast in layers that are perpendicular to the substrate planes, which are differentiated by one or two micrometres, and in the layers the molecules are slanted, as demonstrated in the figure. The host liquid crystal has optically active molecules, and a slight outcome of the optical activity and the tilt of the molecules is the appearance 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 throughout the plane of the layers. Hence, 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 corresponding sign can reverse the direction of this dipole in tens of microseconds and so reverse the tilt direction of the molecules. The corresponding change in optical properties can effect a change from light to dark if one or more polarizers are employed.

SSFLC devices have been commercialized for big passive-matrix presentations, but their high cost and complex detail has hindered them from making any significant progress on the market. Small transmissive and reflective active-matrix SSFLC displays, however, show some promise for use as parts in projection systems or as viewfinders in digital cameras. Their immediate reacting allows them to be made use of in time-sequential colour systems, in which highly expensive colour filters are emulated by a coloured backlight that flashes red, green, and blue in rapid succession (around 100 cycles per second). For example, the liquid crystal can be switched to a transmissive state for the red and green periods then to a nontransmissive state in the blue period, displaying the outcome that the eye sees an average of red and green light, or the colour yellow.

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