Shadow Squadron

The LCDs built for projection systems are most often small reflective or transmissive panels lit up by a powerful arc lamp source. A line of lenses enlarges the reflected or transmitted image and casts it onto a screen. In front-projection systems the LCD is set on the same area of the screen as the viewer, while in rear-projection systems the screen is illuminated from behind. Projectors of higher expense and performance sometimes have three distinct LCD panels, casting separate red, green, and blue images that mesh to create a coloured image on the screen.

The growing demand for video displays has had a special emphasis on the switching speed of liquid crystals. This has necessitated the development of devices using smectic liquid crystals, certain kinds of which give a faster electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is currently the most sophisticated smectic device. In it the liquid crystal molecules are set out in layers perpendicular to the substrate planes, which are separated by one or two micrometres, and within the layers the molecules are slanted, as demonstrated in the figure. The host liquid crystal possesses optically active molecules, and a slight turn up of the optical activity and the angle of the molecules is the presence of a permanent charge separation, or ferroelectric dipole, comparable 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. Thus, there exists a permanent charge separation throughout the liquid crystal layer in the SSFLC, and its sign is directly partnered 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 make a change from light to dark if or when one or more polarizers are employed.

SSFLC devices have been produced for large passive-matrix displays, but their high cost and detail has stopped them from creating any particular progress on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have shown some promise for use as elements in projection systems or as viewfinders in digital cameras. Their quick reacting allows them to be utilised in time-sequential colour systems, in which highly expensive colour filters are emulated by a coloured backlight that flashes red, green, and blue in fast speed (approximately 100 cycles every second). For example, the liquid crystal could be switched to a transmissive state between the red and green periods and to a nontransmissive state for the blue period, with the upshot that the eye sees an average of red and green light, or the colour yellow.

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