The Development of Data Projectors

The LCDs utilised 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 displays it onto the screen. With front-projection systems the LCD is located on the same area of the screen as the viewer, but in rear-projection systems the screen is set off from behind. Projectors of more expense and capability can be found with three separated LCD panels, casting separate red, green, and blue images that come together to make a coloured display on the screen.

The increase in need for video presentations has granted a particular emphasis on the switching speed of liquid crystals. This has demanded the manufacture of devices using smectic liquid crystals, particular types of which possess a speedier electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is currently the most sophisticated smectic device. Within it the liquid crystal molecules are arranged in layers that are perpendicular to the substrate planes, which are separated by one or two micrometres, and throughout the layers the molecules are tilted, as shown in the figure. The host liquid crystal has optically active molecules, and a minor outcome of the optical activity and the angle 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 within the plane of the layers. Hence, there must be a permanent charge separation over 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 correct 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 used.

SSFLC devices have been marketed for large passive-matrix displays, but their expensiveness and complex detail has impeded them from enjoying any great impact on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have some promise for use as aspects in projection systems or as viewfinders in digital cameras. Their fast reaction allows them to be used in time-sequential colour systems, in which expensive colour filters are replaced by a coloured backlight that flashes red, green, and blue in quick succession (around 100 cycles in a second). For example, the liquid crystal might be switched to a transmissive state during the red and green periods then to a nontransmissive state for the blue period, creating the result that the eye sees an average of red and green light, or the colour yellow.

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