The Development of Data Projectors

The LCDs put in projection systems are usually small reflective or transmissive panels set off by a powerful arc lamp source. A line of lenses enlarges the reflected or transmitted image then sends it on a screen. With front-projection systems the LCD is placed on the same area of the screen as the viewer, but in rear-projection systems the screen is illuminated from behind. Projectors of greater expense and capability might be found with three discrete LCD panels, creating separate red, green, and blue images that mesh to create a coloured picture on the screen.

The increase in need for film displays has granted a particular emphasis on the switching speed of liquid crystals. This has necessitated the development of items employing smectic liquid crystals, certain ones of which give 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. Within it the liquid crystal molecules are managed in layers perpendicular to the substrate planes, which are distanced by one or two micrometres, and in the layers the molecules are on a tilt, as displayed in the figure. The host liquid crystal has optically active molecules, and a slight consequence 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 in the plane of the layers. Therefore, there is a permanent charge separation throughout the liquid crystal layer in the SSFLC, and its sign is directly paired up 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 therefore reverse the tilt direction of the molecules. The resultant change in optical properties can cause a change from light to dark when one or more polarizers are used.

SSFLC devices have been marketed for bigger passive-matrix displays, but their high cost and intricacy has impeded them from creating any remarkable impact 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 fast reacting allows them to be made use of in time-sequential colour systems, in which costly colour filters are replaced with a coloured backlight that flashes red, green, and blue in quick pulsing (approx 100 cycles a second). For example, the liquid crystal may be switched to a transmissive state during the red and green periods but to a nontransmissive state for the blue period, with the outcome that the eye sees an average of red and green light, or the colour yellow.

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