The Development of Data Projectors

The LCDs utilised in projection systems are usually small reflective or transmissive panels lit up by a powerful arc lamp source. A series of lenses enlarges the reflected or transmitted image and sends it onto a screen. In front-projection systems the LCD is located on the same side of the screen as the viewer, although in rear-projection systems the screen is lit up from behind. Projectors of higher expense and performance might have three separated LCD panels, forming separate red, green, and blue images that come together to create a coloured picture on the screen.

The growth in requirement for film displays has placed a growth in emphasis on the switching speed of liquid crystals. This has led to the invention of objects employing smectic liquid crystals, certain ones of which emit a better electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is currently the most developed smectic device. Inside it the liquid crystal molecules are managed in perpendicular layers to the substrate planes, which are differentiated by one or two micrometres, and throughout the layers the molecules are tilted, as displayed in the figure. The host liquid crystal contains optically active molecules, and a minor turn up of the optical activity and the slant of the molecules is the appearance 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 in the plane of the layers. So, 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 corresponding sign can reverse the direction of this dipole in tens of microseconds and hence reverse the tilt direction of the molecules. The resultant change in optical properties can effect a change from light to dark if one or more polarizers are utilised.

SSFLC devices have been commercialized for large passive-matrix presentations, but their expensiveness and complex nature has stopped them from enjoying any particular progress on the market. Small transmissive and reflective active-matrix SSFLC displays, however, display some promise for use as elements in projection systems or as viewfinders in digital cameras. Their speedy response allows them to be employed in time-sequential colour systems, in which costly colour filters are emulated by a coloured backlight that flashes red, green, and blue in quick speed (approx 100 cycles every second). For example, the liquid crystal can be switched to a transmissive state for the red and green periods then to a nontransmissive state for 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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