The Development of Data Projectors

The LCDs built in projection systems are generally small reflective or transmissive panels set off by a powerful arc lamp source. A number of lenses magnifies the reflected or transmitted image and then sends it on a screen. With front-projection systems the LCD is located on the side of the screen as the viewer, however in rear-projection systems the screen is illuminated from behind. Projectors of greater expense and capability may be found with three separated LCD panels, creating separate red, green, and blue images that mesh to form a coloured picture on the screen.

The increase in requirement for pictographic presentations has placed a growing emphasis on the switching speed of liquid crystals. This has required the invention of items build with smectic liquid crystals, certain ones of which have a better electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this time the most sophisticated smectic device. In 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 within the layers the molecules are on a slant, as illustrated in the figure. The host liquid crystal has optically active molecules, and a subtle consequence of the optical activity and the shape of the molecules is the appearance 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. So, there has to be a permanent charge separation over 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 by doing so reverse the tilt direction of the molecules. The resultant change in optical properties can create a change from light to dark if or when one or more polarizers are utilised.

SSFLC devices have been publicized for bigger passive-matrix displays, but their expense and complex detail has impeded them from making any great movement on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have displayed some promise for use as parts in projection systems or as viewfinders in digital cameras. Their quick responding allows them to be used in time-sequential colour systems, in which highly expensive colour filters are taken out for a coloured backlight that flashes red, green, and blue in fast pace (around 100 cycles a second). For example, the liquid crystal could be switched to a transmissive state for the red and green periods but to a nontransmissive state for the blue period, creating the outcome that the eye sees an average of red and green light, or the colour yellow.

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