The Development of Data Projectors

The LCDs utilised in projection systems are most often 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 casts it on the screen. In front-projection systems the LCD is placed on the same side of the screen as the viewer, although in rear-projection systems the screen is illuminated from behind. Projectors of greater expense and capability sometimes use three distinct LCD panels, reflecting separate red, green, and blue images that blend to reflect a coloured image on the screen.

The growing requirement for video presentations has had a growing emphasis on the switching speed of liquid crystals. This has necessitated the invention of objects employing smectic liquid crystals, some kinds of which possess a speedier electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this point the most complex smectic device. With it the liquid crystal molecules are set out in perpendicular layers to the substrate planes, which are separated by one or two micrometres, and throughout the layers the molecules are on a slant, as shown in the figure. The host liquid crystal possesses optically active molecules, and a slight consequence of the optical activity and the angle 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 through the plane of the layers. Thus, there has to be a permanent charge separation across 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 corresponding sign can reverse the direction of this dipole in tens of microseconds and hence reverse the tilt direction of the molecules. The corresponding change in optical properties can cause a change from light to dark if or when one or more polarizers are used.

SSFLC devices have been commercialized for larger passive-matrix presentations, but their cost and intricacy has stopped them from having any remarkable movement on the market. Small transmissive and reflective active-matrix SSFLC displays, however, display some probability for use as elements in projection systems or as viewfinders in digital cameras. Their immediate reaction allows them to be employed in time-sequential colour systems, in which high cost colour filters are removed for a coloured backlight that flashes red, green, and blue in rapid succession (about 100 cycles every second). For example, the liquid crystal might be switched to a transmissive state between the red and green periods and then to a nontransmissive state for the blue period, displaying the upshot that the eye sees an average of red and green light, or the colour yellow.

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