The Development of Data Projectors

The LCDs put for projection systems are typically small reflective or transmissive panels set off by a bright arc lamp source. A line of lenses magnifies the reflected or transmitted image and displays it on a screen. In front-projection systems the LCD is located on the same area of the screen as the viewer, although in rear-projection systems the screen is set off from behind. Projectors of higher expense and capability may have three separated LCD panels, creating separate red, green, and blue images that come together to reflect a coloured picture on the screen.

The growing desire for video displays has put a growing emphasis on the switching speed of liquid crystals. This has demanded the creation of items using smectic liquid crystals, some types of which give a better electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is in the current day the most complex smectic device. Within it the liquid crystal molecules are managed in perpendicular layers to the substrate planes, which are separated by one or two micrometres, and throughout the layers the molecules are on a tilt, as demonstrated in the figure. The host liquid crystal possesses optically active molecules, and a subtle result of the optical activity and the shape of the molecules is the presence 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. So, there is a permanent charge separation through 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 correct sign can reverse the direction of this dipole in tens of microseconds and so reverse the tilt direction of the molecules. The resultant change in optical properties can make a change from light to dark when one or more polarizers are utilised.

SSFLC devices have been publicized for larger passive-matrix presentations, but their expensiveness and complex nature has stopped them from creating any significant effect on the market. Small transmissive and reflective active-matrix SSFLC displays, however, show some probability for use as elements in projection systems or as viewfinders in digital cameras. Their immediate responding allows them to be employed in time-sequential colour systems, in which dear colour filters are emulated with a coloured backlight that flashes red, green, and blue in fast pulsing (about 100 cycles in 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 in the blue period, having the result that the eye sees an average of red and green light, or the colour yellow.

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