The Development of Data Projectors

The LCDs utilised in projection systems are usually small reflective or transmissive panels illuminated by a powerful arc lamp source. A series of lenses enlarges the reflected or transmitted image and then displays it on the screen. For front-projection systems the LCD is situated on the same area of the screen as the viewer, although in rear-projection systems the screen is illuminated from behind. Projectors of greater cost and performance sometimes be found with three separate LCD panels, reflecting separate red, green, and blue images that mesh to reflect a coloured picture on the screen.

The increase in desire for film displays has had a growing emphasis on the switching speed of liquid crystals. This has led to the invention of devices build with smectic liquid crystals, particular kinds of which have a quicker electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is in the current day the most developed smectic device. In it the liquid crystal molecules are managed in layers perpendicular to the substrate planes, which are separated by one or two micrometres, and throughout the layers the molecules are on a tilt, as shown in the figure. The host liquid crystal has optically active molecules, and a subtle turn up of the optical activity and the shape of the molecules is the presence of a permanent charge separation, or ferroelectric dipole, similar 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 has to be a permanent charge separation throughout the liquid crystal layer in the SSFLC, and its sign is directly attracted 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 so reverse the tilt direction of the molecules. The resultant change in optical properties can create a change from light to dark in the case that one or more polarizers are used.

SSFLC devices have been produced for larger passive-matrix displays, but their cost and detail has stopped them from creating any significant impact on the market. Small transmissive and reflective active-matrix SSFLC displays, however, show some promise for use as parts 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 dear colour filters are taken out for a coloured backlight that flashes red, green, and blue in quick speed (approximately 100 cycles every second). For example, the liquid crystal can be switched to a transmissive state for the red and green periods and then to a nontransmissive state in 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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