The Development of Data Projectors
The LCDs put in projection systems are usually small reflective or transmissive panels lit up by a powerful arc lamp source. A line of lenses enlarges the reflected or transmitted image and then displays it onto a screen. For front-projection systems the LCD is placed on the same area of the screen as the viewer, while in rear-projection systems the screen is lit from behind. Projectors of higher expense and capability can have three separated LCD panels, reflecting separate red, green, and blue images that combine to reflect a coloured image on the screen.
The increase in desire for film displays has had a growing emphasis on the switching speed of liquid crystals. This has demanded the development of devices using smectic liquid crystals, particular types of which possess a quicker electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this point the most developed smectic device. In it the liquid crystal molecules are set out in layers that are perpendicular to the substrate planes, which are separated by one or two micrometres, and in the layers the molecules are tilted, as demonstrated in the figure. The host liquid crystal holds optically active molecules, and a scarcely perceptible result of the optical activity and the tilt of the molecules is the presence 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 throughout the plane of the layers. So, there has to be a permanent charge separation across 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 in so doing reverse the tilt direction of the molecules. The respective change in optical properties can make a change from light to dark when one or more polarizers are utilised.
SSFLC devices have been marketed for big passive-matrix displays, but their high cost and complexity has hindered them from making any remarkable movement on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have displayed some possibility for use as elements in projection systems or as viewfinders in digital cameras. Their fast responding allows them to be used in time-sequential colour systems, in which dear colour filters are taken out for a coloured backlight that flashes red, green, and blue in rapid pace (approx 100 cycles per second). For example, the liquid crystal can be switched to a transmissive state in the red and green periods and then to a nontransmissive state for the blue period, having the end result that the eye sees an average of red and green light, or the colour yellow.
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