The Development of Data Projectors
The LCDs used for projection systems are most often small reflective or transmissive panels set off by a powerful arc lamp source. A series of lenses enlarges the reflected or transmitted image then casts it onto a screen. With 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 set off from behind. Projectors of greater expense and capacity sometimes be found with three separate LCD panels, forming separate red, green, and blue images that combine to form a coloured display on the screen.
The growing need for visual displays has put a special emphasis on the switching speed of liquid crystals. This has required the invention of objects using smectic liquid crystals, some of which possess a better electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is currently the most developed smectic device. Inside it the liquid crystal molecules are managed in layers that are perpendicular to the substrate planes, which are distanced by one or two micrometres, and within the layers the molecules are on a tilt, as displayed in the figure. The host liquid crystal holds optically active molecules, and a subtle outcome 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 in the plane of the layers. So, there must be a permanent charge separation over 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 in so doing reverse the tilt direction of the molecules. The respective change in optical properties can cause a change from light to dark in the case that one or more polarizers are used.
SSFLC devices have been marketed for big passive-matrix displays, but their cost and complexity has prevented them from having any great movement on the market. Small transmissive and reflective active-matrix SSFLC displays, however, display some promise for use as parts in projection systems or as viewfinders in digital cameras. Their fast reaction allows them to be utilised in time-sequential colour systems, in which costly colour filters are emulated by a coloured backlight that flashes red, green, and blue in rapid speed (approx 100 cycles a second). For example, the liquid crystal could be switched to a transmissive state in the red and green periods but then to a nontransmissive state in the blue period, displaying the result that the eye sees an average of red and green light, or the colour yellow.
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