The Development of Data Projectors
June 30th, 2010
The LCDs used for projection systems are most often small reflective or transmissive panels set off by a powerful arc lamp source. A number of lenses enlarges the reflected or transmitted image then sends it onto a screen. For front-projection systems the LCD is placed on the same side of the screen as the viewer, while in rear-projection systems the screen is lit from behind. Projectors of greater expense and capability sometimes utilise three separate LCD panels, creating separate red, green, and blue images that mesh to form a coloured picture on the screen.
The increasing requirement for pictographic displays has placed a growing emphasis on the switching speed of liquid crystals. This has necessitated the manufacture of devices utilizing smectic liquid crystals, some types of which give a faster electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is in the current day the most progressive smectic device. With it the liquid crystal molecules are managed in layers perpendicular to the substrate planes, which are separated by one or two micrometres, and in the layers the molecules are on a tilt, as displayed in the figure. The host liquid crystal holds optically active molecules, and a slight 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 within 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 partnered to the tilt direction of the molecules. An applied voltage of the right 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 effect a change from light to dark in the case that one or more polarizers are used.
SSFLC devices have been marketed for larger passive-matrix presentations, but their expense and intricacy has prevented them from having any particular progress on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have some probability for use as elements in projection systems or as viewfinders in digital cameras. Their speedy responding allows them to be utilised in time-sequential colour systems, in which high cost colour filters are emulated by a coloured backlight that flashes red, green, and blue in rapid pace (around 100 cycles per second). For example, the liquid crystal may be switched to a transmissive state during the red and green periods then to a nontransmissive state for the blue period, with the end result that the eye sees an average of red and green light, or the colour yellow.
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