CTO Blog

The LCDs built for projection systems are generally small reflective or transmissive panels lit by a bright arc lamp source. A series of lenses magnifies the reflected or transmitted image and then casts it on the screen. With 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 greater cost and capacity can have three separated LCD panels, forming separate red, green, and blue images that combine to create a coloured picture on the screen.

The increasing requirement for visual presentations has granted a particular emphasis on the switching speed of liquid crystals. This has necessitated the creation of objects utilizing smectic liquid crystals, particular kinds of which emit a speedier electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is at this point the most developed smectic device. Within it the liquid crystal molecules are arranged in layers perpendicular to the substrate planes, which are distanced by one or two micrometres, and inside the layers the molecules are on a slant, as displayed in the figure. The host liquid crystal contains optically active molecules, and a subtle turn up of the optical activity and the shape of the molecules is the appearance 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 throughout the plane of the layers. Hence, there has to be a permanent charge separation throughout 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 consequential change in optical properties can effect a change from light to dark in the case that one or more polarizers are employed.

SSFLC devices have been produced for larger passive-matrix presentations, but their high cost and complex detail has prevented them from creating any great progress on the market. Small transmissive and reflective active-matrix SSFLC displays, however, show some possibility for use as elements in projection systems or as viewfinders in digital cameras. Their quick reacting allows them to be made use of in time-sequential colour systems, in which highly expensive colour filters are emulated by a coloured backlight that flashes red, green, and blue in quick succession (approx 100 cycles every second). For example, the liquid crystal might be switched to a transmissive state in the red and green periods but then to a nontransmissive state for the blue period, with the result that the eye sees an average of red and green light, or the colour yellow.

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