Data Projector Evolution

The LCDs used in projection systems are typically small reflective or transmissive panels illuminated by a forceful arc lamp source

. A series of lenses magnifies the reflected or transmitted image then displays it on a screen.

With front-projection systems the LCD is set on the same area of the screen as the viewer, however in rear-projection systems the screen is illuminated from behind. Projectors of higher cost and performance sometimes use three separate LCD panels, creating separate red, green, and blue images that blend to reflect a coloured display on the screen.

The increase in desire for video displays has granted a particular emphasis on the switching speed of liquid crystals. This has demanded the manufacture of items utilizing smectic liquid crystals, particular kinds of which give a better 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. Inside it the liquid crystal molecules are arranged in perpendicular layers 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 possesses optically active molecules, and a scarcely perceptible consequence of the optical activity and the shape 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 in the plane of the layers. So, there must be a permanent charge separation through 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 correct sign can reverse the direction of this dipole in tens of microseconds and therefore reverse the tilt direction of the molecules. The corresponding change in optical properties can effect a change from light to dark if one or more polarizers are employed.

SSFLC devices have been commercialized for larger passive-matrix displays, but their expense and intricacy has impeded them from having any significant progress on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have shown some possibility for use as parts in projection systems or as viewfinders in digital cameras. Their immediate response allows them to be utilised in time-sequential colour systems, in which highly expensive colour filters are taken out for a coloured backlight that flashes red, green, and blue in fast succession (approx 100 cycles a second). For example, the liquid crystal might be switched to a transmissive state in the red and green periods 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.

by: Tammy Frazer

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