Mack and Mark

The LCDs built in projection systems are most often small reflective or transmissive panels set off by a powerful arc lamp source. A line of lenses enlarges the reflected or transmitted image and casts it onto a screen. With front-projection systems the LCD is placed on the side of the screen as the viewer, but in rear-projection systems the screen is illuminated from behind. Projectors of greater cost and performance can have three discrete LCD panels, reflecting separate red, green, and blue images that blend to make a coloured image on the screen.

The growth in requirement for film presentations has had a growth in emphasis on the switching speed of liquid crystals. This has necessitated the creation of objects using 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 currently the most progressive smectic device. Inside it the liquid crystal molecules are managed in layers perpendicular to the substrate planes, which are separated by one or two micrometres, and throughout the layers the molecules are on a slant, as illustrated in the figure. The host liquid crystal holds optically active molecules, and a subtle result of the optical activity and the slant 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. Hence, there is a permanent charge separation across 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 consequential change in optical properties can make a change from light to dark if one or more polarizers are employed.

SSFLC devices have been publicized for large passive-matrix displays, but their expensiveness and complexity has hindered them from creating any significant effect on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have some promise for use as aspects in projection systems or as viewfinders in digital cameras. Their fast reacting allows them to be used in time-sequential colour systems, in which costly colour filters are replaced by a coloured backlight that flashes red, green, and blue in quick pace (around 100 cycles per second). For example, the liquid crystal can be switched to a transmissive state between the red and green periods and then to a nontransmissive state for the blue period, with the outcome that the eye sees an average of red and green light, or the colour yellow.

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