The LCDs utilised for projection systems are generally small reflective or transmissive panels illuminated by a forceful arc lamp source. A line of lenses enlarges the reflected or transmitted image then casts it onto the screen. In front-projection systems the LCD is set on the same side of the screen as the viewer, but in rear-projection systems the screen is set off from behind. Projectors of higher expense and capacity might utilise three separate LCD panels, reflecting separate red, green, and blue images that combine to make a coloured image on the screen.
The growth in requirement for visual presentations has granted a growth in emphasis on the switching speed of liquid crystals. This has necessitated the manufacture of devices employing smectic liquid crystals, particular types of which possess 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. Inside it the liquid crystal molecules are cast in layers that are perpendicular to the substrate planes, which are differentiated by one or two micrometres, and in the layers the molecules are tilted, 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, likeable 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. Thus, there has to 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 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 resultant change in optical properties can create a change from light to dark if or when one or more polarizers are used.
SSFLC devices have been publicized for larger passive-matrix presentations, but their expensiveness and complexity has impeded them from enjoying any remarkable progress on the market. Small transmissive and reflective active-matrix SSFLC displays, however, have displayed some probability for use as aspects in projection systems or as viewfinders in digital cameras. Their quick reacting allows them to be used in time-sequential colour systems, in which high cost colour filters are replaced by a coloured backlight that flashes red, green, and blue in quick succession (about 100 cycles a second). For example, the liquid crystal can be switched to a transmissive state between the red and green periods but to a nontransmissive state in the blue period, creating the upshot that the eye sees an average of red and green light, or the colour yellow.
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