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 January 1, 2005 Q: Can you explain what LCD means, and what an LCD television is? A: LCD stands for liquid-crystal display. In their most elementary forms, LCDs are simple devices used in the displays of many calculators, cell phones, and the like, mostly to communicate alphanumeric information. The LCD’s operative principle is that of polarized light. As in some sunglasses, light passing through an LCD is polarized to have a particular orientation. Anyone who has experimented with polarized glasses has observed this effect: If you hold one lens in front of another so that light passes through both, then rotate one lens, the light will eventually be almost totally blocked. In the first position, both lenses are oriented the same way and light passes through both. As you turn one lens, more and more of the light that passes through the first is blocked by the second, until -- at 90 degrees -- almost nothing gets through. An LCD polarizes light the same way, but as you apply electricity to it, it "twists" the polarization, the angle depending on the amount of current. An LCD placed in front of a fixed polarized panel will allow varying amounts of light to pass through, varying in step with changes in signal level. In a direct-view display, the LCD arrangement is placed in front of a backlit panel; in an LCD projector, an external light source and lenses are used. An active matrix LCD display contains thousands of individual pixels, arranged in rows that duplicate the scan lines of a regular television, and color clusters that duplicate the phosphor dots in a cathode-ray tube (CRT). However, instead of an electron beam sweeping back and forth, as in a CRT, the LCD’s tiny transistors -- one for each pixel -- vary the opacity of each element in sequence, creating the picture as it lets through, to varying degrees, the light from behind. The direct-view version of this technology is familiar in laptop computers, camcorder monitors, and the like. LCDs are also used in front and rear projectors. Because, unlike CRTs, they are not light sources but modify the light from an external lamp, sort of like a photographic slide projector, LCDs are not subject to the burn-in -- the permanent visibility of a repeated image -- that plagues CRT and plasma displays. On the other hand, LCDs are relatively poor transmitters of light, much of which they reflect back to the bulb -- simply cranking up the wattage can ultimate destroy the panel. The dimness of the original LCD projectors, which used single panels, has been largely cured by the use of multiple panels, one for each of the three primary colors. Unlike CRT front projectors, however, these are aligned in an LCD projector by means of mirrors or prisms so that, optically, the three panels are in the same virtual plane, and can be focused and zoomed as if they formed a single panel. The first LCD front projectors were far more flexible than their CRT equivalents because they didn’t need the critical alignment of multiple tubes. Like a slide projector, an LCD unit could simply be aimed at a screen and focused with a simple twist of the lens. The image could even be zoomed to fit the screen. Those early LCD projectors had low resolution, however, with easily visible individual pixels. That has largely been dealt with by refining manufacturing techniques to make much smaller pixels -- an absolute must in digital television, especially of the hi-def variety. ...Ian G. Masters
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