What is the difference between LCD and LED?

Difference Between LED & LCD

Liquid crystal display (LCD) technology was developed in the 1970s---think digital wristwatches---and its use in televisions and computer monitors means thinner profiles and larger screens. Light-emitting diode (LED) is a newer LCD technology.

Technology

Understanding the basic functions of LCD and LED technology provides the basis for understanding the differences between them.

LCD technology used in televisions and computer monitors functions by light shining through a layer of crystals in a liquid solution. The crystals twist, when voltage is applied, to allow varying levels of light to pass through red, green and blue filters. The lighting comes from a cold cathode fluorescent lamp (CCFL).

LED technology is a type of LCD in which multiple diodes---tiny semiconductor tubes---rather than fluorescent tubes provide backlight. LED lights turn off and on independently, allowing control of where light passes.

LED screens use white lights passing through color filters, or red, green and blue lights (RGB LED) with no filter. RGB LED controls light in groups---or banks---producing an effect known as local dimming. Edge-lit LED screens---where light shines from the sides rather than from behind---do not have local dimming abilities, but since the light source surrounds the panel rather than coming from the rear, screens can be thinner.

Contrast and Color

Standard LCD technology does not produce pure black, since crystals create black by blocking light, not turning it off. So, some light always leaks through, softening the appearance of black to dark gray.

LED screens with local dimming, however, give the appearance of pure black by totally turning off light in the black areas. This ability also makes colors on LED screens appear brighter. RGB LED televisions control color distribution, potentially displaying a broader color range.

Viewing Angles

LCD usable viewing-angle technology improvements allow for clear viewing at about 30 degrees off center. Beyond 30 degrees, contrast degradation (the ability to discern black/white and color contrast) distorts or completely blocks the view. Back-lit LED viewing angles correct contrast degradation, allowing much wider viewing options.

Refresh and Response Rates

The speed at which LCD crystals or LED diodes emit or block light determines how fast images move and change on the screen. Refresh and response rates, however, are not affected by differences in back-lighting technology.

Burn-In

Image burn-in happens when a specific image remains on the screen for long periods, "burning" patterns onto the screen. LCD and LED technologies are immune to burn-in.

Size/Weight

The smaller LED lighting system allows a two-thirds reduction in the thickness of a standard LCD and, therefore, LED monitors and televisions are flatter and weigh less.

Environmental Effect/Price

The lighting processes of standard LCD screens use mercury, which negatively impacts the environment when screens are discarded in landfills. LED technology does not use mercury. In addition, edge-lit LED televisions consume about 40 percent less electricity than standard LCD televisions, although some local dimming LED models use more electricity than ordinary LCD models.

As of early 2010, LED televisions cost about 50 percent more than a comparably sized LCD television.


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