LEDs contain a semiconductor crystal that gets excited by a DC power supply, leading to the emission of light in one spectrum (color).
Thus white light can’t be emitted directly: one can use a combination of different colored LEDs making use of a coating. White LEDs emit in the blue or UV specter, phosphor coating makes it appear white.
LEDs turn on instantly, run cooler than alternatives (heating is not required to produce light), and are very small (5 mm). They’re also much less susceptible to vibration, and last almost 50 times longer (typically 50 to 100,000 hours).
Moreover, they can be dimmed, operate silently and require low-voltage power supply (better security).
Bright white LED lights have been the real breakthrough these last years.
Current applications include retail display, exterior lighting and all places where the integration in architectural elements is critical. LEDs are widespread among exit signs and traffic signals.
Organic LEDs (OLED) are currently appearing for displays, such as electronic papers (soft screens).
New, efficient technologies have been developed lately to increase efficiency (lumens per watt or lm/W) and compete with incandescent or fluorescent lamps. For the same comfort, 200 LEDs were required in 1995 but only 18 LEDs today (with 25 lm/W, twice the efficacy of incandescent), with prediction to only 10 soon (according to Philips Lighting).
100 lm/W (equivalent to the best CFLs) has been achieved in laboratories, and further improvement to 150-200 lm/W could be expected for the near future, making LEDs one of the most promising light sources.
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