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Solid-State Lighting and LEDs

Cree XLamp XP-G LED

Cree® XLamp® XP-G LED

Solid-State Lighting (SSL) uses semiconductor materials to convert electricity into light. Some associate SSL only with with LEDs. In reality SSL is an umbrella term encompassing different types of technologies including Light-Emitting Diodes (LEDs) and Organic Light-Emitting Diodes (OLEDs). More recently, Light Emitting Capacitors (LEC) and Light-Emitting Polymers (LEP)  technologies are emerging.

While some of the technologies are evolving rapidly, LEDs are the more mature technology, particularly for white-light general illumination applications [1].

The best white LEDs are similar in efficiency to CFLs, but most of the white LEDs currently available in consumer products are only marginally more efficient than incandescent lamps. Lumens per Watt (lm/W) is the measure of how efficiently the light source is converting electricity into usable light. The best white LEDs available today can produce about 80-120 lm/W [2]. For comparison, incandescent lamps typically produce 12-15 lm/W; CFLs produce at least 50 lm/W [3].

Unlike other light sources, LEDs don’t “burn out;” they simply get dimmer over time. Although there is not yet  an official industry standard defining “life” of an LED, the leading manufacturers define it as the point at which light output has decreased to 70% of initial light output. Using that definition, the best white LEDs have been found to have a useful life of around 35,000 hours (that’s four years of continuous operation). For comparison, a 75-watt incandescent light bulb lasts about 1,000 hours; a comparable CFL lasts 8,000 to 10,000 hours. LED lifetime depends greatly on operating temperature [4]. An increase in operating temperature of 10°C can cut the useful life of an LED in half; with a good thermal management, the LED life can be increased to 50,000 hours or even more (almost 6 years of continuous operation).


  1. Light-Emitting Diode on Wikipedia
  2. The Most Efficient LEDs and where to get them!
  3. Luminous Efficacy on Wikipedia
  4. Philips – Understanding Power LED Lifetime Analysis

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