A Light Emitting Diode (LED) is a P-N junction of semi-conducting materials that emits light from the passage of an electric current. Technically, light is produced when the electron-hole pairs re-combine. The particular material chosen for the junction has optically active impurities added, a process called doping. Types of material at the junction, and their doping, will determine the dominant wavelength of light produced for a monochromatic LED. Aluminum Indium Gallium Phosphide (AlInGaP) is used for red, amber and orange; Indium Gallium Nitride (InGaN) is used for green, blue, or cyan. The junction of an LED can be degraded by excessive temperature, usually only when it is operating.
White LEDs are somewhat different; they typically have a yellow phosphor layer added above the junction of a blue-emitting LED. The yellow phosphor is excited by the blue emission to produce yellow light. The yellow light is combined with the blue light leaking through from behind, and since yellow combines red and green, provides the red-green-blue stimulation to satisfy our three retinal receptors to appear as white light. This type of LED is sometimes known as "phosphor converted" or PC, and may utilize phosphor colors other than yellow. The phosphor in a white or PC LED can be degraded by excessive temperature even when the LED is not operating.
Conventional light producing devices firstly produce heat, whether by electric filament or high voltage ionized gas, and their light production is, in simplest terms, a bi-product of heat. Light from an LED is not a bi-product of heat, but like any diode, an LED will produce heat as a side effect of electric current flow. The light from an LED is a phenomenon that occurs when electrons and holes re-combine during the flow of electric current. Thermal management to maintain proper safe operating temperature is very important for LED longevity.
Unlike a conventional epoxy diode package, the LED is packaged with a lens beside the junction to permit light to escape. Most LED packages have internal reflectors around the junction to collect and direct the maximum amount of light through the lens.
Small 5-millimeter display-type LEDs often have a built-in magnifying lens to focus the light in a tight beam for visual applications.
Some specifiers consider them as "the LEDs used in toys".
High power LEDs typically provide a wide angle Lambertian pattern exceeding 120° beam width, and usually require secondary (external) optics to further shape the light for a particular application.
LEDs have several advantages over other lighting devices. The LED is a solid state device having no filament to heat, or gas to ionize; therefore; no glass envelope, no gas or high voltage electrodes, and no mercury. LEDs are virtually immune to shock and vibration. They are very well suited for high vibration environments and extremely low temperatures where other light sources are short-lived and costly to replace. LEDs are truly a "green" product.
In theory, in a perfect LED with its junction temperature maintained well below 25°C, one might suggest the LED is a never ending light source. However, in the real world, the materials and doping cannot be perfect, and LEDs are almost always operated with junction temperatures well above 25°C. Naturally, heat will eventually alter most materials (even perfect materials). The LED luminous efficacy is inversely proportional to heat, and unlike any other light source, the LED produces more light at colder temperature, about 25% more at -40°C than at 25°C. They thrive and last much longer in extremely cold environments. The cooler they operate the happier and more efficient they are.
LEDs don't "burn out", they last the longest of nearly any other light source. Their output merely deteriorates to some unacceptable level; it is said, they fail gracefully. The typical monochromatic LED junction in a proper design, operating cooler than 75°C can provide 100,000 hours of operation to approximately 70% of its initial luminance. White or PC (Phosphor Converted) LEDs in a proper design, typically operating lower than 85°C will provide in excess of 50,000 hours of operation to approximately 70% of their initial luminance.
Since an LED primarily produces light, it is the most efficient full-spectrum light source we have (in 2010), with 110 lumens per watt efficacy when its junction temperature is maintained close to 25°C. Thus, the overall system design controls efficacy and lifespan, and can provide the longest lasting and most efficient light source.