LED TECHNICAL DESCRIPTION
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, this occurs 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, although an LED like any diode 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 display-type LEDs often have a built-in magnifying lens to focus the light in a tight beam for visual applications. 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.