Quality System
LED KNOWLEDGE
An LED is a very small (dot-sized) electrical device that produces light through the semi-conducting properties of its metal alloys. LEDs have been around since the 1960s, but were used mainly as simple indicator lamps in electronics and equipment. White LEDs are now approaching performance levels that make them attractive for use in automobiles, aircraft, elevators, and some task light applications.
Based on this research, the Alliance for Solid State Illumination Systems and Technologies (ASSIST), a group led by the Lighting Research Center (LRC), recommends defining useful life as the point at which light output has declined to 70% of initial lumens (abbreviated as L70) for general lighting and 50% (L50) for LEDs used for decorative purposes.
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. An increase in operating temperature of 10 C can cut the useful life of an LED in half. When evaluating LED product life claims, ask about the assumed operating temperature and any measures to mitigate heat in the device.
The total luminous flux emitted by the light source divided by the lamp wattage; expressed in (lm/W).
The total luminous flux emitted by the luminaire divided by the total power input to the luminaire, expressed in lm/W.
Three things affect the junction temperature of an LED: drive current, thermal path, and ambient temperature. In general, the higher the drive current, the greater the heat generated at the die. Heat must be moved away from the die in order to maintain expected light output, life, and color. The amount of heat that can be removed depends upon the ambient temperature and the design of the thermal path from the die to the surroundings.
The typical high-flux LED system is comprised of an emitter, metal-core printed circuit board (MCPCB), and some form of external heat sink. The emitter houses the die, optics, encapsulant, and heat sink slug (used to draw heat away from the die) and is soldered to the MCPCB. The MCPCB is a special form of circuit board with a dielectric layer (non-conductor of current) bonded to a metal substrate (usually aluminum). The MCPCB is then mechanically attached to an external heat sink which can be a dedicated device integrated into the design of the luminaire or, in some cases, the chassis of the luminaire itself. The size of the heat sink is dependent upon the amount of heat to be dissipated and the material's thermal properties.
Heat management and an awareness of the operating environment are critical considerations to the design and application of LED luminaires for general illumination. Successful products will use superior heat sink designs to dissipate heat, and minimize Tj. Keeping the Tj as low as possible and within manufacturer specifications is necessary in order to maximize the performance potential of LEDs.
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