Key Factors in Choosing An LED System

Managing the heat in an LED light source is called Thermal Management. It is extremely important in an LED system. Excess heat directly affects both short-term and long-term LED performance. The short-term effects are color shift and reduced light output while the long-term effect is accelerated lumen depreciation (or light loss) and shortened useful life.

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Junction temperature is the highest temperature of the actual semi-conductor in an LED. 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 chip. Heat must be moved away from the chip 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 chip to the surroundings (thermal management).

Energy efficiency of light sources is typically measured in lumens per watt (lm/W), meaning the amount of light produced for each watt of electricity consumed by the light source. This is known as luminous efficacy or source lumens.

Luminous efficacy is an important indicator of energy efficiency, but it doesn't tell the whole story, particularly with regard to directional light sources like LEDs. Due to the directional nature of their light emission, LEDs potentially have higher application efficiency or delivered lumens compared to other light sources in certain lighting applications. Why is that? Fluorescent and standard "bulb" shaped incandescent lamps emit light in all directions. Much of the light produced by the lamp is lost within the fixture, reabsorbed by the lamp, or escapes from the fixture in a direction that is not useful for the intended application.

For many fixture types, including recessed downlights, troffers, and under-cabinet fixtures, it is not uncommon for 40-50 percent of the total light output of the lamp(s) to be lost before it exits the fixture. LEDs emit light in a specific direction, reducing the need for reflectors and diffusers that can trap light, so well-designed fixtures can deliver light more efficiently to the intended location.

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All types of electric light sources experience lumen depreciation, defined as the decrease in lumen (light) output that occurs over time as a lamp is operated. See the graph above. LEDs experience far less lumen depreciation over time compared to traditional light sources, making it a very efficient and consistent light source.

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Correlated color temperature (CCT) is the measure used to describe the relative color appearance of a white light source. CCT indicates whether a light source appears more yellow/gold/orange or more blue, in terms of the range of available shades of "white." CCT is given in Kelvins, which is the unit of absolute temperature. You've heard the term “warm white” and “cool white” used to describe fluorescents. This refers to the temperature (CCT) in Kelvin and the light source's place on the Kelvin scale.

Color rendering index (CRI) indicates how well a light source renders colors of people and objects, compared to a reference source. This index has typically been used for traditional light sources (non-LED) but was found to be less significant a measure with LED.

Learn more about color quality of white LEDs

Learn more about color quality and CQS