Volume 7 Issue 3
May 2003    
beam angle - The angle at which luminous intensity is 50 percent of the maximum intensity. bin - To sort or classify light sources (such as light emitting diodes) into groups according to their luminous intensity or color appearance. conduction - The process of removing heat from an object via physical contact with other objects or materials, usually metals. convection - The process of removing heat from an object through the surrounding air. cosine distribution - A property of a light source such that its luminous intensity in a particular direction is proportional to the cosine of the angle from the normal to the source. driver - For light emitting diodes, a device that regulates the voltage and current powering the source. heat sinking - Adding a material, usually metal, adjacent to an object in order to cool it through conduction. illumination - The process of using light to see objects at a particular location. indication - The process of using a light source as something to be seen as in signaling. junction temperature - For light emitting diodes, the temperature of the light-emitting portion of the device (see PN junction), which is inversely correlated with its light output. lumen maintenance - The lumens produced by a light source at any given time during its operating life as a percentage of its lumens at the beginning of life. monochromatic - For light, consisting of a single wavelength and having a very saturated color. PN junction - For light emitting diodes, the portion of the device where positive and negative charges combine to produce light. pulse-width modulation - Operating a light source by very rapidly (faster than can be detected visually) switching it on and off to achieve intermediate values of average light output; the frequency and the duty cycle (percentage of time the source is switched on) are important parameters in the modulation. semiconductor - A material whose electrical conductivity is between that of a conductor and an insulator; the conductivity of most semiconductors is temperature dependent. spectral power distribution (SPD) - A representation of the radiant power emitted by a light source as a function of wavelength. substrate - For light emitting diodes, the material on which the devices are constructed. tri-phosphor - A mixture of three phosphors to convert ultraviolet radiation to visible light in fluorescent lamps; each of the phosphors emits light that is blue, green or red in appearance with the combination producing white light.
Do all LEDs have the same lumen maintenance characteristics?

No; lumen maintenance depends on several factors:

  • the LED package, as shown in Figure 1
  • the operating conditions such as ambient temperature or current through the LED
  • the LED color (different semiconductor materials will have different degradation properties; additionally, short-wavelength light will tend to cause more degradation in epoxy materials used to encapsulate the junction element; see Figure 12)

It is also important to note that the performance of a single LED in a system might not accurately represent the performance of the entire system. For example, LEDs near the center of an array might experience higher overall temperatures and therefore experience greater reductions in performance than LEDs near the edge of an array (Narendran and Bullough 2001).

Various colored LEDs have different rates of lumen maintenance. As an example, Figure 12 shows lumen maintenance of red, green, blue and white indicator-type LEDs.

Figure 12. Lumen maintenance of several colors of indicator-type LEDs as a function of operating time.
Illumination-grade LEDs have higher overall lumen maintenance but the relationships among the colors are similar.

The current through an LED is also a large determinant of its lumen maintenance characteristics. Operating LEDs at higher than rated currents accelerates the degradation mechanism by creating higher junction temperatures (Figure 13).

Figure 13. Overdriving phoshpor-based white indicator LEDs and the impact on lumen maintenance.

For some applications such as some decorative applications, lumen maintenance lower than 70% might be acceptable (Narendran and Bullough 2001). Until the industry develops consistent criteria for evaluating life for these types of light sources, the specifier or lighting system manufacturer must consult with the LED manufacturer to ensure that the sources will provide sufficient light output for the specific application.

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