Lighting Research Center Lighting Research Center
    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.
How do the color rendering characteristics of LEDs compare to other light sources?

Typical phosphor-based white LEDs have color rendering index (CRI) values comparable to discharge lamps (fluorescent and high intensity discharge) used in many lighting applications. Common belief is that high CRI means good color rendering properties. CRI is actually an index of how similar a light source makes colors appear in comparison to a reference source such as incandescent (which is why an incandescent lamp has a CRI near 100). For mixed-color LED systems, CRI is very sensitive to the wavelengths of the component colors; however, this sensitivity is not necessarily representative of one’s preference for color appearance in an actual application. Recent studies have shown that mixed-color white LED systems with CRI in the 20s can result in higher color preference than systems with CRI in the 90s (Narendran and Deng 2002b). Knowing the limitations of CRI, international standard-setting bodies are working to explore newer metrics for better characterizing the color rendering properties of all light sources, including LEDs.

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