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 is white light made with LEDs?

Presently, there are two approaches to creating white light.

Mixed-color white light: One approach is to mix the light from several colored LEDs (Figure 4) to create a spectral power distribution that appears white. Similarly, so-called tri-phosphor fluorescent lamps use three phosphors, each emitting a relatively narrow spectrum of blue, green or red light upon receiving ultraviolet radiation from the mercury arc in the lamp tube. By locating red, green and blue LEDs adjacent to one another, and properly mixing the amount of their output (Zhao et al. 2002), the resulting light is white in appearance.

Figure 4. Spectral power distributions of several types of LEDs.

Phosphor-converted white light: Another approach to generating white light is by use of phosphors together with a short-wavelength LED. For example, when one phosphor material used in LEDs is illuminated by blue light, it emits yellow light having a fairly broad spectral power distribution. By incorporating the phosphor in the body of a blue LED with a peak wavelength around 450 to 470 nanometers, some of the blue light will be converted to yellow light by the phosphor. The remaining blue light, when mixed with the yellow light, results in white light. New phosphors are being developed to improve color rendering as shown in Figure 5.

Figure 5. Spectral power distributions of early phosphor-based white LEDs (left), and white LEDs using more recently developed phosphors (right) with increased output between 600 and 650 nanometers.

 

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