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.
What now makes LEDs suitable for illumination applications?

Early LEDs, such as those often used as indicator lights on electronic equipment, created very narrowband, but not quite monochromatic light ranging in color from yellow-green to red. But it was not until the development of AlGaInP and InGaN LEDs with much higher light output than the early indicator lamps, that useful quantities of light could be generated from LEDs. In addition, these materials allowed, for the first time, LEDs with peak wavelengths at any part of the visible spectrum to be made. White light can be made by mixing light from different parts of the spectrum (see also How is white light made with LEDs?).

Larger devices and packages have increased the overall light output of LEDs to levels that are useful for some lighting applications. In addition to increased size of the semiconducting elements, LED construction has also changed to make them more efficient. The crystals forming early LED junctions were grown on light-absorbing substrate materials. Using transparent substrates and optimizing the shape of the semiconducting element have increased the amount of light able to leave the device, as shown in Figure 3.

Figure 3. Improved design of LEDs to increase efficiency.
The substrates are shown as shaded areas. Early LEDs used light-absorbing substrates (left); later, transparent substrates were developed that permitted light to be emitted in additional directions (center); subsequent shaping of the semiconducting elements (right) has resulted in improved efficiency.

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