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 is the difference between indication and illumination?

Indication refers to the use of a light source that is to be viewed directly as a self-luminous object, such as in signs, signals, and indicator lights on electronic equipment. Examples of successful LED indication applications include exit signs (Boyce 1994; Bierman 1995; Bierman and O'Rourke 1998) and traffic signals (Conway and Bullough 1999). Illumination refers to the use of a light source to view other objects by the light reflected from those objects, such as the general lighting found in most rooms, or task lighting found on many desks. Figure 1 shows a typical indicator LED and a typical illuminator LED.

Figure 1. LED typically used for indication (left); LED typically used for illumination (right). Not to scale.

LEDs are quite effective and efficient for colored light applications. Unlike conventional signs and signals which use a nominally white light source and a colored glass or plastic filter or lens to create the sign or signal, colored LEDs require no filtering. The light absorbed by the filters in the conventional products is essentially wasted, and because of this waste, the luminous efficacy of LED signs and signals is often higher than those using conventional white light sources.

Recent technological advances (Nakamura 1999), such as the development of white light LEDs in the mid-1990s, have made LED illumination systems feasible for some applications, and a number of products are now available on the market. At present, typical indicator LEDs have light outputs on the order of one to several lumens, whereas LEDs for illumination produce on the order of tens to hundreds of lumens.

LED lighting systems continue to evolve rapidly (LRC 2003), and specific benchmarks for performance (e.g., luminous efficacy, light output) are being exceeded on a regular basis. Therefore this issue of Lighting Answers focuses on issues relating to the technology of LEDs and issues that are likely to be important in specifying them for lighting applications, rather than statements about the suitability of specific LED packages for specific applications.

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