Does higher luminous intensity mean higher light output?

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Volume 7 Issue 3	May 2003

Beam angle - The angle at which luminous intensity is 50 percent of the maximum intensity. 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. 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. 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; 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.

Almost all indicator-type LEDs are rated by their manufacturers in terms of luminous intensity in candelas, rather than light output in lumens. Luminous intensity is a function of the angle from which an LED is seen, so this value should be considered carefully when used to characterize the light output of a particular LED. Two LEDs with the same luminous flux output can have very different peak luminous intensities, if they are designed to produce different beam angles. A narrower beam angle means a higher maximum luminous intensity for the same light output. LED packages are available in a range of beam angles from very narrow (near 6°) to quite wide (more than 100°); most illumination-grade LEDs are rated in terms of light output in lumens.

When using narrow-beam LEDs, it is important to note that small variations in mounting or aiming angle can have a large impact on the appearance of an array of these devices, if the array is designed to be viewed directly.