LED Radiometry – Measuring Total Light Output

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When operated in a forward biased direction, a light emitting diode (LED) emits a monochromatic (single color) light. An LED consists of the light emitting semiconducting material or die, and a lead frame where the die rests. For protecting the die, the LED also comprises the encapsulation epoxy that surrounds the die.

LEDs are now used everywhere. They are used on stereo equipment, toys and laptops as standard indicators, and in traffic lights, automotive lighting and variable message signs. LEDs have witnessed an enormous growth during the last several years.

This growth is mainly due to the increasing brightness levels that have been realized with wafer fabrication processes and new materials, and the development of white and blue LEDs for general illumination and RGB (full color) applications. There is a need for precise measurement of the optical properties of the LEDs, due to the rising levels of sophistication for using LEDs. This article deals with radiometric measurements for LEDs.


The measurement of light, whether in visible, ultraviolet or infrared spectrum, or the total radiation, is referred to as radiometry.

Radiometric optical power (also called as radiant power) is measured in watts (W). Since it is not dependent on wavelength, it is an absolute unit. One watt of visible light has the same power as one watt of infrared light. Radiant intensity measured in watts/steradian, irradiance (W/m2), and radiance (W/m2-sr) are some of the other common measurable radiometric terms. The main method to measure luminous flux/total radiant power is by employing an integrating sphere.

The sphere can measure the LED emitted light in every direction. Usually these measurements are not dependent on the viewing angle, and are accurate in terms of angular measurements, unlike photometric testing. Possibilities of errors still exist. Spheres with 3 inch and 6 inch diameters are mostly used. If accuracy is of importance the larger diameter spheres are chosen, because of the favorable ratio of the area of the sphere, to the size of the LED and ports. The position of the LED inside the sphere is a major source of measurement error. According to the latest specification adopted by the CIE, Publication 127, the entire LED package needs to be inside the integrating sphere, which is known as a 2 luminous flux measurement.

All the precautions required for photometric LED measurements have to be followed for radiometric measurements as well. Similar to photometric conversions, there are various radiometric conversions that can be realized when appropriate information is given. Radiometric values are needed for applications where a photo detector is also utilized, such as scanning, fiber optics or sensing.

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This information has been sourced, reviewed and adapted from materials provided by Marktech Optoelectronics.

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