Light Measurement Using an Integrating Sphere

photopic response curve

Lorne Loudin, Product Manager

One of the main tools for light measurement is an integrating sphere. An integrating sphere captures all the radiation from a light source at the same time. When it comes to light measurement, there are a few major characteristics of a lamp that manufacturers and end-users find most important. These are the number of lumens, the color, and the efficiency (how many lumens of output per watt of energy). Lumens are determined by weighting the spectral radiant flux by the photopic response of the eye. In this article we’ll take a quick look at the two basic types of instruments used to determine luminous flux, photometers and spectroradiometers.

A photometer is a light measurement instrument that directly measures light in accordance with the photometric system. A photometer requires the use of a filtered detector that approximates the relative spectral response of the human eye. The associated spectral response is often referred to as the CIE luminous efficiency function, the V-lambda function, or more commonly, as the photopic response curve.

Spectroradiometers, like our IllumiaPlus2 systems, allow you to directly measure the spectral radiant flux of the light source then apply the photopic response to the spectral data and calculate a highly accurate lumen value. The ability to obtain spectral information from the integrating sphere spectroradiometer is advantageous for several reasons: 1) better lumen calculation, 2) calculation of chromaticity coordinates, 3) calculation of color rendering indices.

A spectroradiometer is your best option to accurately determine the luminous flux of a light source. When using a spectroradiometer you have all the spectral data at your fingertips. The spectral data are easily converted to yield important color properties such as chromaticity, correlated color temperature, and the color rendering indices. On the other hand, when using a photometer with a filtered detector a perfect simulation of the CIE luminous efficiency function is not possible and leads to measurement error. To learn more about light measurement using an integrating sphere spectroradiometer check out this video series where we demonstrate the procedure using our IllumiaPlus2 light measurement system.

Labsphere is the industry leader in light measurement and integrating sphere design. We look forward to learning more about your application challenges.


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