Four Steps to an Absolute Spectral Radiant Flux Measurement

Four Steps to an Absolute Spectral Radiant Flux Measurement

Lorne Loudin, Product Manager

As we learned in my last article, there are two basic light measurement instruments used to determine the luminous flux of a test lamp, photometers and spectroradiometers. A spectroradiometer allows you to directly measure the spectral radiant flux of a light source then apply the photopic response to the spectral data and calculate a highly accurate lumen value. In this article, we’ll take a quick look at the four steps needed to make an absolute spectral radiant flux measurement using an integrating sphere spectroradiometer.

When using an integrating sphere spectroradiometer for light measurement, we are using the substitution method. This means that we are comparing the output of the spectroradiometer with a test lamp in the sphere to the output of the spectroradiometer with a lamp of known spectral flux in the sphere. The substitution method works because the ratio of detector signals is equal to the ratio of lamp flux.

Whenever we place a light source in an integrating sphere, the throughput of the integrating sphere changes due to self-absorption. So, if we want to make an accurate spectral radiant flux measurement, we need to account for the changes to the integrating sphere throughput when we substitute our DUT for the standard lamp in the sphere.

properly designed light measurement integrating sphere spectroradiometer will include an auxiliary lamp. An auxiliary lamp is used to correct for the self-absorption of the reference and test lamps as they are swapped into the sphere. If we put all of this into the form of an equation, we get the following:

Spectral Radiant Flux Equation with Auxiliary Correction

Where:

Terms for the spectral radiant flux equation

If we consider the equation above we want to solve for the flux of the test lamp, and we know the flux of our standard lamp (this value is reported on the calibration certificate of a spectral radiant flux standard lamp). That leaves us with four terms to measure, or the four steps to an absolute spectral radiant flux measurement.

  • Step 1: Measure the signal from the standard lamp. Power on the standard lamp, allow the lamp to stabilize, and record the spectrometer signal from the standard lamp.
  • Step 2: Measure the signal from the auxiliary lamp with the standard lamp in the sphere but powered off. Turn off the standard lamp, power on the auxiliary lamp, allow the lamp to stabilize, and record the spectrometer signal from the auxiliary lamp.
  • Step 3: Measure the signal from the auxiliary lamp with the test lamp in the sphere but powered off. Substitute the test lamp in place of the standard lamp and leave it powered off, power on the auxiliary lamp, allow the lamp to stabilize, and record the spectrometer signal from the auxiliary lamp.
  • Step 4: Measure the signal from the test lamp. Turn off the auxiliary lamp, power on the test lamp, allow the test lamp to stabilize, and record the spectrometer signal from the auxiliary lamp.

That’s it, those are the four basic steps to making an absolute spectral radiant flux measurement. Labsphere’s light measurement systems are equipped with our powerful Integral software. Integral automates this process for you and prompts you every step of the way.

Before I close, here’s a quick note on spectral radiant flux standard lamps. A good spectral radiant flux standard lamp is calibrated in a laboratory accredited in accordance with an international standard such as ISO 17025, and the lamp’s data are traceable to a national metrology institute.

To learn more about light measurement using an integrating sphere spectroradiometer check out our light measurement 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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