How to Measure an LED's Brightness

By Richard Gaughan
If LEDs are to be widely adopted, they must have accurate and consistent test procedures.

The solid-state lighting industry had a problem. It was the early 2000s and solid-state lighting with light-emitting diodes (LEDs) was making great strides in efficiency, color quality, and brightness — but the customers were not showing up. Because the customers were unfamiliar with the new technology, they needed to trust LED manufacturers’ claims about their performance — but everybody had a different measurement method. Finally the industry adopted measurement standards, and LED fixtures — luminaires — are now being actively sought by a variety of customers. One of the most important measurements is light output, which now has a standard governing its measurement.

Preparatory Steps

Put the luminaire on a thermally-isolated mount.

Because LED performance changes with temperature, thermal control is important for any test procedure.

Stabilize the temperature at 25 degrees Celsius. Allow the luminaire and mount to come to thermal equilibrium.

Connect the luminaire to an electrical supply that will provide the operating power specified by the luminaire manufacturer.

Decide on the desired output: total integrated flux (overall brightness) or an intensity distribution (brightness as a function of angle).

Integrated Light Output

A task light that concentrates its output in one direction has different test requirements than an omnidirectional light source.

Place the luminaire mount in the integrating sphere. For directional lamps the mounting should be at the side of the integrating sphere, otherwise, mount the fixture in the center of the sphere.

Place the radiometer detector at a side port of the integrating sphere.

Turn on the luminaire briefly and measure the light output.

Multiply the light output by the calibration factor defined for the specific integrating sphere. This is the total lumen output of the luminaire. The calibration factors will normally be stored within the radiometer during the overall calibration process.

Measuring Intensity Distribution

Place the luminaire in the goniometer mount.

Place the radiometer detector on the measurement arm of the goniometer. The goniometer arm will move the detector around the luminaire, keeping it pointed at the light source at every angle.

Turn the luminaire on briefly and record an output measurement.

A goniometer positions a detector at precise angles with respect to a light source.

Move the goniometer arm to a new angle, wait for the luminaire to cool down, then turn it on briefly and record another output measurement.

Repeat as necessary to measure over the output range of the luminaire. Then add the recorded measurements to determine total lumen output of the luminaire. This method will also provide a measurement of light output for each angle.

Tip

For more details, get a copy of the Illuminating Engineering Society of North America’s LM-79-08 Standard, "IES Approved Method for the Electrical and Photometric Measurements of Solid-State Lighting Products."

About the Author

First published in 1998, Richard Gaughan has contributed to publications such as "Photonics Spectra," "The Scientist" and other magazines. He is the author of "Accidental Genius: The World's Greatest By-Chance Discoveries." Gaughan holds a Bachelor of Science in physics from the University of Chicago.