Turn the light on: how to test high-power LEDs
No longer used solely as indicator lights, LEDs are now available in high-power formats that have applications as light sources, and this higher intensity introduces new test challenges. As one manufacturer learned, testing high-power LEDs isn’t the same as testing LEDs used as indicator lights.
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 4/1/2007 2:00:00 AM
DEVICE UNDER TEST
High-power light-emitting diodes (LEDs) used in lighting signs, warning lights, decorative lights, and as color backlights for cell phones and laptop computers. Colors include red, blue, yellow, green, and white. Wavelengths range from 470 nm for blue to 636 nm for red.
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READ OTHER APRIL ARTICLES: Contents, April 2007 |
THE CHALLENGE
Measure light intensity and color in prototype lots of several hundred pieces as part of engineering evaluations. Look for uniformity in intensity and color that results from manufacturing processes. Change measurement techniques to accommodate high-power devices. Perform statistical analysis on parts to determine variations in device output.
THE TOOLS
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Instek: power supply. www.instek.com.
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Labsphere: LED integrating sphere. www.labsphere.com.
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Photo Research: spectroradiometer. www.photoresearch.com.
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Wei Min Industrial: LED tester. www.weimin.com.tw.
PROJECT DESCRIPTION
Once just bright enough for use as indicator lights, LEDs now illuminate cell phone and laptop screens and have applications in signs as well. “High-power LEDs have thrust LED manufacturers into light sources,” said Harold V. Anagnos, founder and technology consultant at Lumex (Palatine, IL; www.lumex.com). Anagnos predicts that high-power LEDs will become mainstream in applications such as vehicle headlights and room lighting. “In 20 years, the panels used in suspended ceilings will contain LEDs.”
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Engineers test high-power LEDs mounted inside a sphere that reduces the amount of light that reaches a spectroradiometer. |
Each high-power LED design requires lab measurements for numerous parameters; the most important are dominant wavelength, peak wavelength, luminous flux, and chromaticity (color quality). Because of the higher power, engineers must use different measurement techniques than they use for indicator LEDs. High-power LEDs can easily overload a spectroradiometer—a problem that engineers don’t have when testing LEDs used as indicator lights.
To solve the problem, the Lumex engineers use a 4-in. diameter integrating sphere whose interior reflects 98% of a source’s light. The metal sphere not only blocks outside light from interfering with measurements, but also lets only a small portion of an LED’s light exit through its aperture and reach the spectroradiometer. That reduces the amount of light that reaches the instrument, which eliminates the problem of flooding it with too much light.
The LED under test sits at the geometric center of the sphere (figure). On one side sits the spectroradiometer sensor with a baffle that has the same reflectivity as the inside of the sphere. Prior to using the sphere, the engineers had to run the high-power LEDs at low forward current (IF) to avoid flooding the spectroradiometer’s sensor.
Using analytical software supplied with the spectroradiometer, the engineers can calculate the mean and standard deviation of color and intensity parameters for a fixed input current from which they can characterize the LED. They also use an LED tester to make electrical measurements such as forward voltage (VF), reverse breakdown voltage (VZ), leakage current (IR), difference of VF value between two IF levels (VDF), and transient VF deviation (VFD).
LESSONS LEARNED
Because high-power LEDs are often used to light areas, uniformity in color and intensity are more important than they are for indicator lights. “Color measurements are particularly important for yellow and green because our eyes can sense less than 1 nm differences in color wavelength,” said Anagnos. He concluded, “The customer will check for uniformity visually rather than by making measurements.”
Basics of Spectral Measurement, JETI Technische Instrumente, May 2005. www.jeti.com/down/basics/basics.pdf.
“A Guide to Integrating Sphere Theory and Applications,” Labsphere. www.labsphere.com.
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