How to Test LED Lights

White LEDs are now powerful enough to replace many household lamps, but their performance must first be tested.

Ever since significant technological advances in the 1990s made high-power light-emitting diodes possible, the solid state lighting industry has been wrestling with the issues related to LED testing. Each supplier had different methods of testing output power, lifetime, color quality and more. This made it difficult for customers to compare products from different suppliers, and lack of confidence in the stated test results became a barrier to adopting LED lighting.



The Department of Energy, with a charter to help ensure the nation's energy security, stepped in, encouraging industry to adopt standard test methods. In the last few years, the Illuminating Engineering Society of North America (IES) developed two standards for testing, known as LM-79 and LM-80. The two documents outline the comprehensive tests needed to achieve the Energy Star rating.

Things You'll Need

• Electrical power supply
• Mounting hardware
• Spectral radiometric test equipment
• Temperature stabilizing apparatus
• Temperature measuring equipment

Instructions

Testing the LED Chip

1. 

   LED lifetime is tied closely to operating temperature, so monitoring temperature is part of almost all LED tests.

   Attach a temperature sensor directly to the LED case. Maintain the case at 55°C for six- to ten-thousand hours. Monitor the total light output at intervals of one thousand hours or less over that timespan.

2. Using two other identical LEDs, duplicate the measurement in Step 1 at two different temperatures: 85°C and another temperature of choice. This completes the measurement of lumen maintenance.

3. Maintain the case temperature of another identical LED at 25°C. Monitor the output spectrum at least every one thousand hours for six- to ten-thousand hours. This measures chromaticity shift over time.

Testing the LED Luminaire

1. 

   LED lights are supposed to be efficient and effective, but only testing can guarantee they meet their goals.

   Put the LED luminaire (a fancy name for light fixture) on a thermally isolated mount.

2. Place the luminaire in an integrating sphere, and operate it at the manufacturer's recommended drive current. Measure the total light output.

3. As an alternative to Step 2 above, place the luminaire in a goniometer, which allows the output power to be measured at specific angles. Measure the output at a variety of angles, determining both total output power and output power as a function of angle.

4. Divide the total luminaire output determined in Step 2 or Step 3 by the electrical power. This is a measure of efficiency, called efficacy in the lighting industry, and reported in lumens per watt.

5. Measure the optical spectrum of the LED luminaire.

   Use the optical spectrum to determine the chromaticity of the source. Chromaticity is the color the source appears if you turn and look at it.

6. Use the same optical spectrum to calculate the correlated color temperature (CCT). The CCT is more or less a measure of how closely the LED resembles an incandescent bulb. A 100-watt incandescent lamp, for example, has a temperature of 2865K.

7. Again use the optical spectrum, now to calculate the color rendering index (CRI). A CRI of 100 means the source makes colors appear exactly as they do under the mid-day sun.

Tips & Warnings

• The test procedures and calculations outlined here can be found in more detail in the two standards documents: LM-79-08, "IES Approved Method for the Electrical and Photometric Measurements of Solid-State Lighting Products," and LM-80-08, " IES Approved Method---Measuring Lumen Maintenance of LED Light Sources."

References

• U.S. Dept. of Energy: Understanding IES LM-79 & IES LM-80
• U.S. Dept. of Energy: Understanding Photometric Reports for SSL Products

Resources

• ANSI: Method for the Electrical and Photometric Measurements of Solid-State Lighting Products
• ANSI: Measuring Lumen Maintenance of LED Light Sources