LM-85 Light Measurements for High Power LEDs

LM-85 is an Illuminating Engineering Society (IES) approved method for the Electrical and Photometric Measurements of High Power LEDs. The Commission Internationale de L’Eclairage (CIE) has a similar method named TC2-063. LM-85 focuses on the special needs related to getting light measurements for high power LEDs that require heat sinks for their normal operation.   White LEDs and single color LEDs are included.

There are three methods for taking LM-85 Light Measurements for High Power LEDs.

  1. DC
  2. Single Pulse
  3. Continuous Pulse

All three methods including DC require pulsed current drive to the LED under test. Typical pulse specifications for each method might be:

 MethodTypical Pulse WidthNeeded Rise TimeDelivered Power
1DCStep change<10uSContinuous
2Single Pulse10-500mS<100uSLong duration
3Continuous Pulse50-200uS<10uSShort duration

These parameters place high-performance demands on the current source. The two key current source specifications are rise time and the continuous available power. Popular source meter instruments sometimes deliver high peak power levels, but only for short durations. They also lack the fast rise times necessary for the DC and Continuous Pulse methods. These limitations mean source-meters are only appropriate with the Single Pulse method, and only when the LED under test is below the source-meter’s continuous power rating.

The SpikeSafe 400 (SS400) can easily perform any LM-85 measurements The SS400 has no power limitations and it has rise times in the 2-6uS range. The chart below compares popular models from Vektrex and Keithley:

VendorToolMax VoltageMax CurrentContinuous PowerLM-85 DCLM-85 Single PulseLM-85 Continuous Pulse
VektrexSS400-DCP400V2A800WYesYesYes
VektrexSS400-DCP200V8A1600WYesYesYes
Keithley240020V1A20WNoNoNo
Keithley2425100V3A100WNoYesNo

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Jeff Hulett

Jeff Hulett is the Founder and CTO of Vektrex Electronic Systems, Inc. Because of his vision and leadership, the company grew from a start-up to the market leader. Currently, Hulett is the Chief Architect for Vektrex products placing the highest emphasis on technology and quality. Hulett holds a BSEE from the Illinois Institute of Electronics, and has been awarded four U.S.patents. He is a member of the Illumination Engineering Society of North America (IESNA), and chairs the IESNA LM-80 working group that is focused on LED flux maintenance testing. He has been active in the LED testing and reliability field since 2004.

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