Guest post by Jim Brodrick, Department of Energy
Some of you may have come across references to IES LM-79 that imply that the test procedure is flawed or unnecessary or both, and that there’s a better alternative. You may have heard or read terms like “effective lumens” or “equivalent lumens,” or seen sales literature claiming equivalencies to conventional products on the basis of illumination measured directly beneath luminaires, with no information on illuminance measurements at other points. Let’s set the record straight: there is no substitute for LM-79.
In order to understand why, it’s helpful to start at the beginning. What is LM-79? It’s an approved method for taking electrical and photometric measurements of solid-state lighting products, and it was released in 2008 by the Illuminating Engineering Society of North America (IES) after a great deal of hard work and deliberation on the part of the joint IES-American National Standards Institute committee that developed it. You can learn more about it in the DOE fact sheet “Understanding Photometric Reports for SSL Products,” but I’ll give you the basics here.
Why was LM-79 necessary? The complex relationship between LED light sources and the components of luminaires and replacement lamps don’t lend themselves to traditional photometric methods, so we needed a new one. We got one with LM-79, which is based on absolute photometry. Unlike relative photometry, which has been traditionally used to evaluate lighting products and involves separate tests for lamps and luminaires, absolute photometry looks at a luminaire or replacement lamp as a whole, taking into consideration its unique thermal, optical, and electrical properties. This is important, because an LED light source typically can’t be evaluated separately from its luminaire, due to the heat effects. So the actual light distribution and intensity are measured directly from the complete luminaire, instead of being pieced together from separate measurements of lamp and luminaire.
LM-79 looks not only at light output, but also at electrical characteristics, luminous intensity distribution, and color characteristics. (Importantly, the luminous intensity distribution data allows one to calculate fixture spacing, pole spacing for pole-mounted luminaires, or uniformity of illuminance.) LM-79 was created by a dedicated team of experts representing industry, research institutions, and testing laboratories, who truly “sweated the small stuff” in the interest of making the big picture come out right.
For example, LM-79 prescribes the power supply characteristics and electrical instrumentation setup for SSL product testing, and requires that the tested product be operated at its rated voltage. And it allows for two different methods for measuring total luminous flux, either or both of which may be used. One method involves an integrating sphere, which integrates the total light output to produce a single measurement, and the other involves using a goniophotometer to take multiple measurements of luminous intensity around the horizontal and vertical axes, which are then converted and summed as total luminous flux.
Of course, no one is claiming that LM-79 is perfect. What’s more, SSL technology is evolving so fast that things are constantly changing. That’s why an IES committee is in the process of revisiting LM-79 with an eye to updating, refining, and making revisions – as is routinely done with technologies that haven’t yet reached maturity. But there’s no question that when it comes to measuring SSL products, LM-79 works extremely well right now and is the best we’ve got. In addition, it’s crucial at this early stage of the game that those of us who are involved with SSL use the same criteria to talk about these technologies. Otherwise, instead of white light, we might just get white noise.
