New LED Performance Measurements

The Illuminating Engineering Society (IES) has published two new documents related to measuring the performance of Light Emitting Diodes (LEDs). The titles, as well as the aspects that are included and excluded, reveal the complexity of LEDs.

The basic problem is that LEDs typically do not fail the way other lamps do. Instead of a failure that results in the end of light output, LED output fades over time. The result is that at some point, although the LED is still producing light, it is no longer producing enough light for the application so we would say that it has reached the end of its useful life. LEDs have very long lives and relatively short development cycles so it is entirely possible that by the time testing of an LED is complete a newer product has already replaced it. This is compounded by the sensitivity LEDs have to temperature, voltage, and other factors that can mean lab measurements differ greatly from real world measurements. This gives rise to the need for clearly defined testing procedures that reproduce conditions found in typical installations so that designers can rely on the information from the manufacturers.

The first document is LM-84-14 IES Approved Method for Measuring Luminous Flux and Color Maintenance of LED Lamps, Light Engines, and Luminaires. (In the IES naming system LM stands for lumen maintenance, 84 is the document number, and 14 is the year it was issued or updated.) It describes the procedures to be followed in obtaining luminous flux (light output) and color maintenance measurements under standard operating conditions. However, it does not provide information on sampling, or extrapolation of the data for longer time frames.

The second document is LM-85-14 IES Approved Method for Electrical and Photometric Measurements of High Power LEDs, which describes the procedures to be followed in performing accurate measurements of light output of white and colored high-power LEDs. The procedures do not cover LED arrays or modules, AC driven LEDs, among other things.

These two documents join several others that describe the testing and measuring of LEDs. The first is LM-79-08 Approved Method: Photometric Measurements of Solid State Lighting Products, which describes the procedures for testing and reporting of: total flux (light output); color temperature; color rendering index, electrical power characteristics; efficacy (in lumens/watt). LM-79 requires testing of a complete lighting fixture. It does not apply to bare LED packages. LM-79 does not measure the distribution, only the total light output. As a result, it does not provide us with complete photometric performance of the fixture tested.

The next standard is LM-80-08 Approved Method: Measuring Lumen Maintenance of LED Light Sources, which is intended to measure only the LED package, not a complete fixture. LM-80 does not define the end of life for an LED package. It is simply method for determining the light output degradation. LM-80 outlines the testing conditions and the measurement methods that are to be used to measure, track and report the lumen maintenance of an LED package over the course of 6,000 hours. it does not provide a means of estimating life expectancy or light output beyond 6,000 hours.

TM-21-11 Projecting Long Term Lumen Maintenance of LED Light Sources picks up where LM-80 leaves off. (TM stands for Technical Memorandum) It recommends a method for projecting the lumen maintenance of LEDs using the data obtained from LM-80 testing. TM-21 is used to derive L70, which is the number of hours, or life, before the LED package is emitting 70% of the initial lumens. L70 is the number most frequently used by manufacturers as the life, or the useful life, of their LEDs.

DOE Suspends PAR38 L-Prize Competition

On June 13, 2014 the U.S. Department of Energy (DOE) suspended the L-Prize PAR38 Competition. The LED PAR38 products currently on the market fall far short of reaching the rigorous L -rize targets, making it unlikely the DOE will receive a qualifying entry in a reasonable amount of time. The DOE cannot lower the efficacy target because it was set by Congress. The DOE will continue to monitor the PAR38 market for performance and price improvements, to consider reopening the competition at a later date. The graph below illustrates the market vs the L-Prize goals as of November 2013.

More information is available on the DOE web site.

Basking in a New Glow

The New York times has an “I Heart LEDs” article in today’s paper that leaves out some important information about evaluating them. Here are some additional thoughts.

The government hasn’t done a very good job of publicizing or explaining that the Energy Independence and Security Act of 2007 (EISA) set minimum efficiency requirements for general use light bulbs (the act excluded decorative and colored products). The incandescent lamp that’s been around for over 100 years doesn’t meet the energy efficiency standard. Rather than re-engineer incandescent lamps, the lamp manufacturers have focused on expanding and emphasizing compact fluorescent (CFL) and light emitting diode (LED) technologies. Again, you can still purchase 40 – 100 watt decorative incandescent lamps but not A-lamps, the most common shape in use.

The easiest substitution, one that requires no thinking about rewiring, dimming, etc., is the halogen lamp. Halogen lamps are an improvement on standard incandescent lamps, and many of them meet the EISA energy efficiency requirements.

If you’re looking for higher energy efficiency, and are willing to pay a higher price up front to get it, CFL and LED lamps are available in a wide range of wattages and shapes. However, they need to be approached with caution. Both technologies can be difficult to dim, especially with older dimmers that were designed with incandescent lamps in mind, so your existing dimmers may need to be replaced. They can also produce unsatisfactory tints of white light. LEDs are especially notorious for not matching the information provided on the packaging, as demonstrated through the Department of Energy’s CALiPER program.

Here’s what to look for. Every light bulb package should have a Lighting Facts Label that looks like this.

The orange/yellow/white/blue color bar is where you’ll find information about the warmth or coolness of the light, both with an arrow on the color bar and with a number. The number is called the Color Temperature (actually the correlated color temperature) and measures the warmth or coolness in Kelvin. The important thing to know is that a lower number (2700 to 3000 K) is roughly equal to an incandescent light bulb. As the number gets higher the light gets cooler.

Warmth/coolness isn’t the only measurement of the quality of light. Another consideration is how well the light source allows us to see the colors of objects. This is called Color Rendering (Color Accuracy on the Lighting Facts Label) and is indicated by a Color Rendering Index number. Higher numbers (with a maximum of 100) indicate better color rendering, so a light with a Color Accuracy of 95 should be visibly better than one of 80.

The Color Rendering Index is not very specific, however, and is known to misrepresent LEDs. Therefore you are the best, final test of whether or not a given light bulb is appropriate. I recommend purchasing only one or two and trying them out for a few days before committing to changing over your entire house.

My other recommendation is to stick with the major manufacturers (GE, Philips, Sylvania) for most lamps that you test. These companies have a track record of product consistency and quality that many of the newer manufacturers don’t. I can almost guarantee that with an off-brand 5-pack of lamps for $10 you’ll get what you pay for and hate the results. It’s not the technology that you’ll hate, but the manufacturer’s poor execution of the technology.

I hope this helps.