Commentary | Jan 20, 2014

"Beyond LM Testing - A New Metric for Quality over Lifetime" by Thor Scordelis

In a recent blog post, the United States Department of Energy’s Jim Brodrick noted that solid-state lighting standards and testing methods “may not be glamorous, but they’re essential for the success of any technology, and SSL is no exception.” Certainly, this is true with respect to lumen quality, and for many years, the LM-80 standard, used to measure lumen maintenance of LED packages and modules, has sufficed. However, with the growing adoption of LED technology has come higher customer awareness and expectations. “Good enough” isn’t good enough anymore.

Color maintenance, another element of light quality, has also been undergoing greater scrutiny. As customers moved from halogen to metal halide accent lighting, they assumed that the color between lamps would remain relatively consistent until the lamp stopped working.

However, “lifetime” was conveniently being redefined to mean lumen lifetime, not necessarily quality of light lifetime. Digging into a data sheet may have yielded an esoteric note about lumen life of, say, 12,000 hrs. and a color life of less than 8,000 hrs. But few would go to this trouble. Quality was sliced off the datasheet under the rationalization that customers wouldn’t know or care. And then the lamps shifted color or failed well before expected, or, despite being rated a premium “90” CRI, it turned out that they weren’t really “90” after all.

Enter “C”
The lighting industry is familiar with the convention for describing the lumen maintenance of a LED luminaire or light source. The “L” designation stands for the percentage of light output in lumens that remains at a point of time in the future, e.g., “L70” represents 70% lumens. For many applications, however, useful life is best measured not by lumen maintenance, but rather by color maintenance, and locating the color maintenance specification for an LED source can be difficult. Many manufacturers don’t provide this information, or they use different notations, such as “5 SDCM” or “+/- 200K in CCT over life.”

Enter the “C” notation. Devised as a simple, real-world alternative to other color maintenance specifications, the “C” notation stands for a light source’s future color point, compared to its original color point at the date of manufacture, as measured in CIE 1976 color space. In layperson terms, it’s a metric for color maintenance.

For example, “C3 50,000hrs” is 0.003 ∆ u’,v’, or a distance of 0.003 as measured on the CIE 1976 color space, compared to its original color point, at 50,000 hrs. The CIE 1976 color space is used because the distance between points on the diagram is proportional to the perceived color difference, which is a significant advantage. The U.S. Environmental Protection Agency (EPA) “EnergyStar” program also uses CIE 1976, and most lighting academics worldwide agree that it is the most appropriate color space to use for measuring color change.

Similar to the metric that enables customers to compare two sources’ lumen life – e.g., L70, 35,000 hours vs. L90, 25,000 hours – adoption of the “C” metric provides a common yardstick for comparing the sources’ color maintenance over time.

Thanks to the Illuminating Engineering Society of North America (IESNA),
we have tools to back up data sheet lumen claims. These include the aforementioned LM-80, as well as TM-21, which establishes a standard way to use LM-80 data to make consistent lumen lifetime projections beyond the testing period.

For color consistency, there’s no industry-accepted projection standard akin to TM-21, so some individual manufacturers use their own projection methodologies. Without a standardized color maintenance projection methodology and results, examining the warranty is the best way to determine whether a manufacturer values the source’s ability to maintain color over time.

Adoption of the “C” notation will allow the industry to better move forward with educating customers on the basic aspects of light quality. It’s a simple device, yet grounded in established science, and enables a true comparison of products.

Thor Scordelis
Thor ScordelisThor Scordelis has more than 20 years of experience leading lighting design, engineering and marketing teams. Prior to his current role, Thor was PG&E’s Emerging Technologies-Lighting Portfolio Manager where he led the identification and assessment of lighting technologies to determine market viability and readiness. Thor was a steering committee member and contributor to the development of California’s Strategic Lighting Plan.

This article was published in the LED professional Review - Issue 40 (Nov./Dec. 2013) on page 4.

FEATURED

Lumileds RGBW Module Achieves Breakthrough Flux and Color Control

Lumileds RGBW Module Achieves Breakthrough Flux and Color Control Lumileds, a global leader in innovative lighting solutions, today introduced the Luxeon MultiColor Module 2.5 W, a compact RGBW module that produces leading flux and achieves exact color points in architectural and general outdoor lighting applications. The 4-in-1 module simplifies system design by ... Read more »

FEATURED

American Bright introduces Smart RGB Addressable Rope Light Solutions

American Bright introduces Smart RGB Addressable Rope Light Solutions American Bright brings to the market their smart RGB+IC LED flexible rope lighting solutions allowing for boundless application opportunities. The smart LED light source includes the controller and RGB chip in one package allowing for each LED to be addressable. Additional adaptability is gained ... Read more »

FEATURED

Goniometer LGS 650: Higher Accuracy and Reproducibility

Goniometer LGS 650: Higher Accuracy and Reproducibility Goniophotometry is an absolute measuring procedure that can achieve a higher level of accuracy than sphere photometry. It does not require a luminous flux standard, but in contrast is often used for calibrating standard lamps for luminous flux. In addition to the luminous flux, photometric spatial ... Read more »

FEATURED

Cree® XLamp® High Current LED

Cree® XLamp® High Current LED The XLamp High Current LED Array family is optimized for best-in-class lumen output, efficacy and reliability at high drive currents. XLamp CMA LEDs share the same package design and LES sizes as Cree's industry-leading CXA2 Standard Density LEDs, enabling lighting manufacturers to address a range ... Read more »

EDITORIAL

High Conductive Foils Enabling Large Area Lighting

High Conductive Foils Enabling Large Area Lighting Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP as one of the leading partners for research and development for surface technologies and organic electronics and Sefar AG, a leading manufacturer of precision fabrics from monofilaments, developed a roll-to-roll ... Read more »

page_peel