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NEMA LSCR-PP 1-2015 Light Source Color Rendition, 2015
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- A NEMA Lighting Systems Division Position Paper on IES TM-30-15
- LSCR-PP 1-2015
- Light Source Color Rendition
- Prepared by
- NEMA Luminaire Section
- National Electrical Manufacturers Association
- 1300 North 17th Street, Suite 900
- Rosslyn, Virginia 22209
- Approved: November 12, 2015
- www.nema.org
- NEMA supports the concept of providing information on the color gamut/saturation of LED light sources after international agreement on a calculation method has been reached. At this time, NEMA opposes any mandatory reporting or performance requirements for IES-Rf or IES-Rg. Characterizing color quality is difficult because there are many aspects to be considered. A full evaluation of color quality should include application, color saturation, chromaticity preference, and white light perception in addition to a color fidelity metric, such as the International Commission on Illumination’s (CIE) general color rendering index (Ra). Any single-number fidelity measure (such as Ra or the new Rf) that averages the results of many colors in a light source could possibly have a high numerical value and yet perform poorly with some specific colors.
- The Illuminating Engineering Society (IES) has recently published TM-30-15, “IES Method for Evaluating Light Source Color Rendition.” Similar to other recent proposals (see for instance), it addresses the limitations of CIE’s Ra by defining two measures: a fidelity index, IES-Rf, and a gamut area index, IES-Rg. TM-30-15 also defines a “gamut area polygon” created from the average chromaticity coordinates of 99 color evaluation samples within 16 color regions. The reference illuminant is nearly identical to an incandescent lamp operating at the same temperature. The IES-Rg metric can have a value greater than 100 and yet saturation might be lower than the reference light source for certain colors.
- The IES fidelity and gamut area indices are interrelated. If the fidelity index Rf is 100, then the tested light source is equal to the reference illuminant for all color evaluation samples, and the resulting color saturation Rg is fixed at 100. In this case, there is no freedom to adjust the amount of saturation to enhance user preference. Saturation can be increased above 100 only when the test source is different from the reference illuminant, which inevitably reduces the fidelity index. A fidelity index alone cannot assess the preference for color rendering of objects in a specific environment. In side-by-side tests of light sources having equal chromaticity and Ra, in many cases, observers find light sources with lower fidelity index, Rf, to be more natural. For example, in some applications, people tend to prefer light sources with extra red. There are trade-offs between fidelity and gamut area indices based on any reference illuminant and human color preference.
- IES TM-30-15 is a Technical Memorandum and is an initial step toward new metrics to describe light source color. It is not a required standard, nor does it provide design guidance or criteria for best practices. It presents an invitation to the lighting community to evaluate the proposed IES methods. The IES, in its position statement on color rendering index, “does not endorse mandatory color rendering measures in energy regulations until there is a national or international consensus regarding an appropriate metric and range of values.” The IES method has been proposed to CIE for international adoption. The CIE technical committees must carefully study the IES proposal, which might result in some adjustments of the calculation procedure, color space, color evaluation samples, and reference illuminant.
- It is premature to consider IES TM-30-15 as a mandatory requirement or regulation because the metrics are likely to evolve. Color Rendering Index (CRI) has been with us for 50 years. Whatever the lighting community adopts as its replacement likely will be in use for decades, as well. The industry has an opportunity to develop a significantly improved alternative to CRI. Adoption of a new metric without a thorough vetting process will result in unintended and negative consequences.
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