By BizLED Bureau
Mar 16, 2016: The Color-Rendering Index (CRI) is a preferred tool to showcase the accuracy of a given light source while rendering color when compared to a reference light source. Better color-rendering ability refers to sources’ highlight CRI. It is rated at a 0% to 100% scale. National Lighting Bureau panelists are giving their views on the IES TM-30-15.
Lighting professionals around the globe agree that CRI is a simple yet inaccurate rating system. For example, a ‘high-CRI’ light source that will help consumers to perceive an object’s colors as they originally are, may actually distort the colors. This equation exists because the color-rendering process contains considerations than the ones incorporated in CRI. This widely recognized deficiency led lighting researchers to find a better metric. Industry leaders support the idea while few critics don’t find the find substantial enough.
The Bureau’s annual lighting colloquy and the development of colloquy videos were possible due to Bureau’s sponsors’ support. They include professional societies, trade associations, manufacturers, a labor union, and agencies of the U.S. government, now including:
The new metric is prepared by the Illuminating Engineering Society’s (IES’) Technical Memorandum 30 (TM-30-15), the subject of spirited dialogue among three well-known lighting-industry panelists. They are:
- Mark S. Lien, LC, CLEP, CLMC, HBDP, LEED BD&C (Director, Government and Industry Relations, Osram Sylvania, Inc.);
- Kevin W. Houser, Ph.D., P.E., LC, LEED AP (Professor of Architectural Engineering, The Pennsylvania State University); and
- Mark S. Rea, Ph.D. (Professor and Director, Lighting Research Center, Rensselaer Polytechnic Institute).
“TM-30” sets a computational procedure for evaluating light-source color rendition. The IES – a long-time Bureau sponsor – features TM-30 as a metric that consolidates and synthesizes multiple research efforts going on for decades. It measures a light source’s fidelity (closeness to a reference color) and gamut (increase or decrease in chroma or color purity). It further generates a color-vector graphic that indicates chroma shifts and average hue, to help users interpret the values of Rf (the fidelity index) and Rg (the gamut index). TM-30 includes a software tool for calculating display results for users.
Houser does not see TM-30 is a final answer as it is a significant step forward that manufacturers, researchers, and lighting designers can use now to improve lighting-system specification, research on color perceptions and light-source spectral design. He believes that these groups can help establish criteria for the TM-30 fidelity and gamut measures with experience, and relevant inputs may lead to betterment in the system.
Rea confessed to being “let down” by TM-30, as the metrics have not yet been validated. It is not clear that the proposed metrics substantially reduce uncertainty as what forms good CRI. Dr. Rea went on to state his belief that there is no difference between well-established CRI and the proposed fidelity metric. More seriously, he worries that associated graphics and proposed saturation metric could actually mislead users.
Lien noted that proper application of TM-30 will require education as the new procedure is relatively difficult than CRI. While lighting specifiers will be inspired to learn and use improved color metrics, that’s not likely to be the case for most electrical distributors, electrical contractors, architects, interior designers and others for whom lighting is just one of many outdoor and indoor systems they are dealing with. For them, mastering the increased complexity of TM-30 will require a steep and lengthy learning curve.