NEW LIGHTING COLOR RENDERING METRIC
The Illumination Engineering Society has recently published an new and improved color rendition metric called TM-30. This highly precise and science based method is a step up from it's predecessor method, CRI.
WHAT IS THE COLOR RENDERING INDEX? CRI
Objects don’t have an inherent color, but rather reflect different amounts of energy over the visible spectrum. That means that the spectrum emitted by a light source changes the way objects appear. Color rendering describes this phenomenon. By using color rendering metrics, we can assign ratings to light sources using a set of standardized color samples. Although the objects in a given space won’t exactly match those of the standardized color samples, metrics can give us a good idea of performance and simplify comparisons.
SCIENCE BASED, ACCURATE & MORE SPECIFIC
TM-30 uses state-of-the art scientific advances in color science. As a result, it is more accurate than the CRI, especially for narrow-band spectra where the CRI can fail. TM-30 provides more accurate information such as a fidelity index Rf which replaces the CRI index and indicates whether colors look natural. Lastly, TM-30 also provides more insight into the gamut index Rg which indicates a color's saturation and an accuracy graphic which gives advanced information about the rendering of specific colors.
WHO DEVELOPED TM-30?
TM-30 was developed by the Illuminating Engineering Society’s (IES) Color Metrics Task Group. The Task Group was comprised of seven voting members with backgrounds in lighting research, manufacturing, or specification. The Task Group worked under the IES Color Committee, which subsequently voted to approve the document, along with the IES Technical Review Council and IES Board of Directors. Majority approval was required at each balloting step. In total, more than 30 people voted on the TM-30 document, with more than 90% approving.
WILL TM-30 REPLACE CRI?
TM-30 includes a measure of color fidelity that is analogous to the CIE general color rendering index Ra, which is commonly referred to as CRI. It is possible that TM-30’s Rf will supplant Ra, if it achieves widespread use in the industry. However, CRI was developed by the CIE, whereas Rf was developed by the IES. The CIE is currently developing new color rendering measures, which may or may not replace CRI.
HOW DO CRI AND TM-30 DIFFER?
CRI is the common name for the CIE’s general color rendering index Ra. Like CRI, TM-30 Rf is a measure of average color fidelity; however, TM-30 Rf addresses many of the scientific shortcomings of the older metric, which was first adopted in 1965. Two of the main differences are the color space in which the colors are evaluated (CIE U*V*W* versus CAM02-UCS), and the number and type of samples considered (8 versus 99). Whereas Rf was formulated to have approximately the same scale as CRI, there is usually a difference in scores for individual sources, due to the underlying spectral differences. In particular, sources with narrow spectral features that were optimized for the samples used in CRI typically have lower Rf scores than CRI scores. Similarly, CRI is systematically biased against sources that increase red chroma, which can be particularly detrimental when attempting to identify preferred sources; this is primarily due to the color space used for CRI.
The color space, chromatic adaptation transformation, quantity of color spaces, and type of color samples all contribute to differences in score for Rf and CRI [1, 2]. Each contributes anywhere from minimal to more than 8 points difference in score for any given source.
- Smet KAG, David A, Whitehead L. 2015. Why color space uniformity and sample set spectral uniformity are essential for color rendering measures. Leukos 12(1–2):39–50.
- David A, Fini P, Houser K, Ohno Y, Royer M, Smet K, Wei M, Whitehead L. 2015. Development of the IES method for evaluating the color rendition of light sources. Opt Expr 23(12):15888.