The color quality of light sources is critical in a broad range of applications, from making colors pop in retail merchandise to promoting social interaction by properly rendering skin tones….
The color quality of light sources is critical in a broad range of applications, from making colors pop in retail merchandise to promoting social interaction by properly rendering skin tones.
To evaluate, predict and discuss color quality, the industry relies on two metrics, correlated color temperature (CCT) and the color rendering index (CRI). Varying these metrics can dramatically change the visual appearance of objects and spaces.
CRI, a standard developed by the International Commission on Illumination (CIE), expresses color fidelity, or how closely a source renders colors compared to an ideal light source. Despite its limitations, CRI last received a major revision in 1974, with much of the science behind it going back to 1937.
The proliferation of LED lighting, with its unique characteristics, accelerated demand for a new and improved metric. In 2006, CIE began working on one but hasn’t achieved consensus. In 2013, the Illuminating Engineering Society (IES) formed the Color Metrics Task Group, which developed TM-30-15, IES Method for Evaluating Light Source Rendition.
This extraordinary method creates new metrics, backed by the latest color science research, that provide greater accuracy and more information about color rendering. These metrics are intended to be used alongside CRI, be revised based on industry feedback, support CIE’s efforts, and ultimately replace the CRI metric.
TM-30 introduces three major tools:
• Fidelity Index, which expresses color fidelity or rendering;
• Gamut Index, which expresses average color saturation; and
• color vector and distortion graphics, which indicate relative saturation and muting of individual colors.
Color fidelity (rendering)
The Fidelity Index (Rf) (0-100 scale) is analogous to CRI (Ra) but is based on average fidelity across 99 color samples instead of eight. This provides more accuracy while incentivizing manufacturers to design light sources that optimize average color rendering across a broader palette instead of just eight test colors.
As with CRI, a score of 100 means the light source renders all colors as well as the reference source, assuming they have the same CCT. With 99 colors, however, and with manufacturers previously incentivized to optimize color rendition of CRI’s eight colors, many traditional light sources may have a lower Rf value than CRI. A triphosphor fluorescent lamp, for example, could have a CRI of 86 but an Rf of 80.
A major limitation of relying on a color fidelity metric alone is it doesn’t cover color distortion. We could have two light sources with the same Rf and CRI but where one results in reds visually popping because its emission enhances reds, or the other lamp is desaturating (muting) that color.
To address this, a second color metric, Gamut Index (Rg), is used. This metric expresses the extent of average color saturation or desaturation compared to the reference source. If Rg is higher than 100, the light emission is producing an average increase in saturation; if lower, an average decrease.
The below graphics, courtesy of the Department of Energy and Randy Burkett Lighting Design, provide a simulated example. The two graphics on the right indicate how a space looks under light sources with the same CRI but with different levels of saturation, which causes reds to visually pop on the far right.
Individual color distortion
While average gamut is useful, it is often important to know which colors are saturated or desaturated, not just the average. For this, we use the third major tool offered by TM-30, which is color vector and distortion graphics.
Looking at the color distortion graphic for a sample light source with an Rf or 81 and an Rg of 101 (Department of Energy), colors outside the white circle have increased saturation, while a lack of color (black) inside the circle indicates desaturation. This light source, on average, enhances saturation, though it specifically saturates blues and other colors while muting reds and other colors.
And here we see the space shown earlier with color vector graphics for the light sources used:
For those who want to take their analysis further, TM-30-15 offers additional indexes including skin fidelity (Rf,skin), fidelity by hue (Rf#), chroma shift by sample (Rf,CES#) and fidelity by sample (Rf,CES#).
TM-30 is a major advance in the lighting industry, providing more accurate and informative tools allowing lighting professionals to predict, evaluate and communicate color in their projects. Now comes adoption. It remains to be seen how well accepted these new metrics will be by specifiers, manufacturers and owners.
Click here to view/listen to an archived Department of Energy presentation on how TM-30 was developed and how to apply it.