Show Your True Colors: Employ the TM-30 method to find the right light

Published On
Oct 15, 2020

From ensuring that merchandise looks vibrant in a retail store to including proper flesh tones in an office space, color quality is a key specification characteristic for light sources. For electrical contractors, understanding light source color rendering and using it with accuracy and precision can differentiate you from the competition.

To do this, we need metrics. A common metric is correlated color temperature (CCT), which describes the color appearance of the light source and its emission on the Kelvin (K) scale, typically characterized as “warm,” “neutral” or “cool.” Another is color rendering index (CRI), a measure of color fidelity that averages eight pastels and six optional colors to express how natural colors appear under the given source.

For decades, CRI was considered good enough for typical applications using traditional light sources. A CRI of 80+ was considered good for many commercial general lighting applications, while a CRI of 90+ was better for color-critical applications such as retail and hospitality.

With the proliferation of LED lighting, previously accepted flaws in the CRI metric became accentuated. With LED, you can have two products with the same CRI but very different color-rendering abilities. In other words, different effects on finishes, furnishings and flesh tones. To address this, in 2015, the Illuminating Engineering Society in New York produced TM-30, “IES Method for Evaluating Light Source Color Rendition,” a proposed method for color evaluation. The idea behind this method was more information and greater accuracy.

TM-30 has three main outputs. The first is fidelity index (Rf), which is similar to CRI but averages 99 color samples and is limited to a 0–100 scale; it’s considered more accurate than CRI. Gamut index (Rg) expresses average color saturation. This addresses situations where you might have two light sources with the same CRI/Rf, but one accentuates reds while the other mutes them. An Rg greater than 100 shows an average increase in color saturation, while less than 100 shows a decrease. The third output is particularly useful: color vector and distortion graphics that get past averages to visually reveal exactly which colors are saturated and muted.

Why is saturation important? An important study by Pacific Northwest National Laboratory, Richland, Wash., confirmed that saturation, notably in the red spectrum, is a key attribute of human color preference. Study participants entered a space filled with objects under a 3,500K CCT. As spectral power distribution changed, they were asked to rate each scene. The researchers learned that color preference—what a typical person would regard as a “pleasing” light source—correlated with somewhat high fidelity (>74 Rf) coupled with oversaturation (>100 Rg), particularly red.

So if you’re looking for greater accuracy, information and control of color rendering, including saturation, TM-30 is generally superior to CRI. CRI continues to persist, however, as it is simple and well known. While some companies quickly adopted TM-30 to optimize their products, other manufacturers and many specifiers have lagged. Some specifiers only use TM-30 for discerning applications, recognizing that CRI remains suitable for a significant number of general lighting applications. Some adapt CRI by ensuring a sufficient R9 (saturated red, one of the optional CRI colors) rating.

CRI also remains institutionally embedded in the industry. While TM-30 is represented in the Well Building Standard and will be reported for products complying with the new 5.1 version of the Medford, Mass.-based DesignLights Consortium’s technical requirements (used to qualify products for many utility rebate programs), CRI is also used in these programs in addition to others such as Energy Star and local codes such as California’s Title 20 efficiency regulations.

For example, with a few exceptions, the Well color rendering option requires all lighting to have a CRI >90; a >80 CRI with an R9 >50; or have a TM-30 average Rf of at least 78, an average saturation rating of at least 100, a red chroma saturation of -1% ≤ red chroma shift (Rcs), and an h1 ≤ 15%. TM-30 will benefit from broader representation in codes, standards and programs.

Clearly, CRI still has game, though TM-30 certainly offers compelling reasons for use. For contractors, it enables close and accurate discrimination between lighting products, which can serve as a point of differentiation and as a way to better service customers and projects requiring a discerning level of color quality. TM-30 value is arguably proportional to the value of a project’s finishes and furnishings, ensuring the light source renders their colors exactly as they should and that mixed sources more closely match.

Overall, TM-30 offers a superior method for predicting, evaluating and talking about color, offering ECs who specify light sources a potentially powerful tool for serving customers looking for just the right light.

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