Developed by the Illuminating Engineering Society (IES) as a method for evaluating and predicting light source color performance, TM-30 turned 10 this year. Discerning lighting designers have begun using it to specify lighting products that more closely match customer needs and preferences. What’s in it for electrical contractors to get to know and adopt its metrics?

Color is a central element of lighting quality. The color qualities of a light source are important to make produce visually appealing in a supermarket or make clothes vibrantly pop in a retail display. When selecting the right lighting product for a project, workers must be able to predict its performance after installation, and, for that, we use metrics. In the case of color, these are traditionally correlated color temperature (CCT) and the color rendering index (CRI). Varying these values can make a big difference in how objects, spaces and people appear to the human eye.

Choosing the right source

For more than 50 years, the current version of CRI has expressed how closely a light source renders colors compared to an ideal reference light source. It always had its limitations, partly mitigated by adding an R9 value for applications where properly rendering saturated reds is important. In the LED era, however, these limitations have become more pronounced, resulting in calls for a more accurate metric.

In 2015, the IES published TM-30, IES Method for Evaluating Light Source Rendition, subsequently amended in 2018 to harmonize with International Commission on Illumination (CIE) standards and provide guidance on how to use the method. TM-30 is designed to address many of the CRI’s limitations, providing more information with greater accuracy.

The method quantifies color fidelity (closeness to a reference) through the Fidelity Index (Rf) (0-100 scale) that is analogous to CRI but is based on average fidelity across 99 color samples instead of 8–14. The higher the Rf score, the more accurately colors will be rendered as they would under the reference light source. As with CRI, an 80+ score is considered “good” for many indoor general lighting applications and a 90+ score “excellent.”

A high number doesn’t automatically mean the light source is better for more demanding jobs, however. Suppose there are two light sources with an equal Rf and CRI but one results in reds visually popping because its spectral emission enhances reds, while the other source is muting reds. In discerning color applications, such as retail and healthcare, this becomes very important.

To predict this, we can use a second color metric in TM-30, Gamut Index (Rg), “gamut” being a term that describes a change in chroma. An Rg greater than 100 means there’s an average increase in saturation, while less than 100 means there is an average decrease. This is valuable to know, as we might have light sources each with an Rf of 90, but where one has an Rg of 110, increasing saturation, while the other has an Rg of 90, which can cause some colors to be muted. By using this second metric, we can more accurately predict how objects and spaces will look.

Even with these metrics, however, there is still the problem of averages. A source with an Rg of 110 may enhance overall saturation, but only certain colors may be saturated, while others may even be muted.

To address this, TM-30 provides a method to produce color vector and distortion graphics that visually depict hue and saturation changes. In the color distortion graphic, for example, colors outside the white circle indicate increased saturation for these colors, while a lack of color (black) inside the white circle indicates these particular colors will be muted. So, if the customer is trying to sell more apples, we would of course want a source that has good or enhanced saturation of reds, indicating ripeness.

Since its first publication, TM-30 has gained some traction among specifiers, manufacturers and building standards such as WELL, though it hasn’t yet achieved mainstream use in North America. In January 2025, the CIE issued a position statement recommending its Rf metric be adopted in all relevant global standards, which may spur adoption.

What’s in it for electrical contractors? Contractors benefit when they bring value to the customer. Using Rf instead of CRI would be a natural first step, as it’s essentially the same metric but a more accurate predictor of color fidelity. As for the rest of what TM-30 offers, its tools would be handy for applications where color is important, such as museums, car dealerships and some hospitality and offices. In these projects, selecting the right solution would be a matter of applying the tools to the customer’s needs using manufacturers that provide TM-30-based information for their products.

TM-30 represents a major leap forward for understanding and predicting color performance. As such, it offers electrical contractors useful tools for customer engagement and support.

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