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Spectral tuning and IES TM-30-15

The coming of age of solid-state lighting and associated smart controls has opened up a world of possibilities. LEDs are more efficient, better for the environment and more easily controlled than other sources. And because they offer greater control over the spectral wavelength, LEDs have stimulated new measures for evaluating color rendering – a longtime goal of the lighting industry.

In 2015, the Illuminating Engineering Society (IES) produced a new system to evaluate and communicate a light source’s color rendering properties. This system, IES TM-30-15, can be used by specifiers, manufacturers and others to identify appropriate light sources and evaluate efficacy and color rendering.

Eaton’s lighting division team talked with Kevin Houser, PhD, professor of architectural engineering at Penn State University and editor-in-chief of LEUKOS, The Journal of the Illuminating Engineering Society of North America, about the new system and how it expands opportunities for spectral engineering. Dr. Houser is a member of the IES Color Metric Task Group, which was primarily responsible for the development of TM-30-15.

What is spectral tuning?

Every electrical or natural light source is composed of different wavelengths of optical radiation. Spectral tuning is our ability, through engineering and product development, to control light at different wavelengths. For example, we can make light fractionally more red or blue or create gaps by omitting optical radiation. Spectral tuning gives us granular control over the location of optical radiation, which affects not just how a source renders colors, but also luminous efficacy.

How does spectral tuning help shape the lighting experience?

Now, when we look at light sources, we can think about them more in regard to their application. For example, fluorescent lights have historically been sold as “commercial grade,” “specification grade” and “premium grade.” They haven’t been sold as “good for residential, hospitality or grocery store settings.” Spectral tuning allows us to match spectrums of light with settings and activities or applications in a more specific, thoughtful, purposeful way.

What is IES TM-30-15?

Released in 2015, IES TM-30-15 is the 30th technical memorandum published by the IES. The short answer is that it is a comprehensive method for evaluating light source color rendition. It includes high-level indices for characterizing color fidelity and color gamut, and a graphic that visually depicts color distortions. For a power user, the system includes many sub-indices that quantify color distortions for different colors such as reds, greens and blues.

IES TM-30-15 has a single calculation engine and one set of color samples that were selected to uniformly sample all common object colors. It can be used to replace or supplement the color rendering index (CRI), which was developed by the International Commission on Illumination (CIE) and has been used as the worldwide standard for evaluating the color rendition of light sources since 1964. The CIE method was never intended to be comprehensive; it was only designed to characterize color fidelity, a measure of similarity to daylight or incandescent light. One problem with the CRI is that it disregards the nature of color distortions in comparison to a reference. For example, we may want a person’s complexion to look ruddier or a bowl of cherries to look redder. But this isn’t captured by the CRI. In fact, any distortion – even one perceived as positive – is given a lower score by the CRI. Within the TM-30-15 system, this is also true for the measure of fidelity, Rf. But rather than stopping here, Rf is complemented with a measure of gamut, Rg, and a color vector graphic (CVG).

In part because CRI is limited to color fidelity, the industry needed a method that takes into account other attributes of color rendition. The IES TM-30-15 method can help us determine how a light source will enhance saturation of or mute certain colors.

IES TM-30-15 includes:

  • Color fidelity – The Rf index quantifies the accurate rendition of colors, or how similar they will be in comparison to a familiar, or reference, illuminant.
  • Color gamut – The Rg index quantifies the average level of saturation relative to familiar, or reference, illuminants.
  • Color vector graphic – These charts provide a visual description of hue and saturation changes.

Below is a spectral power distribution (SPD) for a source composed of multiple narrow-emitting LEDs and the CVG for that source. Inset on the CVG are scores for Rf (top left), Rg (top right), CCT (lower right) and Duv (lower left). Positive Duv indicates a source has chromaticity above the blackbody; negative Duv is for sources below. CIE CRI and R9 are provided below the CVG.

The TM-30 method takes an objective and statistical approach, quantifying the fidelity and gamut of a light source. It is a comprehensive way of evaluating and thinking about color and light.

TM-30-15 provides a designer with the tools to evaluate color rendition, but it does not offer design guidance or application-specific criteria. Still, since the publication of TM-30-15, a few studies have been carried out that relate human perceptions to TM-30-15 measures. 

Here are a few examples of individual use cases:

  • In a candy store with a highly colorful environment, it would be appropriate to employ sources that enhance colorfulness of the merchandise. This would suggest a high value for Rg (e.g., 110, 120 or higher) but only a modest value of Rf (e.g., a score as low as 60 may be acceptable). The color vector graphic will show an increase in saturation for many colors.
  • A physician’s examination room is best illuminated with sources that render skin with high fidelity. The Rf score should be above 90, and the Rg scores should be near 100. The color vector graphic will show minimal distortion.
  • A hospitality setting may be best served using sources that have at least modest fidelity (e.g., Rf of 80 or above) but that enhance chroma (e.g., Rg greater than 100, with a value of 110 or so likely appropriate). The color vector graphic should show a modest increase in red-saturation, consistent with a desirable rendition of ruddy flesh tones.
  • A working environment, such as an office or school, will be best served with sources that have high fidelity (e.g., Rf of 90 or above) and little change in chroma (e.g., Rg of around 100, or slightly greater). It may be equally acceptable to use sources with modest fidelity (e.g., Rf as low as 70) that are designed to enhancement gamut and that are engineered to increase red-saturation. At an Rf of 70, Rg should be in the range of about 110, and the color vector graphic should show an increase in red-saturation.

These are just a few examples, and the specific numbers should only be considered as general guidance. With additional research and input from the design community about what works in practice, it may become possible to move from general guidance to more specific numerical criteria.

Why is accurately quantifying the color rendition characteristics of a light source such a complex issue?

Though it has been the standard for a long time, the CRI frequently provides information many people would consider incorrect. That’s because a measure of color fidelity does not always align with visual perception. A lower CRI source may make objects look very good and may be preferred to a source with a higher CRI. Though counterintuitive, this occurs because CRI simply averages a small number of color distortions without considering whether the distortion enhances or deteriorates saturation. Importantly, this is not a fringe phenomenon, but something that can easily be harnessed with today’s LED technologies.

Ignoring the direction of color distortion, and relying solely on an average, removes nuances and subtleties that are extremely important to human color perceptions. The TM-30 method provides a more comprehensive set of tools, allowing us to more carefully match light sources with applications.

How have modern LEDs enabled and even revolutionized spectral tuning?

Recent advancements in LEDs and controls have been huge for this field, because LEDs are far more controllable than any other light source. In the past, we had to use phosphors, such as inside fluorescent lamps, or even filters to create colored light, but these aren’t energy-efficient solutions. Metal halide fixtures are another existing option, but their chemistry prohibits them from being as tunable as LEDs. Now, we can use multiple LEDs in conjunction with phosphors to create a wide variety of spectral power distributions.

We have the incredible growth of solid-state lighting, a growing spectrum of LED sources, and tools like TM-30-15 to thank for our newfound ability to characterize spectral characteristics and performance consistently. TM-30-15 is a communication tool that helps create a level playing field for innovation in the spectral design of light sources.

What are the main benefits of spectral tuning?

Spectral tuning allows us to more fully understand existing light sources and will help in the design and engineering of applications-specific light sources. It’s a great product development opportunity, especially with the capabilities of LEDs, since attributes of color rendition can be translated into design criteria for new products.

I can imagine a future where, for example, grocery retailers can use spectral tuning to make product packages look a certain way, and product manufacturers can influence the way lighting is designed on the aisles where their packages are displayed. Think about pumpkins in the fall and wreaths at Christmas. Spectral tuning provides the ability to render those products in richer orange and green colors. We’ll be able to go down the aisles and light individual products differently, which can differentiate merchandise and motivate purchases.

What’s next?

IES introduced the TM-30 method in August 2015, so it’s still quite new. I think a lot of education is required in order to ensure we’re making the most of it. It’s a more comprehensive system than the CRI, and that comes with a learning curve. Fortunately, because the system was developed by the IES, the impressive resources and credibility of the IES are behind it.

Taking the time to learn the method is well worth the effort. It provides a common framework such that architects, engineers, interior designs and others have a single method to review, use and rely upon. It’s especially useful for companies that manufacture light sources since they can engineer and market the performance of their products in better and more accurate ways.

It will take push-and-pull over the next few years, as manufacturers introduce new products and customers come to manufacturers with new applications or problems they couldn’t solve in the past. Fortunately, TM-30-15 adds no new photometric measurement requirements, meaning the effort required to start using the included measures should be nominal. Many lamp and luminaire manufacturers like Eaton are already providing TM-30-15 data for their new products, handheld instruments are available to measure and report TM-30-15 data, and numerous software packages are available that will perform the TM-30-15 computations.

The bottom line is that strong communication between manufacturers and customers will lead to light sources and lighting design solutions that wouldn’t have been possible a few short years ago, and that should be exciting to everyone.