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What is the Reference Illuminant?

May 11, 2021 in Uncategorized

Over the past few months I’ve had a manufacturer, a sales rep, and a lighting designer all tell me they think CRI compares a light source to daylight. When I tried to correct one of them the reply was an acknowledgment that an incandescent source is normally used, but daylight can be used, too. Given that the lighting industry has been using CRI since 1965, all three should have known better. On the assumption that they’re not alone in their misunderstanding, let’s talk about reference light sources.

The International Commission on Illumination (CIE for Commission Internationale de l’Eclairage) and the Illuminating Engineering Society (IES) both define color rendering as, “The effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant.” In other words, we evaluate the color rendering of a given light source by comparing it to another light source. The other light source is called the reference illuminant.

In 1965 the CIE published CIE 13 Method of Measuring and Specifying Colour Rendering Properties of Light Sources. The current version is CIE 13.3-1995. Its General Color Rendering Index, R_a, is usually referred to as CRI. CIE 13.3 says, “the reference illuminant for light sources with correlated colour temperatures below 5000 K shall be a Planckian radiator, and from 5000 K one of a series of spectral power distributions of phases of daylight.”

So, the reference illuminant must have the same color temperature or correlated color temperature (CCT) as the light source being tested. For all light sources with a CCT below 5000 K we use the spectrum of a Plankian, or blackbody, radiator. For all light sources with a CCT of 5000 K or above we use a CIE model of daylight, again at the same CCT as the light source we’re testing.

Regular readers of this blog know that I’m not a fan of CRI, greatly preferring the increased accuracy and depth of information provided by ANSI/IES TM-30 IES Method for Evaluating Light Source Color Rendition. What about TM-30’s reference light source? It’s nearly the same. For CCTs of 4000 K and below it’s a Plankian radiator. For CCTs of 5000 K and above, it’s the CIE model of daylight. TM-30 avoids CRI’s sudden jump between reference illuminants by using a graduated blend of Plankian radiator and daylight over the range of 4001 to 4999 K.

CIE 224 Color Fidelity Index for Accurate Scientific Use is identical to TM-30’s Fidelity Index (R_f) and uses the same reference light sources.

Tags: color rendering
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TM-30 Is Not Too Hard To Learn!

March 22, 2021 in Color, Design, Education, Light, Lighting Profession

Recently, a well-known lighting designer gave a presentation at a well-known lighting conference. During the Q&A he was asked his opinion of TM-30 and replied that it was too hard so he just specified CRI>90. At the risk of sounding like a jerk I have to say that maybe it was too hard for him, but it’s not too hard for most of us. Here is a brief list of new things lighting designers have had to learn over the years.

The introduction and transition to electronic ballasts and transformers meant that we had to learn about reverse phase dimming and control protocols.
The T5 lamp meant we had to change our layout patterns to accommodate lamps that weren’t standard 2’, 4’, and 8’ lengths.
Metal Halide lamps, especially PARs, meant that in exchange for energy savings we had to learn about the color rendering of a new type of lamp, and give up dimming.
Daylight harvesting and daylight responsive designs meant we had to learn about daylight zones, photosensors, and daylight harvesting control systems.
White LEDs meant we had to learn about another light source and its specific pros and cons, including different color rendering properties due to its SPD.
Circadian lighting means we are all in the process of learning how and when to apply the most current scientific evidence to certain project types. Since the science is constantly advancing on this topic, we must be aware and continue to educate ourselves.
Regularly updated energy conservation codes mean that as we begin to memorize the lower LPDs and changes to control and daylighting requirements, we have to relearn that information because it changes every three years.
Most recently, we’re supposed to enthusiastically embrace IoT, adding new hardware and controls to our lighting control systems.

There is a ton of TM-30 educational material available, including posts on this blog here, here, here, here, here, and here. There’s this article on the IES’s FIRES Forum, and this page on the Department of Energy web site. Manufacturers are also providing education including DMF Lighting, Soraa, Premier Lighting, Alphabet, and Lighting Services Inc. Then there are the articles in trade magazines and sites such as Lux Review and Architect Magazine, not to mention many articles in Lighting Design and Application and Leukos (no links because they’re behind the IES login). In addition, there have been presentations at other conferences (some given by me) at the IES Annual Conference, LightFair, and LEDucation.

If that’s not enough for you, let me know. I have a presentation approved for one AIA HSW LU, so if you’re architectural firm wants to learn more let’s set up a presentation. Ditto for lighting design firms and teachers of lighting. If I’m not available there are a half dozen others on the IES Color Committee who regularly give TM-30 presentations. You can learn TM-30. I’m here to help.

Tags: color rendering, TM-30
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Standard 189.1 Now Includes TM-30 Requirements

August 19, 2020 in Codes, Color, Documentation, LEDs, Luminaires, Sustainability

Yesterday an addendum to ANSI/ASHRAE/ICC/USGBC/IES Standard 189.1-2017 Standard for the Design of High-Performance Green Buildings was published. The addendum makes changes to Section 8.3.5, which covers lighting. One of the biggest changes is to add TM-30 color rendition criteria to the section on Indoor Lighting Quality. Here’s the relevant text:

8.3.5.3 Color Rendition. At least 95% of lighting power of nominally white lighting within each enclosed space shall be provided by luminaires that meet the following criteria at full light output in accordance with IES-TM-30, Annex E, P2 and F3:
1. R_f of at least 85
2. R_f,h1 of at least 85
3. R_g of at least 92
4. R_cs,h1 of at least -7% but no greater than +19%
Nominally white lighting is lighting that has chromaticity within the basic or extended nominal color correlated temperature (CCT) specifications of ANSI C78.377.
Where a lighting system is capable of changing its spectrum, it shall be capable of meeting the color rendition requirements within each nominal CCT of 2700 K, 3500 K, 4000 K, and 5000 K, as defined in ANSI C78.377, that the system is capable of delivering.

I hope that this is going to put more pressure on manufacturers to improve the color rendering of their luminaires as measured by TM-30, not CRI, and to provide TM-30 information on their cut sheets. If not, they’ll risk not being considered on projects that have TM-30 requirements.

Tags: color rendering, LED color, TM-30
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Using TM-30 to Improve Your Lighting Designs

February 6, 2020 in Color, Design, Lighting Profession

Recently, ANSI/IES TM-30 was improved with the addition of Annexes E and F. Annex F reviews and summarized five studies that explored using TM-30 metrics to predict subjective visual outcomes. Annex E uses that research to establish recommended specification criteria when the designer’s color rendering goals are Preference, Vividness and/or Fidelity.

The IES Forum for Illumination Research, Engineering, and Science (FIRES) has an article I wrote with Michael Royer and Tony Esposito explaining the Annexes and how to use the information in Annex E. Here’s the link: Using TM-30 to Improve Your Lighting Design – Illuminating Engineering Society

I’ve been using Annex E on projects and have spoken to other designers who have begun to use it. It provides useful, accurate information that allows me to evaluate the color rendering results of light sources in a way that hasn’t been possible until now. It lets me make informed decisions about my projects, and explain those decisions to colleagues and stakeholders in (relatively) easy to understand terms.

TM-30 and the TM-30 calculators continue to be a free download from the IES here. Annexes E and F are also free on the Errata and Addenda page here and here.

Tags: color rendering, TM-30
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Do LEDs Make You Look Orange?

September 16, 2019 in Color, Design, LEDs

Last Thursday Donald Trump spoke to a group of Republicans in Baltimore. One of the things he said caught my attention: “The lightbulb. People said what’s with the lightbulb? I said, here’s the story. And I looked at it, the bulb that we’re being forced to use, No. 1, to me, most importantly, the light’s no good. I always look orange. And so do you. The light is the worst.”

Now, I’m not aware of being made to look orange under LEDs, nor have I ever noticed LEDs making my friends, colleagues, or students appear orange. You can’t imagine how embarrassed I’d be if it turned out that a real estate developer and entertainer had more astute color perception than me, a lighting designer and Co-Chair of the IES Color Committee. If our only means of evaluating the color rendering of a light source, and evaluating the orange content specifically, was CRI we would have no objective way of testing his statement. CRI, technically R_a, is a single value that gives us an average of the match between the light source in question and its reference source (either a blackbody radiator or a CIE definition of daylight, depending on CCT) using only eight color samples.

Colors used to calculate CRI R_a

Since R_a is an average value there’s no way to understand the rendering of any particular hue. I’ve talked about this here. However, one of the wonderful things about ANSI/IES TM-30 IES Method for Evaluating Light Source Color Rendition is that we can use it to test that claim. TM-30 uses 99 color samples that are distributed across the color space and the visible spectrum.

TM-30 CES spectral reflectance functions

It also breaks the color space up onto 16 Hue Bins, each one covering a specific range of the color space. In the case of orange, we want to look at Hue Bin 3. Specially, we want to look at R_cs,h3 (the subscript CS stands for Chroma Shift) which quantifies the increase or decrease in the saturation or vividness of orange compared to the reference light source.

Example of TM-30 chroma shift bar graph by hue bin

So, let’s put the science of TM-30 to work and see if we really do know that LEDs make us look orange!

The TM-30 calculator contains a library of 300 SPDs (spectral power distributions), of which 137 are commercially available white LEDs. The CCTs range from 2776 K to 6123 K. If white light LEDs really did make us look orange we’d expect to see a large majority of them have a positive R_cs,h3, probably with an average chroma shift in excess of 10%. In fact, the 137 SPDs have R_cs,h3that range from -8% to 1% with an average of -3.6%, a decrease (not an increase) in the saturation of orange. It’s not me, it’s him. TM-30, which uses the most modern models of human vision and a set of colors that cover the color space and visible light spectrum, proves it. What a relief!

Don’t believe me? Download TM-30 and the calculator for free from the IES web site and see for yourself.

Of course, I’m not saying LEDs are perfect light sources. Like any other product there are good ones and bad ones. However, TM-30’s measurements of fidelity and gamut (as averages) and measurements of fidelity, chroma shift, and hue shift (by hue bin) permit us to make a thorough evaluation of a light source to understand its color rendering characteristics. Using this knowledge, we can determine if a particular light source distorts colors and is appropriate for a project, or not.

I should take a moment to note another error he made when he said, “And very importantly—I don’t know if you know this—they have warnings. If it breaks, it’s considered a hazardous waste site. It’s gases inside.” Perhaps you’ve heard the acronym SSL or the phrase solid state lighting. LEDs are a version of SSL, which means that they are…well, solid. Unlike previous light producing technologies LEDs are a solid combination of materials. As such, if one were to physically break (which is unlikely since LEDs are small, are mounted to a heat sink and often covered with a lens, so you’d have to break a lot of materials simultaneously) no gas, hazardous or benign, is emitted. He’s thinking of fluorescent lamps and the small amount of mercury they contain. Even then, a broken fluorescent lamp doesn’t turn the area into a” hazardous waste site.” Here are the EPA’s instructions for cleaning up a broken fluorescent lamp.

Tags: Color Perception, color rendering, LED color
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Misunderstanding CRI

April 8, 2019 in Color

Last Friday I took my class on a visit to a fixture manufacturer’s showroom. The visit was pretty successful, but I had one issue with the information that was presented. This manufacturer’s rep presented their CRI 80 and CRI 90 products by saying that CRI 80 dulls colors and CRI 90 makes colors “pop”. I can’t blame him too much, after all it’s a common misconception that higher CRI is “better.” However, it’s not true so let’s take a look.

CRI (or more formally, CIE 13, Method of Measuring and Specifying Colour Rendering Properties of Light Sources, R_a) is a fidelity metric. That means it calculates the color rendering of a light source in comparison to the color rendering of a reference light source of the same color temperature or correlated color temperature (CCT). A light source with a CRI 80 renders colors with more color error (that is, a larger mismatch or a larger color appearance distortion) than a light source with a CRI 90. That’s all. One of the problems with CRI, which is addressed in TM-30, is that a single number value doesn’t tell us the hue(s) where there is a color rendering error compared to the reference light source, nor do we learn the direction or the degree of color rendering error(s). In other words:

What hues are not rendered accurately? CRI doesn’t tell us.
Are those hues made to appear more or less saturated? CRI doesn’t tell us.
Are those hues shifted toward an adjacent hue? CRI doesn’t tell us.
How big are the color distortions? CRI doesn’t tell us.

TM-30 (ANSI/IES TM-30-18 IES Method for Evaluating Light Source Color Rendition) does give us this information, which immediately puts to rest the notion that higher fidelity is “better” color rendering in all cases.

It’s entirely possible for a light source with a CRI 80 to render a set of colors more vividly than a CRI 90 light source if the color errors increase saturation and minimize hue shifts. It’s even possible for two light sources of the same CRI to render colors differently. Here’s an example. The first light source has a TM-30 R_f(fidelity) of 90 and an R_g(chroma) of 99, meaning that on average colors are rendered slightly less vividly than the reference light source. The TM-30 Color Vector Graphic shows us clearly that the rendering of red (Bin 1) is less saturated than the reference, and that the rendering of warm blue (Bin 12) is more saturated. The other colors are a nearly perfect match to the reference source.

The second source also has an R_f91. However, the green and purple hues are rendered with increased saturation so that it has an R_g105. (Yes, the CCTs are different, but that doesn’t matter because in the calculation a light source is compared to a reference light source of the same CCT, cancelling out any color errors due to CCT.)

Understanding this information opens the door to considerations other than fidelity. The first is vividness. Are you lighting the M&M store in Times Square? If so, your design goal may be to increase saturation of the candy, not accurately render it. In that case you’re going to want a lower fidelity (R_f) so that you can get higher chroma (R_g). The light source shown below might be just the one for this application.

The second is preference. Studies have shown that in many applications people prefer slight increases in chroma, especially in the red range. Are you lighting a restaurant? If so, and if preference and increased red chroma are important, this might be the light source for your project:

The increased information TM-30 provides is both more accurate and more detailed than CRI. Not only that, it gives us a deeper understanding of the color rendering capability of a light source and allows us to consider design goals other than fidelity. Designers who care about these color considerations need to keep pushing manufacturers to provide TM-30 information and train their employees in its meaning and use.

Tags: color rendering, TM-30
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TM-30 and Daylight

August 6, 2018 in Daylighting

An architect recently emailed me asking if it was possible to use TM-30 metrics with daylight. My short answer was something like, “I suppose you could, but why would you?”

The long answer is that all of the TM-30 measurements (Rf, Rg, the 16 chroma shifts, the 16 hue shifts) are relative – comparing the light source in question with the reference light source. With daylight, you’d be comparing the daylight SPD you captured at a moment in time with the CIE definition of daylight at the same CCT. On an average day I doubt that any of the measurements would deviate from 100 by more than a couple of points. So, using TM-30 (or CRI Ra) is like measuring a ruler with another ruler. You’re essentially comparing one thing to a definition of itself.

Tags: color rendering, CRI, TM-30
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Specifying Color Quality With TM-30

June 11, 2018 in Calculations, Color, Design, Education, Light

By now most of us have attended one or more seminars or webinars about IES TM-30 and understand that it is a method of measuring various color rendering properties of a light source and reporting those measurements. The thing that’s been missing is a recommended set of values that set minimums, maximums and/or tolerances for the various measurements. This has been true for two reasons. First, TM-30 is a method and as such was never intended to set recommended values. The second is that while the science behind TM-30 is solid, the science doesn’t offer any predictions of acceptability.

Good news! After almost three years of research and tests around the world we’re much closer to establishing a set of recommended values. At this year’s IES Annual Conference in Boston, Tony Esposito, Kevin Houser, Michael Royer and I will be presenting the seminar “Specifying Color Quality With TM-30” The description of the seminar is, “This presentation will discuss several research projects which have used the IES TM-30 color rendition framework, and whose results have been used to develop various specification criteria. We will discuss UFC 4-510-01, The Department of Defense Unified Facilities Criteria for Military Medical Facilities, which has already implemented IES TM-30-15 specification criteria.”

During the seminar we’ll review some TM-30 basics, look at several research projects that are helping to establish TM-30 thresholds, and review how to use the TM-30 calculator. Don’t miss it!

Tags: color rendering, TM-30
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Design for Color and Illumination Webinar April 19th

April 2, 2018 in Color, Education, Lighting Profession

On April 19th Wendy Luedtke (my co-chair of the IES Color Committee) and I will be presenting a free IES webinar called Design for Color and Illumination. Here’s the blurb from the IES site:

When developing a lighting design, lighting specifiers determine the lamp and fixture combination that best suits the design’s requirements based on many factors. While some considerations are largely technical, such as power consumption, the amount of light generated, and how light is distributed, one consideration is both technical and artistic and can be approached in a number of ways. Participants to this webinar are eligible for one (1) IES Continuing Education Unit (CEU).

The webinar is at 12 noon EDT. Register here.

Tags: Color Perception, color rendering
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I’m Presenting at LEDucation 2018

January 22, 2018 in Color, Education

This year LEDucation, the largest LED only trade show and educational forum in the U.S., will be on March 13 and 14. On Tuesday the 13th from 11 am to 12:30 pm I’ll be presenting a forum called How To Use TM-30, along with Dr. Michael Royer of Pacific Northwest National Laboratory and Wendy Luedtke of ETC. Here’s a summary of the event.

This presentation discusses a number of design trends that are currently shaping the industry and promising to redefine the role of lighting as we know it. New technologies are making dimensions such as color and dynamic behavior over time viable for main-stream lighting, and that constitutes exponential change. This presentation explores a few of the dimensions that appear to be gaining traction, and attempts to illuminate some of the reasoning behind their development. Implications to the user experience are discussed, along with the inadequacy of present lighting metrics.

See you there!

Tags: color rendering, TM-30
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