TM-30 at ArchLIGHT Summit 2021

My colleague Tony Esposito and I will be giving a new TM-30 seminar and demonstration at ArchLIGHT Summit 2021 in Dallas on September 21st and 22nd. We’re working on a new, and we hope more attendee friendly, presentation and an all new set of demonstrations to explain TM-30s Annex E specifications. The demo will include, for the first time, live models of different ethnicities so attendees can evaluate the impact of of the specifications on skin tone. I hope to see you there!

New TM-30 Tutorial Available

Many of us on the IES Color Committee, myself included, have written and spoken about TM-30 and how to use it. I’ve written posts on this blog (click on the color rendering tag to see them all), authored articles, spoken at IES Annual Conferences, given webinars to architects and lighting designers, and assisted manufacturers in adding TM-30 data to their cut sheets. Despite our efforts, and those of others, TM-30 is still not as well understood and broadly implemented as it could be.

A recent issue of Leukos featured an excellent tutorial by Michael Royer of Pacific Northwest National Laboratory. In it, he describes the development of TM-30, color rendering fundamentals, the workings of the TM-30 calculation framework, TM-30 measures and their meaning, and more. That article is now available on the US Department of Energy’s website here. Anyone who’s unsure about TM-30 will find it immensely useful.

On a related note, many members of the IES Color Committee, myself included, can make themselves available to answer questions or present webinars to architects, interior designers, lighting designers, electrical engineers, sales reps, and manufacturers. If you’re interested, use the Contact Jason Livingston link above to send me a message. If I’m not available or the right person for your organization I’ll find someone who is.

TM-30 Is Not Too Hard To Learn!

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.

It’s Time for a Unified Chromaticity Diagram

The pandemic has certainly distracted me from regular posting here.  I’m probably not back to posting weekly, or even monthly, but I do have a new topic and a few things to say about it.  The topic is color science as it applies to lighting.

No doubt you’ve seen something like Figure 1 before.  It’s the CIE 1931 (x, y) chromaticity diagram and is the most common graphic for showing the range of tunable white luminaires and LED colors and their color mixing possibilities. 

CIE 1931 (x, y) Chromaticity Diagram
Figure 1 CIE 1931 (x, y) Chromaticity Diagram

The thing is, we keep using this diagram even though it has problems and has been replaced twice.  The problem is that it isn’t perceptually uniform, which means that the distance between any two color points doesn’t correspond to the perceptual difference between those two colors.  This was famously demonstrated in 1942 by David MacAdam. Using 25 chromaticities he had a trained observer, using a device that allowed for the color adjustment of light, attempt to create a side-by side match from different starting points – for example match a yellow sample starting from green, then match it again starting from red, etc.  When he plotted the results in CIE 1931 (x, y) the area where color differences could not be detected formed an ellipse as shown in Figure 2.  This demonstrated that the color space was not perceptually uniform.  If it was the ellipses would have been circles.

CIE 1931 (x, y) Chromaticity Diagram with MacAdam Ellipses
Figure 2 CIE 1931 (x, y) Chromaticity Diagram with MacAdam Ellipses (at 10x size)

These “MacAdam ellipses” have become the default way manufacturers talk about color consistency of their products.  You’ll often see statements on cut sheets saying that the LEDs for a particular product line all fall within an X-step MacAdam ellipse (2-step, 3-step, etc.).  Want to hear something crazy?  In 2014, the International Commission on Illumination (CIE), which sets the standards for most things related to color and light, recommended ending the use of MacAdam ellipses.  Why?  Look at Figure 2 again.  The size of MacAdam ellipses changes as we move around the chromaticity diagram. So does anything related to them, such as Standard Deviation Color Matching (SDCM) another, although less common, measure.

The first attempt to address the uniformity problem resulted in the CIE 1960 (u, v) uniform chromaticity scale (USC) diagram (Figure 3).  Correlated color temperature was originally calculated in the CIE 1960 (u, v) UCS.

CIE 1960 (u, v) Chromaticity Diagram
Figure 3 CIE 1960 (u, v) Chromaticity Diagram

It was later discovered that the CIE 1960 (u, v) USC diagram also was not uniform.  To improve uniformity the v-axis was scaled by 1.5, resulting in the CIE 1976 (u’, v’) UCS diagram shown in Figure 4.  As the most uniform UCS diagram, CIE 1976 (u’, v’) is the one recommended for use when calculating or evaluating color differences, not CIE 1931 (x, y).

CIE 1976 (u', v') Chromaticity Diagram
Figure 4 CIE 1976 (u’, v’) Chromaticity Diagram

Correlated color temperature was originally calculated in CIE 1960 (u, v).  However, since that diagram is no longer recommended for any purpose by the CIE, we use CIE 1976 (u’, v’) but scale it back to CIE 1960 (u, v).  This is described as CIE 1976 (u’, 2/3 v’).

The CIE’s 2014 recommendation mentioned earlier replaced MacAdam ellipses with a circle in the CIE 1976 (u’, v’) UCS.  A rough rule of thumb is that one MacAdam ellipse corresponds to a circle with a radius of 0.0011. Unfortunately, it doesn’t seem that any manufacturers have made this transition.

So, our industry is in a situation where we commonly use a 90 year old first generation diagram that was replaced 61 years ago.  We calculate CCT in a third generation chromaticity diagram that is 45 years old but tweek the math to refer back to a second generation 61 year old diagram.  It’s crazy!  No other industry uses a system this convoluted.

Why am I mentioning this?  I was recently reminded of a paper that was presented at last August’s IES Annual Conference.  Presented by Michael Royer of Pacific Northwest National Laboratory, it proposed using the latest color science to make a fresh start with a single new chromaticity diagram that is very similar to CIE 1976 (u’, v’) where we would calculate CCT, the color temperature bins for LEDs, color differences and the rest.  IES members can access the archived presentation after logging in to the IES website.

Full disclosure, I’m on the IES Task Group that is developing this new system.  The Task Group is made up of people in academia, design, manufacturing and research from three countries.  We’ve refined our work since August and expect to publish these refinements soon.  I encourage all of you to look for and learn about this proposal, to attend seminars when available, and to weigh in on this topic.  Would our industry benefit from moving to a unified chromaticity system?  Is this the right one?  How do we educate specifiers and manufacturers?  How do we phase in a new system?  We can all have a voice in bringing the science we rely on into the 21st Century.


CIE. (2014). TN 001:2014 Chromaticity Difference Specification for Light Sources. Vienna: International Commission on Illumination.

CIE. (2018). CIE 015:2018 Colorimetry, 4th Edition. Vienna: International Commission on Illumination.

MacAdam, D. (1942). Visual Sensitivities to Color Differences in Daylight. Journal of the Optical Society of America, 32(5), 247-274.

Royer, M. et. al. (2020).  Improved System for Evaluating and Specifying the Chromaticity of Light Sources. In: Illuminating Engineering Society Annual Conference 2020.

Standard 189.1 Now Includes TM-30 Requirements

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: 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. Rf of at least 85
2. Rf,h1 of at least 85
3. Rg of at least 92
4. Rcs,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.

A Look at Bridgelux’s Average Spectral Difference Metric

Today’s post was going to be a reminder to take manufacturer provided education with a grain of salt.  Last week I sat through a manufacturer’s presentation on color.  There were some big errors and some that’s-not-quite-right errors that angered me. The information presented wasn’t hard to confirm, but whoever created the presentation didn’t so some of it was wrong.  However, before I could start writing I received an email about a new color quality metric that was developed by Bridgelux.  Here’s the scoop.

Last Thursday, May 14th, Bridgelux announced a new metric, Average Spectral Difference (ASD), which they claim quantifies the naturalness of a light source.  The announcement is based on this white paper by Bridgelux.  The white paper asserts that since we evolved under fire light and day light, human-centric lighting should use spectra that mimic these “natural” sources.  Bridgelux says that, “ASD provides an objective measurement of how closely a light source matches natural light over the visible spectrum, averaging the differences of the spectral peaks and valleys between a light source and a standardized natural light source of the same CCT.”  

Basically, ASD is a measurement of the difference between a “natural” spectrum and that of an electric light source.  It is expressed as a percentage, with lower percentages equaling a closer match to the reference source and higher percentages equaling a larger difference between the two.

My first thought was, “Oh, it’s CRI – Natural Edition” but in some ways it’s even worse.  For starters, while Bridgelux presents a definition of “natural” light that is based on the illuminants we use as references for color fidelity calculations, there is no accepted definition of “naturalness” in the lighting industry, or most other industries for that matter. Obviously, a metric for something that has no industry-wide definition is of questionable value.  The white paper says, “The reference source used by Bridgelux is the blackbody curve (BBC) for light sources of 4000K and below, and the daylight spectrum (i.e. standard illuminants such as D50, D57, and D65) for light sources of 5000K and above.”  (Yes, there’s an obvious typo there because they’ve left a gap between 4000 K and 5000 K.)  Second, like CRI it presents a single number with no additional information about where in the spectrum the differences occur, or if they are increases or decreases relative to the reference light source.  Third, as a measurement of spectral difference alone, it disregards the fundamentals of human vision, including the principle of univariance and how perception changes with intensity, among other things.  

I emailed a few colleagues on the IES Color Committee and found that they were already examining ASD.  Some of the comments that came back were, “This is just a refresh of a spectral bands method. It says little about color rendering” and “This is very similar to the Film industry’s SSI developed by the Academy. It also suffers from the same problem. If the result isn’t 0% (or 100%) then it tells you nothing about where the differences are. Thus, it tells you nothing about whether two light sources will work together.”

Michael Royer at PNNL went further by looking at ASD with the sets of data in TM-30 Annex F that were used to develop the TM-30 Annex E recommendations.  Here’s what he had to say. (You may have to right click and open the graphs in a new tab to see them clearly.)

First, spectral similarity metrics are not new at all—they predated CRI (e.g., Bouma spectral bands method from 1940s). For some reason they gained popularity again in the last decade or so. Here are some other examples:

B. H. Crawford. 1959. Measurement of Color Rendering Tolerances J. Opt. Soc. Am. 49, 1147-1156

Crawford, B. H. 1963. Colour-Rendering Tolerances and the Colour-Rendering Properties of Light Sources. Transactions of the Illuminating Engineering Society, 28: 50–65.

Kirkpatrick, D. 2004. Is solid state the future of lighting?” Proc. SPIE 5187, Third International Conference on Solid State Lighting.

J. Holm et al., “A Cinematographic Spectral Similarity Index,” SMPTE 2016 Annual Technical Conference and Exhibition, Los Angeles, CA, 2016, pp. 1-36, doi: 10.5594/M001680. Also:

Acosta I, Leon J, Bustamante P. 2018. Daylight spectrum index: a new metric to assess the affinity of light sources with daylighting. Energies 11 2545

Spectral similarity measures, like ASD, don’t relate to perceived naturalness or preference at all. They’re more closely correlated with color fidelity (e.g., Rf) but perform even worse in terms of correlation with perceived qualities because they don’t account for how the visual system works (they might have more use for understanding cameras, as used by SMTPE with SSI, linked above). I guess people just assume that a Plankian/Daylight spectrum is ideal.  While smooth SPDs have advantages, Planckian/Daylight SPDs aren’t perceived as more natural or more preferred in typical architectural lighting scenarios. This has been shown over and over in experiments, where it’s become quite evident that certain deviations from Planckian are preferred/viewed more natural than others.

Here’s the correlation between ASD and rated naturalness/normalness, preference, and Rf for the three datasets used to develop TM-30 Annex E:

 If you’re not up on your statistics, r2 is a measurement of how well data fits to a prediction or to the data average.  1.0 is a perfect fit.  Generally, 0.7 or above indicate a strong statistical correlation, and values less than 0.3 indicate no relationship.

PNNL (combination of three studies):


Penn State:

Overall, it’s clear that ASD isn’t a tool for characterizing perceived naturalness (or preference) over a wide range of SPDs, and it probably has limited other uses. While spectral smoothness (as exemplified by the reference illuminants in ASD) is sometimes a useful goal, there are other metrics more rooted in human vision to better asses this characteristic. It’s a shame that ASD and the accompanying message will likely lead to confusion, especially when there’s enough to learn about color rendition already.

This is a good example of why it’s important to rely on metrics that have been vetted through a standardization process and to always be skeptical of marketing material.

So there you are. Take manufacturer’s education with a grain of salt. The same is true of their internally developed metrics. I’m not saying that they are intentionally deceiving anyone. but their goal is sales, not education. As Mike points out, this is why metrics need to go through a vetting process before we can use on them with confidence.

By the way, although I’ve mentioned the IES Color Committee and quoted a few of its members, this post doesn’t represent the opinions of the committee or of the IES.

LEDucation Update

LEDucation this year is on March 17 and 18 at the New York Hilton Midtown where I’ll be part of two presentations.  The first, at 9 am on Tuesday morning with Wendy Luedtke of ETC, is a seminar called Specifying Color Rendering with TM-30’s New Annex E.  The session presents the new ANSI/IES TM-30 Annexes E and F, which apply recent research to identify three color rendering design intents (Fidelity, Preference, and Vividness) and provides specifiers with TM-30 values to achieve them alone or in combination. Our goal is to increase awareness of Annexes E and F and to help attendees better understand their contents and use. The seminar is most appropriate for people with some prior knowledge of TM-30, although there will be a brief TM-30 overview for those who are new to the topic.

Then, on Wednesday, we’ll be joined by Jess Baker of Schuler Shook for a daylong demonstration of Annex E.  In the TM-30 Demo Room visitors will experience an immersive mockup illuminated with a variety of light sources illustrating the Annex E design intents. The lighting demonstrations will be paired with TM-30 values to show how TM-30 can be used to select light sources for each intent. Visitors will experience sources that meet different levels of the IES TM-30 specification guidelines outlined in IES TM-30-18 Annex E.  We’ll be presenting the demonstration on the hour and half hour from 9 am to 2 pm.

You can register to attend LEDucation here.

Using TM-30 to Improve Your Lighting Designs

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.

Do LEDs Make You Look Orange?

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 Ra, 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 Ra

Since Ra 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 99 color evaluation samples (CES)
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 Rcs,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.  

TM-30 hue bins
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 Rcs,h3, probably with an average chroma shift in excess of 10%.  In fact, the 137 SPDs have Rcs,h3 that 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.