Category: Color

A Brief History of TM-30

Next Wednesday I’ll be giving a presentation on LP-30 at LEDucation (hope to see you there!), and in preparing for it I’ve been reflecting on how far we’ve come in our understanding and evaluation of color rendering.

 

The Color Rendering Index (CRI) was published by the International Commission on Illumination (Commission Internationale de l’Eclairage or CIE) in 1965, primarily to deal with the problem of poor color rendering by early fluorescent lamps.  It was a rather simple system that evaluated a light source by mathematically comparing the appearance of eight colors under the test light in question to their appearance under a standardized reference light source of the same correlated color temperature (CCT).  It used the latest models of human vision, returned a single value that was easy to understand, and was quickly adopted by the industry.

 

CRI is a fidelity metric, meaning that the CRI value represents how closely the test light matches the reference light.  Since this was the only system endorsed by a standards setting body, and became ubiquitous in our industry, many people came to believe that color rendering and fidelity were the same thing.

 

Perhaps unsurprisingly for a new system, errors were found and corrected in the 1974 update.  The next update, in 1995, made only typographical corrections to the document.  Science has, of course, progressed since then with many components of the CRI calculation being withdrawn from use by the CIE as improved components were developed.  The problems were well known and documented.  For example, the IES Lighting Handbook 9th Edition, published in 2000, included a brief discussion about this, and the 10th Edition, published in 2011, included a table describing nine limitations of CRI.  Unfortunately, for what seem to be political, not scientific, reasons CRI hasn’t been updated since.  In fact, as late as 2017 CIE 224:2017 continued to recommend CRI for general use.

 

The “limitations” of CRI were exacerbated by LEDs, whose distinctive spectral shape frequently resulted in CRI values at odds with observed color rendering.  By the mid-2010s the mismatch between CRI values and observations, combined with the clear trajectory LEDs were on to exceed the efficacy of other light sources and become the dominant source, meant something had to change.  Since it was clear that CIE wasn’t going to update CRI, members of the IES Color Committee formed a Task Group to evaluate the issue and develop a completely up to date, color rendering metric drawing on the best ideas other researchers had proposed over the years.  The result, in 2015, was ANSI/IES TM-30 Technical Memorandum: IES Method for Evaluating Light Source Color Rendition, or TM-30.

 

Initially, we thought that TM-30’s Rf (Fidelity Index) and Rg (Gamut Index) would be the key metrics, because they were similar to the CRI-GAI (Gamut Area Index) system developed at Rensselear Polytechnic Institute as an improvement on CRI alone.  Light sources that met certain values were described as Class A under this system.  TM-30 produces many more measurements, 149 in fact. We didn’t know what they would mean or how they could be used, but thought the additional information would, at a minimum, be useful to researchers.

 

However, by 2018 several studies had been published with exciting results showing that by using four metrics instead of two (Rf, Rg, Rf,h1 (fidelity of red), and Rcs,h1 (chroma shift of red) TM-30 would allow users to evaluate a light source not only for fidelity, but for preference and vividness as well.  The analysis of the studies became TM-30’s Annex F, with the explanation of Preference (P), Vividness (V), and Fidelity (F) provided in Annex E, and the TM-30 reports were modified in 2020 to display the four key metrics and the PVF results.  This was a huge improvement over CRI and gave designers the ability to tailor their color rendering goals to the needs of the project and the occupants.  For example, projects such as hospitality benefit from a light source that renders colors in a way that is preferred vs high fidelity.

 

In the meantime, CIE did a more thorough evaluation of Rf (specifically their version of Rf as defined in CIE 224:2017) and in January of 2025 CIE PS 002:2025 recommended the industry transition away from CRI to Rf.  The IES, on the other hand, takes the position that it doesn’t make sense to simply replace one fidelity metric with another.  Instead, they recommend the industry adopt the entire suite of TM-30 metrics, including the evaluation of PVF.

 

The problem for specifiers, at that point, was how to use this information.  What are preference, vividness and fidelity?  How do I find a preferred source, and how do I specify it?  What TM-30 information do I collect/need at each phase of the design?  What language can I put in my specification?  While many of us on the Color Committee (and others) gave seminars and webinars over the years, there are still many who aren’t certain how to use TM-30.

 

To provide a clear answer to those questions the Color Committee formed a Task Group to write a user’s guide to TM-30.  For more than three years a half dozen people wrote, illustrated, re-wrote and re-illustrated ANSI/IES LP-30 Lighting Practice: A Comprehensive Guide to Specifying Color Rendition — Concepts, Criteria, and Implementation.  LP-30 provides the industry with a guide to specifying color rendition with TM-30 by expanding on the framework of TM-30 Annex E. The goal of this document is to help lighting specifiers understand color rendition considerations for a project and appropriately gather, interpret, and apply TM-30 metrics. It provides a thorough description of how color rendition may be considered in each phase of design, explains color rendition goals and color rendition related steps in the design process, related activities that occur during those steps, and examples of the deliverables in each design phase.

 

LP-30 is available in the IES Online Lighting Library and the IES Store.

 

For those of you not able to make it to LEDucation, I will also be giving a presentation on LP-30 at ArchLIGHT Summit in September.  There, we hope to have a color rendering demonstration to illustrate the ideas discussed during the presentation.

TM-30 Users Guide Now Available

I’m delighted to announce that ANSI/IES LP-30 Lighting Practice: A Comprehensive Guide to Specifying Color Rendition — Concepts, Criteria, and Implementation is now available in the IES Online Lighting Library and the IES Store.

For more than three years, a half dozen IES Color Committee members worked to create a clear, concise, easy to understand document.  LP-30 provides the industry with a guide to specifying color rendition with TM-30 by expanding on the framework of TM-30 Annex E. The goal of this document is to help lighting specifiers understand color rendition considerations for a project and appropriately gather and apply TM-30 metrics. It provides a thorough description of how color rendition may be considered in a given phase of design, explains color rendition goals and color rendition related steps in the design process, related activities that occur during those steps, and examples of the deliverables in each design phase.

Get your copy today!

IES LP-30

TM-30 and LP-30 at Lightovation

The IES is about to publish ANSI/IES LP-30 Lighting Practice:  A Comprehensive Guide to Specifying Color Rendition –  Concepts, Criteria, and Implementation, which is essentially a user’s guide to TM-30.  The IES Color Committee worked on it for over two years to create an easy to read, comprehensive, phase-by-phase guide to integrating TM-30 into a designer’s work flow.

On January 11th I’ll be giving a presentation on TM-30 and LP-30 at Lightovation at the Dallas Market Center.  There will be a book signing afterward, with the first 40 attendees receiving a free copy of my Designing with Light, 2nd Edition.  Hope to see you there!

Lightovations 2026 Logo

False TM-30 Reports

I recently received a set of lighting submittals.  In them, one manufacturer had included TM-30 Full Reports.  At first I was delighted, thinking, “Finally, a manufacturer who’s made TM-30 a default part of their documentation!”  Here’s the report.

false tm-30 report

 

My excitement was followed, a few seconds later, by a sinking feeling as I realized that the report was falsified and was a composite of at least two SPDs.  Can you spot the errors?

  1.  The first odd item is the gray boxes around each graphic. Neither the Excel calculator nor the online calculator have those boxes as part of their graphics.  Something’s wrong.
  2. Look at the CVG and notice that the red shape, which represents the color rendering of the test source, very closely matches the black circle, which is the reference light source.  This is the hallmark of a high fidelity light source, and we would expect both Rf and Rg to be near 100.  Why is Rf 91?  Should it be higher?
  3. Look at Hue Bin 1 (red).  Notice that it’s almost touching the reference source circle.  There’s almost no hue shift or chroma shift.  But…
  4. Look at the graph of Local Chroma Shift.  Hue Bin 1 has a chroma shift of -12%, yet the red shape in the CVG is nearly touching the reference light source and is nowhere near the -10% white ring.  These two graphics are not from the same SPD.
  5. Likewise, the Local Color Fidelity of Hue Bin 1 is 80.  Again, the CVG shows almost no hue or chroma shift, so the Rf should be much higher than 80.  These are not from the same SPD.

What’s happened?  I honesty don’t know.  Obviously, someone cut and pasted TM-30 report components from at least two SPDs to create a false report – there’s simply no way the calculator created this from a single SPD.  Was it done out of ignorance or for a purpose?  Well, there’s no reason to cut and paste elements of a report because they’re generated automatically by the calculator.  I don’t know why this one was edited.  I do know that I rejected all fixtures by this manufacturer in this submittal.  I noted that if I they’re falsifying the spectral data I have no reason to believe they’re not doing the same elsewhere – driver into, housing dimensions, CCT, beam angle…who knows what they’re going to ship?

CIE Recommends Transition from CRI to Rf

Earlier this month, CIE published CIE Position Statement on Color Quality Metrics, in which it recommends the lighting industry transition from the outdated and sometimes inaccurate General Color Rendering Index (CRI) to the General Color Fidelity Index (Rf ) defined in CIE 224:2017.  The position statement notes that problems with CRI (which we’ve known about for years) include use of an outdated color space (CIE 1960 (u, v)), the small number of samples used to calculate CRI (only 8), and that CRI has proven to be especially problematic in evaluating narrow band emitters.

The good news is that CIE is finally recommending retiring CRI from use (which was last updated over 50 years ago in 1974) and adopting a modern, accurate metric for evaluating fidelity for all lighting applications.  That’s a huge step forward for the lighting industry.

The less good news is that it stops there. The position statement acknowledges that fidelity is not the only aspect of color rendering, and that studies have shown preferences for light sources that slightly enhance saturation (and therefore reduce fidelity).  However, it makes no mention of other metrics (such as ANSI/IES TM-30’s Gamut Index and Preference Design Intent) that address the issue.  Since CIE 227’s Rf and TM-30’s Rf are identical, I see this as a belated endorsement of Rf as a fidelity metric and of TM-30 in general.  My hope is that this spurs the industry to greater adoption of TM-30, especially for its evaluation of color preference, vividness, and fidelity described in Annex E.

One side note: Rf as defined in CIE 227 is Rf as defined in TM-30.  In fact, TM-30 was published two years before CIE 227, which was a response to TM-30.  In evaluating TM-30, CIE found that there were a few places where CIE and IES chose different methods of extrapolating certain information.  Since CIE had formalized their procedures and IES had not, the two organizations worked together to harmonize their calculations into one calculation that is used in both systems.  It’s frustrating that the CIE position statement reads as if CIE developed Rf out of whole cloth, rather than as a response to TM-30 and a mutual refinement of the Rf calculation.

How are CCT and Duv Calculated? IES TM-40

If you look up the definition of Correlated Color Temperature (CCT) in IES LS-1 you’ll find, “The absolute temperature of a blackbody whose chromaticity most nearly resembles that of the light source.”  It seems straightforward.  The spectra of non-incandescent light sources don’t exactly match a blackbody radiator.  They’ll plot off the blackbody locus in a chromaticity diagram.  A CCT calculation identifies the color temperature closest to the light source in question – that’s the CCT.  We all know that.

Not long ago someone on the IES Color Committee suggested that we develop and issue a standardized CCT calculation.  My first thought was, “What?  Are you saying that after decades of specifying CCT there’s no industry standard calculation method?  What’s going on?”  It turns out that since the late 1930s at least a dozen CCT calculation methods have been developed but none of them have been adopted by a standards setting body like the IES or CIE.   The same is true for Duv, the direction and distance between the chromaticity coordinates of the light source and the nearest point on the blackbody locus.  This means that the method used is up to the LED or equipment manufacturer, and can vary from one manufacturer to another.

Now, for lighting designers this isn’t a problem.  Variations between the calculation methods generally aren’t large (although they can range from less than 1/1000th to several hundred K), and we are selecting LEDs described by their nominal CCT as outlined in NEMA C78.377, not their exact CCT.  Those chromaticity quadrangles are huge, roughly 400 – 500 K wide and over 7 MacAdam ellipses, as shown below. For lighting designers the problem is the huge variation possible within a single CCT designation, but that’s for another day.

C78.377 chromaticity quadrangles
NEMA C78.377 Chromaticity Quadrangles for LEDs

But, for LED manufacturers, testing equipment manufacturers, and researchers this can be a real problem.  If two people attempt to precisely measure the same LED and arrive at two different CCTs how would they determine who’s right, or which calculation is “better”?

Now we have an answer.  ANSI/IES TM-40 IES Method for Determining Correlated Color Temperature (CCT) and Distance from the Planckian Locus of Light Sources describes a CCT calculation method with an error of less than 0.1 K that calculates Duv based on the result of the CCT calculation.  While there’s no way to compel anyone to use this method, it is an American National Standard developed under the ANSI process, making it the closest thing we have to an industry standard.  Finally.

Do you see blue or green? This viral test plays with color perception | The Guardian

Here’s an interesting article about color perception, specifically blue/green perception, from The Guardian.  There’s also a link to a fun web site where you can test your blue/green perception.

Source: Do you see blue or green? This viral test plays with color perception | Well actually | The Guardian

Healing Light at AIA New York

It’s been a while since I’ve posted anything because I’ve been working on a new book.  More on that as we get closer to the publication date.  Meanwhile, I’ve been asked (somewhat at the last minute) to speak on Monday June 3rd at an upcoming AIA New York event called Healing Light: The Biological and Social Effects of Lighting.  The presentation will be an introduction to color temperature and color rendering and is primarily targeted at architects and interior designers who are new to lighting.  Registration is free to AIA members and students, and only $10 for the general public.