Tag: color rendering

CRI Has One Job. That’s the Problem.

Below is a slightly expanded version of an article I wrote for this month’s IES NYC newsletter.

Color rendering should be an intentional design decision, not a specification checkbox.  I was recently in what should have been a beautiful hotel bar.  The interior was relatively new and skillfully conceived and executed, the materials were luxurious, and the lighting design should have been beautiful. Unfortunately, the overall look of the space was flat and dull.  It didn’t take me long to figure out why – poor color rendering was muting the colors in the space.

I could tell that the SPD, while warm (roughly 2400 K) was still deficient in red and probably had a CRI in the low 80s.  The lighting designer probably relied on CRI, which isn’t up to the job in this application.  CRI only tells you one thing about a light source: fidelity, or how closely it matches the reference light source.  This means the designer can have only one color rendering goal.  TM-30, on the other hand, supports a broad range of possibly overlapping color rendering goals.  The lighting designer then balances the metrics, evaluating of preference, vividness, and fidelity to achieve specific goals.

Start with Blue to Make Red

Most of us know that white light LEDs start with a blue LED as the “pump”.  Phosphors packed on top of the blue LED convert a large portion of that blue light to longer wavelengths.  Converting high energy, short wavelength blue light to lower energy, longer wavelength light comes at the expense of efficacy.  The more blue light converted to green, yellow, orange, and red, the lower the LED’s lumens per watt.  Since most manufacturers prize efficacy over color rendering, the skimp on red compared to the reference light source, resulting in low CRI.

Figure 1 is a TM-30 report for a typical 80 CRI LED.  At the top right hand corner the P3 V- F- means its color rendering meets Priority Level 3 for Color Preference (P3) so it is slightly preferred over a high CRI source, that it doesn’t increase the vividness of colors (V-), and that the color rendering is not very close to its reference source (F-).  In the graphic, the red arrows pointing toward the center of the circle indicate less energy in those color ranges than in the reference source.  This results in the muting or graying of all object colors in those ranges, including the skin of the people in the room.

TM-30 report of a typical CRI 80 light source
Figure 1 – TM-30 report of a typical 80 CRI light source.

 

Figure 2 is a TM-30 Simple Report of a typical 90 CRI LED.  Even here, there’s slightly less red than in the reference light source. P2 indicates that it is broadly preferred over a high CRI source and F2 indicates that it has good color fidelity.

 

TM-30 report of a typical 90 CRI light source
Figure 2 – TM-30 report of a typical 90 CRI light source

 

Use the Tools

So, using a 90 CRI LED instead of an 80 CRI LED would improve things, and many designers and manufacturers are defaulting to 90 CRI these days.  However, there’s another problem – the Hunt effect, which tells us that as illuminance decreases, so does perceived object colorfulness.  The bar was lit to somewhere between 5 and 10 fc, so even under a high CRI source the colors in the room would be muted compared to their appearance at, say, 50 fc.  CRI leaves us powerless to do anything about this, but TM-30 gives us the tools.

Instead of looking for a high CRI source, the designer could have used the PVF measures from TM-30 to find a high preference source, as shown in Figure 3.  A light source that measures P1 (highly preferred compared to the reference source) has a spectrum that increases red saturation, which would enhance the colors of materials in a way that people prefer over high fidelity – and possibly somewhat negate the Hunt effect.

 

TM-30 report for a typical high preference light source
Figure 3 – TM-30 report for a typical high preference light source

 

When attempting to counter the Hunt effect, a light source that also increases the overall vividness of colors (V3 or V2) would do an even better job.  This is just one example of how switching to TM-30 and broadening your color rendering goals can enhance your lighting designs.

Don’t Trade One-for-One

One final note.  In January of 2025 the CIE recommended the lighting industry transition from CRI to Rf, which is a much more accurate measure of fidelity.  My position, and that of the IES Color Committee, is it makes no sense to swap out one fidelity measure for another.  Designers would still be limited to a single color rendering goal.  We’d like the industry to adopt the full TM-30 suite.  Designers can ignore the 149 calculated values and focus on the PVF categories as I’ve done here.

For a full explanation of the color rendering goals possible with TM-30 and Preference, Vividness, and Fidelity, see ANSI/IES LP-30 Lighting Practice: A Comprehensive Guide to Specifying Color Rendition – Concepts, Criteria, and Implementation in the IES Online Store or the Online Lighting Library.  Despite its long title it is an easy to read, phase-by-phase user’s guide to TM-30. From schematic design to construction documentation, this TM-30 user’s guide outlines important color quality considerations and how to incorporate them into your workflow.

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.

Upcoming TM-30 Presentations

I’ll be giving two presentations in the next month.

On September 8th I’ll be giving an online presentation to the Raleigh Section of the IES.  Simplifying, Understanding, and Applying IES TM-30 will be from 12:30 to 1:30 eastern.  I’m going to set aside the color science that normally opens a TM-30 presentation to make more time for Annex E and the PVF specification system.  Admission is free.  Register here: https://www.eventbrite.com/e/simplifying-understanding-and-applying-ies-tm-30-tickets-695339287527?aff=oddtdtcreator

On September 19th and 20th Tony Esposito and I will be presenting Designing with TM-30, our third TM-30 presentation at ArchLIGHT Summit in Dallas, TX.  This 90 minute presentation starts with Tony talking about color science as it applies to TM-30, then I discuss integrating TM-30 into the lighting design workflow, and we wrap up with a 30 minute demonstration of TM-30s capabilities with the best mockup I’ve ever seen (see below for  Tony’s photos of last year’s mockup).  Register here: https://archlightsummit2023.sched.com/

TM-30 demonstration showing how varying the SPD, even within a single CCT, affects color appearance.

TM-30 Update: Challenges and strategies for working with SSL manufacturers – LD+A

I have written a lot about TM-30 from the specifier’s viewpoint – how TM-30 is better than CRI, explanations of the various TM-30 measures. why TM-30 data and reports are so important, how to get TM-30 data or reports, how to use TM-30, etc.

This month’s LD+A has a great article about TM-30 from the manufacturer’s viewpoint that addresses issues such as – why TM-30 reports for every LED option aren’t always available, why some manufacturers are reluctant to prepare TM-30 reports, strategies specifiers can use to deal with these issues, and more.  It’s a very useful article.  Give it a read at TM-30 Update: Challenges and strategies for working with SSL manufacturers – Illuminating Engineering Society %

AI and the Internet Need to Learn Color Rendering

Recently a colleague of mine from the IES Color Committee asked of one of the new AIs “Tell me about color rendering.”  If we assume that the answer was formulated based on information about color rendering that is available on the internet we have a problem because the answer the AI returned was almost completely wrong.  That strongly suggests that most individuals and manufacturers posting information about color rendering don’t fully understand color rendering and are perpetuating outdated or incorrect information.  Here’s what the AI had to say, and what’s wrong with it’s response.

Color rendering is a measure of how well a light source reveals the true colors of objects. (1) A light source with a high CRI (Color Rendering Index) will show objects in their natural colors (2), while a light source with a low CRI will make objects look dull or washed out (3).  The CRI is a scale from 0 to 100 (4), with 100 being the perfect score (5).  A light source with a CRI of 100 will show objects in their true colors (1) while a light source with a CRI if 0 will make all objects look the same color (6).

The CRI (7) is important for applications where it is important to see objects in their true colors (1), such as art galleries, hospitals, and retail stores (8).  It is also important for applications where it is important to see subtle variations in color such as fashion design and photography (9).

There are a few things that can affect the CRI of a light source (10).  The type of light source (incandescent, fluorescent, LED, etc.) will affect the CRI (11).  The color temperature of the light source will also affect the CRI (12).  Light sources with a higher color temperature will have a higher CRI (12).

The CRI (7) is a valuable tool for choosing the right light source for your application.  By understanding the CRI, you can choose a light source that will show your objects in their true colors (1) and help you achiever the desired look and feel for your space.

  1. Objects don’t have “true” colors, they have apparent colors.  The color of an object is determined by the spectrum of light striking it, it’s reflective properties, and the resulting light directed toward the observer.  An object’s reflective properties don’t change, but the spectrum of light changes every time the light source changes.  Changing the spectrum may, therefore, change the color appearance of the object, hence apparent color.
  2. While there is a correlation between fidelity and naturalness, they are not the same thing.  CRI measures fidelity, i.e. how well a given light source matches the color rendering of a defined reference light source.
  3. One of the problems with single measure metrics like CRI is that there’s a lot of information that isn’t conveyed.  As CRI values drop, the only thing conveyed is that the match to the reference light source is worse.  A worse match, however, doesn’t mean colors are made dull. It could be they are increased in saturation since both deviations from the reference are equally penalized.  That’s the advantage of TM-30.  As Rf decreases we can see why by looking at Rg and some of the other 147 measures.
  4. CRI can have negative values.  TM-30 Rf is calculated so that 0 is the lowest value.
  5. 100 is the highest value.  It’s dangerous to call it “perfect” though as that implies that high fidelity is the only color rendering goal, which it isn’t.  TM-30 provides information for the color rendering goals of preference and vividness, and may include more in the future.
  6. A CRI of 0 will certainly make nearly all colors look terrible and very similar, but not all the same.
  7. CRI isn’t a proper noun, and shouldn’t be preceded by “the”.
  8. There are strong arguments for emphasizing preference over fidelity in many applications, including retail.  Again, fidelity isn’t the only color rendering goal, although it is the only one CRI measures.
  9. Research shows that high fidelity isn’t necessarily the best spectrum for detecting color difference.  Additional research is needed, but the IES may eventually add a color difference metric to TM-30.
  10. Only one thing affects CRI value – the spectrum of the light source.
  11. This is true because different light producing technologies have similar quirks in their spectra.  Those similarities can lead us to blanket statements such as “all fluorescents are green” which are not true for all products.  Again, the individual light source’s spectrum determines everything.
  12. A common misconception, but not true at all.  Not in the slightest. CCT and CRI are separate metrics.