Designing Beyond Fidelity

I recently began a project that includes about 8,000 SF of office space that is completely without windows or skylights.  I’ve renovated spaces like this before, and the common complaint from occupants was a disconnect from daylight, weather, and the way they indicate the passage of time.  On this project, I determined that the most appropriate solution was to use a light source that rendered colors in a way that is highly preferred to make the spaces more pleasant to occupy and use.

Of course, as a designer who is very knowledgable about color rendering issues and is a TM-30 advocate, I know two things.  First, highly preferred is not high fidelity.  People prefer a light source that slightly increases the saturation of object colors (especially reds) over a high fidelity source.  Second, TM-30’s Annex E provides specifiers with ranges for certain TM-30 measurements that allow us to accurately specify highly preferred light sources.

The task seemed simple enough.  Forget about fidelity and find a fixture/LED combination with a spectrum designed for preference, i.e. a light source that meets the TM-30 Annex E specification for a highly preferred (aka P1) light source.  After all, we’ve had TM-30 for seven years now, and Annex E for four years.  Surely, by now LED manufacturers have introduced products that meet color rendering goals other than fidelity, right?  Who wouldn’t see that as a huge marketing opportunity?  And surely fixture manufacturers would offer specifiers that LED, again to differentiate their products from the many, many, many similar products from other manufacturers…right?

Alas, the answer is, “No.”  One member of the IES Color Committee shared with me a database of over 1,000 LED products, their SPDs, and their various TM-30 measurements, including the Annex E Preference Design Intent.  Of those, there are a generous handful of retrofit lamps, most of them by Soraa and Cree, that meet the P1 specification, but only one LED in commercially available linear LED fixtures – Focal Point’s Preferred Light series.  That’s it!

Fortunately, this project is for a private firm so I don’t have to worry about developing a three-name or performance spec.  Otherwise, I would have to give up on a preferred spectrum and default to high fidelity not because it would be appropriate for the project but simply because there are more options.

I’ve mentioned fidelity and preference.  You might be asking if there are other color rendering goals.  The answer is, “Yes.”  Other color rendering design goals, with brief explanations, include:

Preference.  Light distorts object colors with slight increases in saturation, especially reds, in a way that is preferred over the reference light source (that is, preferred over high fidelity).   This might be the goal in an expensive restaurant where you want to emphasize the beautiful colors of the food, people, and interior design.

Vividness.  Light renders object colors as more or less vivid, or saturated, than the reference light source. Vividness is different from Preference in the degree of distorted saturation and the design intent – making colors pop, not making colors more attractive.  This might be the goal in the Skittles store in Times Square where you want the colors to leap out at people.  

Naturalness. Light renders object colors as expected, which, surprisingly, is usually not the same as fidelity.  This might be the color rendering goal in a grocery store where you want the food to look ripe and appetizing.

Discrimination.  Light renders object colors so they can be appropriately sorted. This might be the goal in a facility where even slight color variations must be detected.

Specifiers are captives of manufacturers.  We can have design goals oriented toward the needs of the users and the success of the project, but manufacturers only want to sell us fidelity, the same way they’ve been selling us fidelity since CRI was introduced in 1965.  For 50 years we only had a hammer (CRI) so all problems were nails (fidelity).  TM-30 changed that and it’s time for manufacturers to catch up.

IES Publishes “Standards Toolbox”

The IES has added a Standards Toolbox to their web site that features an online TM-30 and TM-21 (projected luminous flux maintenance, i.e. LED lifetime projections) calculators, an interactive illuminance selector (for subscribers to the IES Online Library), and an IES Reference Retriever where members can access all of the documents, articles, and papers referenced in various IES documents.

Of course, I’m most excited about the TM-30 calculator which imports and exports spectral data and reports in a variety of formats and increments, and will always be the most up-to-date version.  As a bonus, the calculator’s code is also available for download on GitHub, which may be of special interest to manufacturers who want to bring calculations in house instead of doing them online.

The TM-30 calculator includes CIE S026 Alpha-Opic calculations (CIE S 026:2018. System for Metrology of Optical Radiation for ipRGC-Influenced Responses to Light) and output, and is expected to include additional spectral calculations in the future.

Manufacturers Don’t Understand Color Rendering

I attended LEDucation in New York City this week.  While there I spoke to over two dozen manufacturers, none of whom understood color rendering beyond the (partially accurate) belief that higher CRI is better.  My conversations when like this.

Lighting Salesperson: “Our color rendering is great.  Our CRI is over 90 and our R9 is over 50.”

Me: “CRI measures color fidelity.  What if my goals are something else?”

Lighting Salesperson: “…”


Lighting Salesperson: “Our color rendering is great.  Our CRI is over 90 and our R9 is over 50.”

Me: “CRI has inaccuracies that have been known for decades while TM-30 is the most accurate and up to date system.  If color fidelity is important to you, why aren’t you using TM-30’s Rf?”

Lighting Salesperson: “Well, lighting designers don’t understand TM-30.”

Me: “Oh, I think many of them do.  But, even if they don’t, don’t you want to be sure that your color rendering claims are true?  Couldn’t you educate designers about TM-30?  The IES Color Committee will help.”

Lighting Salesperson: “…”

So, (big sigh, big eye roll) let’s go over this again.  If we think of CIE 13.3-1995 Method of Measuring and Specifying Colour Rendering Properties of Light Sources, aka CRI, as a technology, then it’s a technology from 1965 which is when the first version was published.  In 1965 Lyndon Johnson was president, Bonanza was the most popular TV show, Wooly Bully by Sam the Sham and The Pharaohs was the most popular song, and the Chevrolet Impala (Jet Smooth Ride!) was the most popular car.  CRI is O-L-D.

As with any other technology that is 57 years old,  the science has advanced.  Unfortunately, CRI has not. There were two minor corrections, the most recent in 1995, but they did little to fix at least a half dozen errors and inaccuracies that have been well documented for decades.  The CIE basically admitted that when, in 2017, they published CIE 224 Colour Fidelity Index for Accurate Scientific Use, which is TM-30’s Rf measure of fidelity.  Why publish 224 and not withdraw CRI?  Why have one measure for accurate scientific use and one for (inaccurate?) general use? The CIE requires unanimous votes for any action to be approved.  I’m told by reliable sources who were in the room that one global lamp manufacturer has been resisting updating or replacing CRI for decades, and that one manufacturer has held up progress or change.

So, CRI is has known inaccuracies resulting from a combination of outdated internal calculations along with other limitations.  Meanwhile, TM-30 is known to be the most accurate measure of fidelity.  If fidelity is your concern, Rf is the measurement you want to use.

What about concerns other than fidelity?  When TM-30 was published in 2015 there wasn’t evidence it could be used for purposes other than fidelity.  However, by 2018 studies provided ample evidence that TM-30 measures could also be used to evaluate light sources for preference and vividness.  The studies are summarized in TM-30’s Annex F, and recommendations based on those studies are in Annex E (yes, that seems backwards, sorry).  Here’s an explanation of all three color rendering goals. (Oh, and TM-30 is still a free download from the IES!)

I think preference is an incredibly interesting color rendering goal.  Color preference means that the light source in question renders colors differently than the reference light source (and therefore has a lower fidelity) but does so in a way that is preferred by most people (usually by slightly increasing the saturation of colors, especially red).  Color preference is usually my color rendering goals in spaces, such as hospitality, where aesthetics are the primary concern.

Manufacturers don’t get it. Designers do, and we have to demand that they educate themselves so they can provide us with the tools we need.

Updating the CCT Calculation

As I noted in Chapter 9 of the 2nd edition of Designing with Light, we calculate color temperature, correlated color temperature, and distance from the Plankian locus in a perverse way.  The calculations are performed in the CIE 1960 (u, v) chromaticity diagram (which is why distance from the Plankian locus is Duv).  However, since 1960 (u, v) is obsolete, we perform the calculation using CIE 1976 (u’, v’) chromaticity diagram, but then scale the v’ axis by .66 so that we’re using 1976 (u’, ⅔ v’) which is 1960 (u, v).

To complicate things, to present information graphically, most manufacturers transpose these calculations to the 1931 (x, y) chromaticity diagram, resulting in the industry using 2 ½  chromaticity diagrams for various calculations and illustrations.  Unfortunately, they also use 1931 (x, y) to illustrate the gamut of multi-colored luminaires even though it isn’t uniform, making the illustration of questionable value (they should be using CIE 1976 (u’, v’), which is perceptually uniform).

In a counter to this fragmented system, yesterday Leukos published a research article called Improved Method for Evaluating and Specifying the Chromaticity of Light Sources.  Among other proposed improvements to how we perform chromaticity related calculations, it introduces a new uniform chromaticity scale (UCS) diagram with coordinates (s, t), a measure of correlated color temperature (CCTst), and a measure of distance from the Planckian locus (Dst).  Importantly, it makes all chromaticity calculations in a single chromaticity diagram instead of the 2 ½ diagrams we use today.  It’s heavy on the science, but is an important step in fixing our current system.

The Limits of a ‘Standard’ Observer

I tell my students that we’re lighting designers not scientists, but that it’s good to understand some of the science that underpins our work.  This is especially true when the science is out of date and produces results that don’t necessarily agree with our vision and/or perception.  It’s frustrating and amazing to me that as individuals we’d never agree to use a broadcast only TV and give up our modern cable and internet channels. We’d never agree to use a flip phone and miss out on all of the upgrades and improvements that have been developed over the years. Yet as an industry we seem perfectly happy to continue to use 75+ year old technology with known flaws when we calculate color rendering, measure brightness, plot chromaticity in color spaces, etc.  Our industry doesn’t seem interested in “upgrading” to get the latest features like less metameric mismatch and measurements that better align with our vision and perception. But, I continue to shout into the void about these things.

One of these topics is the standard observer. This article, online and in the current issue of LD+A, looks at the problems that can arise from continuing to rely on the 1931 standard observer, and not “upgrading” to the 1964 or 2015 standard observers.

Tony and I Talk Color Rendering

Podcast Album Cover

In September at ArchLIGHT Summit, Tony Esposito and I gave a series of demonstrations on the spectral flexibility of LEDs and the possibilities they present with regard to color rendering.  While there we spoke to Sam Koerbel on his LytePod podcast about the basics of the new measures introduced in Annex E, and discuss why TM-30’s multi-dimensional approach to quantifying color preference is superior to the old-standby in the industry: CRI.  Our discussion is now available.  Give it a listen.

The Strength of TM-30

Last week Tony Esposito and I presented seminars at ArchLIGHT Summit in Dallas, TX. The topic was TM-30 and the deep information that it provides us about a light source’s spectrum and the resulting color rendering. CRI, of course, only evaluates fidelity – how close a light source matches its reference light source. But CRI penalizes all deviations and says nothing about the rendering of individual colors. Nor does it help us understand if the deviations from the reference are acceptable to viewers.

A small part of our demo is shown below. It illustrates how two light sources can have the same fidelity (in this case Rf of 70) but wildly different spectra that produce wildly different color rendering results. This is the great strength of TM-30, a deeper insight into the effect of a light source on illuminated objects and their color appearance – not just fidelity, but chroma shift, hue shift, and the perceptual implications of those shifts.

The video below shows the color appearance shifts. The graphic illustrates that even though the Rf is 70, the first light source renders objects in a preferred manner (Preference Priority Level of 3 or P3) and increases vividness (Vividness Priority Level of 2 or V2). At the same Rf the second source mutes colors and fails to achieve any of the Design Intents and Priority Levels specified in TM-30’s Annex E.

Alternating between light sources with Rf 70, Rg 94 and Rf 70 Rg 111

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!