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!
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.
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?
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.
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?
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…
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.
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?
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 isRf 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.
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.
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.
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.
I’ll be giving a presentation on TM-30 (naturally) at the American Lighting Association Conference in Palm Beach, FL on September 23rd. If you’re at the conference and/or a fan of TM-30 stop by and say hi.
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.
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 %
