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 spectrum 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 closes 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

Retail Design Institute Lighting Summit

On Wednesday, September 25th I’ll be on a panel at the Retail Design Institute New York Chapter’s Lighting Summit 2024.  The other panelists are Shoshanna Segal of Hantranft Lighting Design, Carlotta Dove of IA Interior Architects, and Kazumi Tanimura of Atelier Lumiere.  The event is at the Illuminations showroom at 5 Penn Plaza.  Hope to see you there!

Lamp Ban Summary

We’re now up to 14 states and Canada that have enacted one or more restrictions or bans on mercury containing lamps.  Some of the bans are already active, while others go into effect as early as January 1, 2025.  LEDVANCE is maintaining a list on their website along with other helpful resources.

Fluorescent Phase-Out Continues

The big fluorescent news last week was that Minnesota has become the ninth state to pass a phase out of fluorescent lamps (the others are California, Colorado, Hawaii, Maine, Oregon, Rhode Island, Vermont, and Washington). Minnesota’s timeline is aggressive – screw-based compact fluorescent lamps will be banned from sale six months from now, starting on January 1, 2025.  Other fluorescent and mercury vapor lamps will be banned from sale beginning January 1, 2026.

The Minnesota House of Representatives has an information sheet about the new law here.  What’s interesting is that the first topic is about reducing the hazard of exposure to mercury, not increasing energy efficiency.  It’s not wrong, just curious.  The information sheet then goes on to address reduced CO2 emissions, electricity savings, and reduced power plant mercury emissions.

Illinois also has fluorescent banning legislation working through the legislature, so by the end of the year we may see 10, 11 or more states that are phasing out fluorescents.

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.

147 Countries free to Eliminate Fluorescent Lamps by 2027

The Minamata Convention on Mercury, a program of the United Nations with delegates from at least 150 countries, is dedicated to improving global health by phasing out the use of mercury in manufacturing, banning new mercury mines, and limiting mercury emissions.   Last month, 147 countries (out of a global total of 195) agreed to phase out florescent lighting globally and completely by 2027.

According to the appliance efficiency non-profit, CLASP, the phase out will, between 2027 and 2050:

  • Avoid 2.7 gigatons of CO2 emissions,
  • Eliminate 158 tons of mercury pollution, both from the light bulbs themselves and from avoided mercury emissions from coal-fired power plants,
  • Save US$1.13 trillion on electricity bills.

Early fluorescent lamps were being tested by Thomas Edison and Nicola Tesla in the 1890s, but it took several advances before they were ready for commercial use around the 1940s.  According to the Department of Energy, by 1951 more light was being produced in the US by fluorescent lamps than by incandescent lamps.  But, this always came at a cost.  Fluorescent lamps work by passing an electric current through gaseous mercury, which emits ultraviolet light, which in turn is converted to visible light by the phosphors that line the fluorescent tube. When discarded (and eventually broken) the mercury would enter the environment, which is why the EPA began encouraging fluorescent lamp recycling and mercury recovery in the mid-2000s.  Mercury is a neurotoxin, and symptoms of prolonged and/or acute exposures include:

  • Tremors
  • Emotional changes (such as mood swings, irritability, nervousness, excessive shyness)
  • Insomnia
  • Neuromuscular changes (such as weakness, muscle atrophy, twitching)
  • Poor performance on tests of mental function

So, after around 70 years as the dominant commercial light source, and 10 years of decline after the introduction of LEDs, the fluorescent lamp has joined kerosene, whale oil, and others as an historical or legacy light source.

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.