Category: Lamps

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.

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.

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.

New Energy Efficiency Rules Ban Incandescent Light Bulbs: What to Know – The New York Times

We’ve known for a while that the day was coming, and now it’s here.  Effective yesterday retailers will no longer be permitted to sell most incandescent lamps (which includes halogen lamps) in the U.S.  There are some exceptions for things like bug lights, but not many.

Replacement lamps for sale now must produce at least 45 lumens per watt, which is easily achieved by LEDs but impossible with incandescents.

The Department of Energy estimates that consumers will save about $3 billion per year. ( Of course, this assumes they buy quality lamps that don’t fail within a year, which is a real problem with off brand, and even some name brand, lamps.)

Source: New Energy Efficiency Rules Ban Incandescent Light Bulbs: What to Know – The New York Times

Color Rendering and Skin Tone

As part of the conversation around Diversity, Equity, Inclusion, and Respect there’s been a lot of discussion about color rendering and skin tone.  I recently heard one speaker say something like, “We know that historical SPDs are racist.”  I don’t think that’s accurate or helpful.  Here’s why.

Since the development of the fluorescent lamp, the first priority for lamp manufacturers has been maximizing efficacy – getting the most lumens per watt.  That’s still largely true today, even though LEDs are so efficient that there’s a lot of room for other considerations.  An exclusive focus on efficacy inevitably results in poor color rendering, so the second priority has been acceptable (not maximized) color rendering.  In other words, manufacturers have tried to find the right balance between efficacy and quality, but they emphasize efficacy.

When evaluating color rendering, manufacturers only look at the numbers.  Whether it’s a calculation of CRI, Rf, Rg, or something else, it’s all done mathematically.  There’s no interest in comparing the calculated values with empirical observations.  The eight colors used to calculate CRI are a limited range that don’t include a representation of skin, as shown below.

Color used for CRI calculation

The 99 colors used for TM-30 calculations span the color space and are not weighted toward any hue, tint, or value, as shown below.

So, there’s never been a focus on caucasian skin tone to the detriment of others because skin tone isn’t part of the evaluation.

 

Does that mean that all skin tones are rendered equitably?  Honestly, we don’t know.  On one hand, there’s no reason to think that we evaluate skin tone differently than we evaluate other surfaces.  It’s reasonable to expect that a high fidelity source, for example, that give cars, apples, and kittens a good color appearance will do the same for human skin.

On the other hand, we don’t have good studies to confirm that.  It may be that we hold different criteria for evaluating skin than we do for apples, resulting in the need for a separate skin tone rendering metric.  Again, today we just don’t know.

In fact, the IES Color Committee is looking at this right now.  We’ve started with an effort to gather as many studies as we can find – though there are very few that focus on skin rendering.  The next step is to evaluate the literature to determine if additional study is needed, and what such a study (or studies) would require and evaluate.  The hard part is funding the studies, and that would be the next step.  Eventually, we’d have some solid science from high quality studies that would tell us if skin tone is evaluated differently than other surfaces, and if so what the calculation of a skin tone metric should include.  The goal is to use the appropriate TM-30 measures (remember, there are 149 of them) to evaluate skin tone rendering, and to add a skin tone metric (maybe Rs) to TM-30, if needed.

If you’re interested in joining the task group looking at this, please contact me.

Predicting LED Lifetimes

Recently, a corporate client asked me to specify only LED fixtures with a lifetime of 100,000 hours, and preferred fixtures with a life of 200,000 hours.  I don’t know where they came up with these numbers, but my reply was that an L70 of 100,000 hours or more cannot be validated through standard testing procedures.  Here’s why.

To begin with, LEDs themselves don’t experience catastrophic failure the way incandescent and fluorescent lamps do. The don’t stop making light, but their output declines over time.  Today the generally accepted calculation of the life of an LED is called L70, which is the length of time before the light output has fallen to 70% of initial output.

The IES approved lifetime calculation method begins by collecting data using the procedure described in LM-80 (ANSI/IES LM-80 Measuring Maintenance of Light Output Characteristics of Solid-State Light Sources). Please note that LM-80 measures “LED packages, arrays, and modules” not fully fabricated fixtures, and there’s some dispute about whether or not testing bare modules is appropriate.  However, it does permit module manufacturers to test once and derive a lifetime, rather than every fixture manufacturer testing every fixture with every module they want to offer, which would be incredibly burdensome and expensive.

LM-80 requires a minimum collection time of 6,000 hours (250 days) but sets no upper limit.  If manufacturers want to use the data they’ve collected and project future performance they use the calculation procedure in TM-21 (ANSI/IES TM-21 Projecting Long-Term Luminous, Photon, and Radiant Flux Maintenance of LED Light Sources).  Importantly, TM-21 only permits data to be projected to six times the LM-80 data collection time period.  This is because of uncertainties involved with longer predictions (see PS-10-08 IES Position on LED Product Lifetime Prediction at https://www.ies.org/advocacy/position-statements/ps-10-18-ies-position-on-led-product-lifetime-prediction/).  So, an L70 of 50,000 hours is based on at least 8,333 hours of LM-80 testing.  That’s 347 days.

Thus, to say that an LED has an L70 100,000 hour life would require a data collection period of 16,667 hours (695 days), or 1,390 days (3.8 years of continuous testing) for a life of 200,000 hours.  Today, no LED manufacturer conducts LM-80 tests for that extended period of time because the lifetime of a given LED product is too short.  By the time you’ve finished a 4 year long test, the LED being tested is out of production and replaced by something new.  In the future, when the LED industry has matured and we’re no longer seeing continuous improvements in efficacy, color rendering, etc., they may test for that long, but not now.

Where do these 100,000 hour and longer lifetimes come from?  It seems that some manufacturers are using an internally generated prediction to get to these numbers.  The thing is, we don’t know what’s involved in that prediction, which means we can’t validate it or compare it to any other prediction.  We just have to take their word for it. With the LM-80/TM-21 procedure, on the other hand, we know that testing labs, regardless of who or where, are using the same procedure and their results should be consistent and repeatable.  That allows us to reliably, confidently compare fixtures by any number of manufacturers.

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.

26 Consumer Groups Urge D.O.E. to Take Action on Lamps

You may recall that in 2019 the Trump administration blocked a rule intended to phase out incandescent lamps and encourage a conversation to more energy efficient models, namely LEDs.  If you don’t remember the New York Times and NPR both had articles, among many others.

Last week, the Consumer Federation of America and the National Consumer Law Center, along with 24 other groups across the country, urged the U.S. Department of Energy (DOE) Secretary Granholm to implement the efficiency standard for household lighting products mandated by Congress as soon as is practicable.  They claim that “Each month of delay costs American consumers nearly $300 million in lost utility savings and results in another 800,000 tons of climate changing CO2 emissions over the lifetimes of the incandescent bulbs sold in that month.”

You can read the entire press release at Consumerfed.org