New Definition of Kelvin Goes Into Effect Today

Science geeks everywhere are celebrating World Metrology Day today.   Today has also been chosen by Bureau International des Poids et Mesures (International Bureau of Weights and Measures) to implement changes to the International System of Units or the SI.  The changes are to the definitions of the kilogram, ampere, kelvin, and mole.  

Of special interest to lighting designers, the Kelvin is now defined “by taking the fixed numerical value of the Boltzmann constant k to be 1.380 649 x 10–23 when expressed in the unit J K–1, which is equal to kg m2 s–2 K–1, where the kilogram, metre and second are defined in terms of hc and Cs.”

Yeah, I don’t know what that means, either!  Here’s a link to the full definition and formula.

CIE Publishes Position Statement on Blue Light Hazard

On April 23rd the CIE (Commission Internationale de l´Eclairage) published a position statement on so-called Blue Light Hazard. Given the amount of press this phrase has received and the responses, such as the AMA Policy H-135.927 Human and Environmental Effects of Light Emitting Diode (LED) Community Lighting, which was refuted by the IES and the LRC, it’s a timely piece.

The position statement makes a couple of key points. The first is that the phrase itself “has been inaccurately used to represent the risk of actual eye damage and the influence on general well-being. The term ‘blue light hazard’ should only be used when considering the photochemical risk to the retinal tissues of the eye (technically referred to as “photomaculopathy”), usually associated with staring into bright sources, such as the sun or welding arcs”

The second point is that “There is no evidence in humans of any adverse health effects from occasional exposure to optical radiation at the exposure limits.” It’s an important position paper with links to research. Read it for yourself.

Specifying Color Quality With TM-30

By now most of us have attended one or more seminars or webinars about IES TM-30 and understand that it is a method of measuring various color rendering properties of a light source and reporting those measurements.  The thing that’s been missing is a recommended set of values that set minimums, maximums and/or tolerances for the various measurements.  This has been true for two reasons.  First, TM-30 is a method and as such was never intended to set recommended values.  The second is that while the science behind TM-30 is solid, the science doesn’t offer any predictions of acceptability.

Good news!  After almost three years of research and tests around the world we’re much closer to establishing a set of recommended values.  At this year’s IES Annual Conference in Boston, Tony Esposito, Kevin Houser, Michael Royer and I will be presenting the seminar “Specifying Color Quality With TM-30”  The description of the seminar is, “This presentation will discuss several research projects which have used the IES TM-30 color rendition framework, and whose results have been used to develop various specification criteria. We will discuss UFC 4-510-01, The Department of Defense Unified Facilities Criteria for Military Medical Facilities, which has already implemented IES TM-30-15 specification criteria.”

During the seminar we’ll review some TM-30 basics, look at several research projects that are helping to establish TM-30 thresholds, and review how to use the TM-30 calculator.  Don’t miss it!

International Day of Light 2018

Today, May 16th, is the International Day of Light 2018.  It began in 2015 when the U.N. declared  The International Year of Light and Light-based Technologies.  Today, the International Day of Light is

a global initiative that provides an annual focal point for the continued appreciation of light and the role it plays in science, culture and art, education, and sustainable development, and in fields as diverse as medicine, communications, and energy. The broad theme of light will allow many different sectors of society worldwide to participate in activities that demonstrates how science, technology, art and culture can help achieve the goals of UNESCO – education, equality, and peace.

There are events around the globe related to the theme of light.  If you’re interested you can check the web site, and the sites of organizations such as IALD and IES.

Outdoor Night Time Lighting May Reduce Crime

I’ve just learned about a study conducted last year for the New York City Housing Authority (NYCHA).  In a randomized trial 39 NYCHA sites received additional night-time lighting for 6 months, while 38 sites received no additional lighting.  The study showed these reductions in crimes:

Index crimes: 7% reduction in overall index crimes (day and night). This reduction in overall index crimes was driven by a 39% reduction in index crimes that took place outdoors at night.

Felony crimes: 5% reduction in overall felony crimes (day and night). This reduction in overall felony crimes was driven by a 30% reduction in felony crimes that took place outdoors at night.

Assault, homicide and weapons crimes: 2% reduction in overall assault, homicide, and weapons crimes (day and night). This reduction in overall assault, homicide and weapons crimes was driven by a 12% reduction in assault, homicide and weapons crimes that took place outdoors at night.

Misdemeanor crimes: No detectable change in net misdemeanor crimes in treatment communities.

The results of other studies have been mixed, but I’m not clear if they were controlled, randomized studies.

The disappointing thing, from a lighting designer’s perspective, is the data that’s missing.  The report tells us the fixture wattage and lumen output, but doesn’t tell us the area covered or measure the increased light levels.  Instead it treats light fixtures as fixed items and counts them per square block.  This method would be fine if the world had only one type of outdoor fixture, but it doesn’t  So more light is better, but it doesn’t say how much more or what the upper limit should be.

TM-30 Rg, The Gamut Index

In addition to an index that measures the fidelity of a light source to its reference source (Rf) IES TM-30 includes an index that indicates the change in saturation of colors called the Gamut Index and abbreviated Rg.  Rg is calculated using the same Color Evaluation Samples (CES) and underlying calculation engine as Rf, which makes TM-30 a cohesive system.

Here’s how Rg works.  An Rg value of 100 indicates that, on average, the light source in question does not change the chroma, or saturation, of the 99 CES when compared to the reference light source.  An Rg value below 100 indicates that, on average, the light source renders colors as less saturated than the reference source, and an Rg value above 100 indicates that, on average, the light source renders colors as more saturated than the reference source.

Since Rg is an average it says nothing about the possible change in chroma for any individual hue angle bin or for any individual color evaluation sample.  That’s ok, thought, because TM-30 also tells us the Rg values for each hue angle bin, and for each CES.

Here’s an example of the graphic for the hue angle bins using the same light source as the previous post on Rf.

TM-30 doesn’t recommend any particular Rg or set of Rg values.  As with Rf, the interpretation of the information is left to the specifier.  Acceptable or desirable values will vary by application.  Rg doesn’t have a maximum or minimum value, but the possible range increases as Rf decreases, as shown below. The wedge to the left of the gray lines shows the range of possible Rg values, while the red dot represents the lamp we’ve been discussing.

The Rg values are also presented in a Color Vector Graphic (CVG), as shown below.  The white circle is the normalized reference source.  The black circle is the lamp in question.  Where the black circle is inside the white, colors are desaturated.  Where the black circle is outside of the white, colors have increased saturation.  The colored arrows indicate the direction of saturation shift, and the direction of hue shift.  Arrows that point straight in or out show only saturation shift.  Arrows that show rotation left or right also indicate hue shift.  I know!  And, the next version of TM-30 will present a graph showing the hue shift!


Research is revealing that we shouldn’t treat all hue angle bins the same.  Bins 1 and 16, which include the most red, are indicative of preference and it seems likely that they will take on increasing importance in that role.  Some specifications are already acknowledging this.  For example, the Department of Defense recently re-issued the Unified Facilities Criteria for Military Medical Facilities that establishes the following requirements for light sources:

Fidelity Index: Rf ≥ 80,

Relative Gamut Index: 97 to 110,

Fidelity Index, Hue-Bin 1:  ≥ 78,

Chroma Shift, Hue-Bin 1: -9% to +9%.

Clearly, TM-30 permits us to be much more specific about the color rendering that is acceptable or desirable for a project.  Why bother with CRI anymore?

Focal Point Introduces TM-30 Based “Preferred Light”

Today Focal Point Lights of Chicago, IL introduced a series of fixtures that feature what they call Preferred Light.  Preferred Light is based on recent studies at PNNL and Penn State, plus their own study, and uses TM-30’s Rf, Rg, and Hue Bin 16 values to establish a balance of fidelity, saturation, and red rendering that is “visually appealing to humans.”

The overall idea is that people seem to prefer a light source that slightly over saturated most colors, especially red.  “Using a custom LED mix, Focal Point defines Preferred Light using TM-30-15 metrics as having a fidelity (Rf) of 89, a gamut (Rg) of 107, and over-saturating Hue Bin 16, deep red content, by 9% at a [Correlated] Color Temperature of 3500K.”  So, by using the statistical measures of TM-30 and applying them to the related topic of color preference Focal Point has identified an optimized set of LED products to meet their customers’ needs.

I’ll be the first to admit that it may be risky to base all of this on only three studies, but other studies have shown that the TM-30 results can be applied in this way, and are also showing us the relative importance of the various calculated values.  I’m excited to see the industry using the tools, and am looking forward to seeing the Preferred Light for myself.

TM-30 Rf: So Big, So Strong, So Smart!

As we know, CRI Ra and TM-30 Rf are both measurements of color fidelity.  That is, they compare a test light source to a known reference light source and measure how well the test source matches the reference source.  One of the many shortcomings of CRI Ra is that it provides us with a single value.  That single value is easy to use, but doesn’t tell us anything about what colors will have increased saturation, decreased saturation, hue change, or will be unaffected.

TM-30 is a tougher test than CRI, so how do Rf and Ra values relate?  Lamps with Ra values below about 70 tend to have higher Rf values, while lamps with higher Ra values tend to have reduced Rf values.  Of course, this doesn’t mean that the lamps we think of as better have suddenly become worse, it’s just that we’re scoring on a different scale.  This means that we can’t draw direct comparisons.  For example, Energy Star requires that lamps have a minimum CRI Ra of 80, but that doesn’t mean that they should also have a minimum Rf of 80.  Different tests give different results and we have to be careful not to apply the meaning of one to the scores of the other.

IES TM-30’s Rf mathematically compares the appearance, under a test light source, of 99 color evaluation samples (CES) that are derived from real world objects, to the CES appearance under a reference light source of the same CCT.  The distance of the color shift for each CES is measured in the CAM02-UCS color space and averaged.  Throw in a lot of calculus (which we don’t need to get into) and voila, the Rf value.  It’s important to remember that what we get is just a number.  TM-30 doesn’t qualify any of the results as good or bad, desirable or undesirable.  It presents information to the lighting specifier and allows the specifier to apply education, professional experience, and knowledge about the project to determine whether or not a given light source is appropriate.

As with Ra, the single value of Rf conveys limited information.  It is more accurate, but still only tells us the average match or mismatch between the two light sources.  What makes TM-30 so powerful and useful is that it tells us much more if we want to know.  For example, using the Calculation Tool that can be downloaded with the purchase of TM-30 (which I wish the IES would make freely available), we can see that one common F32T8/830 has the following characteristics:

Rf   78
Rg   102
CCT   2943
Duv   0.0014
Ra   85

This lamp has moderately good fidelity (Rf), a slight increase in saturation (Rg), has a CCT of just under 3000K, and is slightly above the black body locus and therefore is slightly green (Duv).  The Advanced Calculation Tool tells us that the R9 value is 2 and that the Rf for skin is 85.  It also tells us the (x, y), (u, v), and (u’, v’) chromaticity coordinates (which, frankly don’t mean anything to me, but the information is there).  This information is immediately useful and isn’t provided as part of the CRI calculation.  In addition, most light source manufacturers don’t tell us the Duv, although understanding it is becoming increasingly important, especially now that NEMA has extended the chromaticity bins for LEDs in ANSI/NEMA C78.377 American National Standard for Electric Lamps – Specifications for the Chromaticity of Solid-State Lighting Products.  That’s a post for another time.

As I’ve already discussed, IES TM-30’s color fidelity metric Rf provides us with as little, or as much, information as we want.  If you just want top line information that the beige office you’re lighting will continue to look beige, you can have it.  An Rf of 78 is probably just fine.  If you want to see the fidelity of each of the 16 hue bins because you’re interested in the fidelity of a particular color range, it’s there.  If you want to know the Rf value of all 99 color samples you can have that, too!  What else?  Well, would you like to see the chromaticity coordinates in (x, y) color space, the SPD vs the reference source, or a pictorial comparison of each of the 99 CES?  No problem.

Pretty pictures but are they useful?  Not as useful as the data given above, but lighting designers do like to see this information, even if it’s difficult to interpret.  The CIE 1931 (x, y) color space isn’t perceptually uniform, so the distance we see between the reference source and the test source isn’t very informative.  Seeing the SPD is interesting, but no one can read an SPD and know what the light looks like or how it renders colors.  The CES Chromaticity Comparison is also interesting, but the red and black dots aren’t connected to one another.  With some light sources it’s easy to tell how they relate so we can see chroma and hue shifts, but as Rf drops and color shift increases it gets harder and harder.  What is useful are the next two graphics: the Rf value by Hue Angle Bin and by CES.


Now we can see how individual color ranges are affected by the lamp in question.  This may be especially useful on certain projects were specific color ranges are present and need to be accurately rendered.  The individual CES scores useful for the same reason.  However, in my opinion if you want information at that level of detail you’re probably better off doing a mockup and looking at project specific color and material samples instead of the CES.

TM-30 arms the lighting specifier with as much or as little information as needed on a particular project.  It also provides additional information that may be important (such as Duv).  It then allows the specifier to apply experience and knowledge about the client and the project to determine whether or not a given light source is appropriate.    Who could say no to that?