Category: Color

Misunderstanding CRI

Last Friday I took my class on a visit to a fixture manufacturer’s showroom.  The visit was pretty successful, but I had one issue with the information that was presented.  This manufacturer’s rep presented their CRI 80 and CRI 90 products by saying that CRI 80 dulls colors and CRI 90 makes colors “pop”.  I can’t blame him too much, after all it’s a common misconception that higher CRI is “better.”  However, it’s not true so let’s take a look.

 CRI (or more formally, CIE 13, Method of Measuring and Specifying Colour Rendering Properties of Light Sources, Ra) is a fidelity metric.  That means it calculates the color rendering of a light source in comparison to the color rendering of a reference light source of the same color temperature or correlated color temperature (CCT).  A light source with a CRI 80 renders colors with more color error (that is, a larger mismatch or a larger color appearance distortion) than a light source with a CRI 90.  That’s all. One of the problems with CRI, which is addressed in TM-30, is that a single number value doesn’t tell us the hue(s) where there is a color rendering error compared to the reference light source, nor do we learn the direction or the degree of color rendering error(s).  In other words: 

  • What hues are not rendered accurately?  CRI doesn’t tell us.
  • Are those hues made to appear more or less saturated?  CRI doesn’t tell us.
  • Are those hues shifted toward an adjacent hue?  CRI doesn’t tell us.
  • How big are the color distortions? CRI doesn’t tell us.

 TM-30 (ANSI/IES TM-30-18 IES Method for Evaluating Light Source Color Rendition) does give us this information, which immediately puts to rest the notion that higher fidelity is “better” color rendering in all cases. 

It’s entirely possible for a light source with a CRI 80 to render a set of colors more vividly than a CRI 90 light source if the color errors increase saturation and minimize hue shifts.  It’s even possible for two light sources of the same CRI to render colors differently.  Here’s an example.  The first light source has a TM-30 Rf (fidelity) of 90 and an Rg (chroma) of 99, meaning that on average colors are rendered slightly less vividly than the reference light source.  The TM-30 Color Vector Graphic shows us clearly that the rendering of red (Bin 1) is less saturated than the reference, and that the rendering of warm blue (Bin 12) is more saturated.  The other colors are a nearly perfect match to the reference source.

The second source  also has an Rf 91.  However, the green and purple hues are rendered with increased saturation so that it has an Rg 105. (Yes, the CCTs are different, but that doesn’t matter because in the calculation a light source is compared to a reference light source of the same CCT, cancelling out any color errors due to CCT.) 

Understanding this information opens the door to considerations other than fidelity.  The first is vividness.  Are you lighting the M&M store in Times Square?  If so, your design goal may be to increase saturation of the candy, not accurately render it.  In that case you’re going to want a lower fidelity (Rf) so that you can get higher chroma (Rg).  The light source shown below might be just the one for this application.

 The second is preference.  Studies have shown that in many applications people prefer slight increases in chroma, especially in the red range.  Are you lighting a restaurant?  If so, and if preference and increased red chroma are important, this might be the light source for your project: 

The increased information TM-30 provides is both more accurate and more detailed than CRI.  Not only that,  it gives us a deeper understanding of the color rendering capability of a light source and allows us to consider design goals other than fidelity. Designers who care about these color considerations need to keep pushing manufacturers to provide TM-30 information and train their employees in its meaning and use.

Fixing CRI, But Not Really

A couple of weeks ago the Global Lighting Association (GLA) published Application of CIE 13.3-1995 with Associated CRI-based Color Rendition Properties.  It proposes TM-30 like metrics to supplement CRI Ra.  Specifically, it proposes a color gamut index, Ga, that is similar to TM-30’s Rg and a set of chroma indices, Ci, similar to TM-30’s Rch,hj.  At first glance I can see some specifiers getting excited about this.  Since it’s based on CRI it’s already somewhat familiar so it should be easier to learn.  But…

The first part is the problem – it’s based on CRI, which has well known and well documented problems and shortcomings.  As described in The Lighting Handbook, 10thEd.and IES DG-1 Color and Illumination, they include:

Averaging the Color Shifts. CRI is computed by averaging the color shifts of the eight color samples.  A light source can render one sample very poorly and still achieve an acceptable score.  The Chroma Indices attempt to resolve this, but there’s a problem with the eight color samples, which is next on our list.

Test Color Samples. The eight color samples are all of moderate saturation, so saturated colors can be rendered poorly even when CRI is high.  More importantly, the eight color samples are A) not based on any real-world objects B) don’t adequately cover the visible spectrum.  The latter is significant because it means that some wavelengths play an outsized role in determining CRI.  Manufacturers can use this to optimize (or cheat) their spectrum to achieve a higher CRI than visual inspection would warrant.  This is why TM-30 uses 99 color samples drawn from real world objects.

Color Space.  CRI is calculated in the CIE 1964 color space, which is no longer recommended for any other use because it is outdated.

Penalties for all Chromaticity Shifts.  As a fidelity metric, CRI penalizes all chromaticity shifts even though research has shown that certain increases in chroma are preferred.  Again, it seems that Ci is intended to address this, but it doesn’t resolve the other problems with CRI. 

Chromatic Adaptation. The chromatic adaptation transform used in CRI has been shown to perform poorly and is no longer recommended for any application.

CRI served the industry (relatively) well, but its time is over.  Layering new calculations on top of CRI’s flawed foundation doesn’t make it better. I know TM-30 can be tough.  But it works.  It tells us what we want to know at the level of detail we need for a given project, and it’s accurate.  The IES has moved on and the CIE seems to be leaning in that direction as well.  So should you!

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!








Design for Color and Illumination Webinar April 19th

On April 19th Wendy Luedtke (my co-chair of the IES Color Committee) and I will be presenting a free IES webinar called Design for Color and Illumination.  Here’s the blurb from the IES site:

When developing a lighting design, lighting specifiers determine the lamp and fixture combination that best suits the design’s requirements based on many factors. While some considerations are largely technical, such as power consumption, the amount of light generated, and how light is distributed, one consideration is both technical and artistic and can be approached in a number of ways. Participants to this webinar are eligible for one (1) IES Continuing Education Unit (CEU).

The webinar is at 12 noon EDT.  Register here.








I’m Presenting at LEDucation 2018

This year LEDucation, the largest LED only trade show and educational forum in the U.S., will be on March 13 and 14.   On Tuesday the 13th from 11 am to 12:30 pm I’ll be presenting a forum called How To Use TM-30, along with Dr. Michael Royer of Pacific Northwest National Laboratory and Wendy Luedtke of ETC.  Here’s a summary of the event.

This presentation discusses a number of design trends that are currently shaping the industry and promising to redefine the role of lighting as we know it. New technologies are making dimensions such as color and dynamic behavior over time viable for main-stream lighting, and that constitutes exponential change. This presentation explores a few of the dimensions that appear to be gaining traction, and attempts to illuminate some of the reasoning behind their development. Implications to the user experience are discussed, along with the inadequacy of present lighting metrics.

See you there!








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.








Samsung Introduces Chip-on-Board LED Packages Optimized for Commercial Lighting – Samsung Global Newsroom

Source: Samsung Introduces Chip-on-Board LED Packages Optimized for Commercial Lighting – Samsung Global Newsroom

An interesting bit of news from Samsung this week.  They’ve developed an LED package especially designed to achieve an Rg value over 110, “a level that ensures lighting with outstanding color and whiteness.”

It’s important to note that increased saturation means decreased fidelity to the reference light source.  This is a lighting solution that will be desirable in some applications, such as retail,and undesirable in others, such as medical facilities.








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?