Designing Light

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?

In this series of posts about IES TM-30-15 I’ve discussed the problems with CRI and the resistance to adopting TM-30.  In this post I’ll discuss the advantages of TM-30 over CRI, and what TM-30 is and isn’t.

Color Samples

Like CRI, TM-30 compares color samples rendered by a given test light source and a reference illuminant of the same correlated color temperature (CCT).  The first advantage of TM-30 is the selection of color samples.  CRI uses the eight samples show in Fig. 1, which are selected from the Munsell color system.

Figure 1 Colors used to calculate CRI Ra

All eight are of medium value and are not evenly distributed across the color space or across the visible spectrum.  This allows lamp manufacturers to “optimize” lamp spectra to score a higher CRI Ra than visual evaluation of the light would indicate.  TM-30 uses the set of 99 color samples shown in Fig. 2.  These color samples range from pale tints to saturated colors, and are drawn from real world objects including textiles, plastics, skin tones, printed materials, natural objects, and paints.

Figure 2 Colors used in the TM-30 calculations

These colors have been selected from a database of about 105,000 objects.  In reducing that number to one that is more manageable, the authors of TM-30 made sure that the color samples were even distributed across the most modern color space (CAM02-UCS) and that their reflectances were evenly distributed across the visible spectrum, as shown in Fig. 3.

Figure 3 Spectral reflectances of the TM-30 color samples

Spectral tuning (gaming the system to achieve a higher score) isn’t possible with these color samples, which means that the resulting scores are honest, and comparisons of light sources are apples-to-apples.

Color Space

The second advantage of TM-30 is the selection of color space.  A color space is a model of a range of possible colors.  In our case we are interested in a color space that encompasses the entire range of visible colors.  CRI uses a color space called CIE 1964 (U*, V*, W*), which is no longer recommended for any other use.  In other words, it’s very outdated.  TM-30, on the the other hand, uses the most up-to-date color space CAM02-UCS.  TM-30 isn’t locked in time, either.  There is a new, more accurate color space under discussion at CIE.  If it is approved, and increases the accuracy of TM-30, I expect it would be included in a future update.

Reference Light Source

Like CRI, TM-30 uses Plankian radiation (blackbody radiator) for lower CCTs and the CIE Daylight (D) Series for higher CCTs for the reference light source.  The difference is that CRI Ra has a pronounced shift at 5000 K from one to the other, resulting in the possibility of a significant shift in Ra values between 4999 K and 5001 K.  TM-30 overcomes this by using a proportional blend of Plankian radiation and the CIE Daylight (D) Series between 4000 and 4999 K, much the way a variable white LED fixture blends LEDs of two different colors to achieve its full range.

Calculation Results

Instead of a single fidelity value, as with CRI Ra, TM-30 give us a wealth of data about the color rendering of the light source in question.  The first is the Fidelity Index Rf.  Like Ra, it is a comparison of the color rendering of the test light source compared to the reference light source.  However, with 99 color samples it is a tougher test that cannot be gamed.  I’ll have more to say about Rf in a future post.

The second is the Gamut Index Rg.  Rg indicates the average change in saturation of the 99 color samples as rendered by the test source compared to the reference source.  I’ll have more to say about Rg in the future, too.

So, from the start TM-30 gives us more information, but it doesn’t stop there.  It also divides the color space into 16 wedges, called hue angle bins, as shown in Fig. 4.  The Rf and Rg values of each bin are also calculated and reported so that if a specifier is interested in the performance of a light source in a particular color range, that information is available.  The information is also presented graphically by showing the average shift of each bin on the same graphic.

Figure 4 TM-30 hue angle bins

In addition, if you really want to dig down deep, the TM-30 calculation tool calculates the Rf and Rg values of the individual 99 color samples.

What TM-30 Is And Isn’t

TM-30 is a calculation procedure that takes an objective and statistical approach to analyzing two aspects of color rendering – fidelity to a reference source, and saturation shift relative to the same reference source.  The calculation also produces information about hue shift, which is presented graphically.  The calculation procedure is a consolidation of years of research by individuals and organizations around the world.  Its authors come from the research, specification,and manufacturing areas of the lighting industry.  Research since its introduction in 2015 has supported its validity as an accurate method of characterizing color rendering.  The CIE has endorsed Rf for scientific use in CIE 224:2017 Color Fidelity Index for accurate scientific use.  Unfortunately, they declined to endorse it for specification or other uses, as I’ve discussed here.  However, quite a few manufacturers see the advantage of TM-30 and are including Rf and Rg information on their cut sheets.

TM-30 isn’t a color rendering guide.  It doesn’t contain recommendations for acceptable values.  It reports calculated values and leaves interpretation of those values to the specifier based on experience and the particulars of the project.  (However, the IES is likely to publish guidance in the future.)  It also doesn’t attempt to evaluate color perception or color preference.  Those two aspects of color are application (and even situation) dependent, so again the specifier will use experience and understanding of the project to determine what values are appropriate and/or acceptable.

TM-30 provides significantly more information about the color rendering of a light source, and the information presented is far more accurate than CRI.  The authors of TM-30, and the IES Color Committee in general, are open to improvements in the calculation and the presentation of its results.  As additional scientific information becomes available, or improved or expanded means of calculation and presenting information are developed, it can be updated as needed or on a regular three to five-year cycle.