Munsell - the Father of Color Science? (part 3)

This series of blogs was foretold in a prophecy of April of 2013:

Someday I will write a blog post about how this guy Munsell laid the foundation for the ever-popular color space CIELAB, and came to be known as the Father of Color Science. He was also the father of A. E. O. Munsell, who carried on his work. I don't intend to write a blog post about how Albert became the father of A. E. O.

What I did not foretell in that blog post is that ISCC will be sponsoring the Munsell Centennial Symposium,  June 10 - 15, 2018 in Boston. Or that I would be keynoting this event.

After two previous attempts (Munsell as an educator, Munsell and 3D color space), I am finally have made my way to looking at the most significant work of Munsell. The Munsell Color Space was a model for CIELAB.

First cursory pass

Exhibit A. Richard Hunter's book The Measurement of Appearance, on page 136.

Photo taken at the Color Difference family picnic

This is a family tree of proposed models for determining color difference. Note that the Munsell Color system is in the upper right hand corner, and all arrows come from that box. The only little boxes that are still active today are the two boxes labeled CIE 1976. A similar diagram is on page 107 of that same book, which shows a family tree of color scales. (I have an image of that in a previous blog posts about color difference.) Again, this shows a straight lineage from Munsell Color Scpce to CIELAB.

Is this reliable testimony? Richard Hunter was a fairly knowledgeable guy when it comes to color. I mean, he has his own entry in Wikipedia for goodness sake. CIELAB is (perhaps) the most widely used tool in the color industry. Since Hunter traces the lineage of CIELAB back to Munsell, then I feel pretty confident about putting Munsell on the shortlist of highly influential figures in the history of color science, at the very least.

But, that hides a lot of the fun stuff that happened between the creation of the Munsell color space and the ratification of CIELAB as a standard for color measurement.

What Munsell did

    Munsell Color Space

Munsell's color space is based on some simple principles.

1. Hue, Value, and Chroma

There are three attributes to color in Munsell's color system. While these are implicit in many of the previous color systems (enumerated in a previous blog post), Munsell was intent on tying these to our intuitive understanding of color. (After studying on this for 25 years, I have come to realize that they are indeed intuitive). 

2. A physical standard produced with simple tools, simple math, and a defined procedure

Munsell described the procedure by which his color system could be developed from any reasonable set of pigments. The procedure included a way to assign unique identifiers to each color.  As a result, all colors within the gamut of the chosen pigments could be unambiguously named.    

3. Perceptual linearity

One of Munsell's secondary aims was to create a color space where the steps in hue, value, and chroma were all perceptually linear. Did he meet his goal? Stay tuned.


This color system was used to create the Atlas of the Munsell Color System, which was a book containing painted samples with their corresponding designations of hue, value, and chroma. This book was to be used as an unambiguous way to identify colors, and thus, to provide a standrd way to communicate color.

     Munsell photometer and the gray scale

Munsell invented and patented a photometer which was capable of measuring the reflectance of a flat surface. Well, provided it was a neutral gray. The user would look into a box and see two things: the sample to be measured, and a standard white patch. The sample was illuminated with a constant illumination, and the white standard was illuminated with light through an adjustable aperture. To make a measurements, the size of the aperture was adjusted so as to match the intensity of the dimmed white standard and that of the sample. The width of the aperture, scaled from 1 to 10, was the Munsell Value for the gray sample.

A shoebox with some holes and stuff

Munsell used his photometer to mix black and white paints in steps from V = 1 to V = 10.

     Maxwell disks and the rest of the colors

James Clerk Maxwell invented a creature called the Maxwell disk around 1855. I spent the better part of a day building my own set of Maxwell disks from colored construction paper as shown below. The cool part is the slit. You can slide two or more disks together, and rotate them so as to get any proportion of the colors to show. In the inset, I show the device that I adapted to rotate the disks. Again, the better part of a day was spent assembling a bolt, a couple of washers, and a nut. I first tried a cordless drill, and found it didn't spin fast enough to merge the colors. I had to use my old drill that plugs into the wall.

The Maxwell disks were the inspiration for PacMan

The picture below shows the results of day 3 of my dramatic reenactment of Munsell's landmark experiment. I selected red, green, and blue construction paper, and adjusted the size of the segments in order to get a facsimile of gray. When I saw that gray, I realized that this was four days well spent.

Me, geeking out on the creation of gray from Red, green, and blue

If I were to be doing this on a government grant, I would have spent another day or two actually measuring the sizes of the red, green, and blue areas. For the purposes of this blog, I will be content with just saying that red and blue are each one-quarter, and green is one-half. In other words, this green is half as strong as the others. Thus, Munsell would conclude that the chromas of this red and blue were twice that of this green. Munsell would also have measured this gray with his photometer. Another opportunity for me to get a little more grant money.

In this way, Munsell was able to assign values to the colors.

     Perceptual linearity?

Linear in Value?

Since Munsell's original Value was measured as the width of an aperture, the amount of light let through is proportional to the square of the Value. Conversely, Value is proportional to the square root of the light intensity. The plot below compares this scale against today's best guess at perceptual linearity, CIEDE2000.

Munsell's original Value was kinda sorta close to perceptually linear

Note: The DE2000 scale in the plot above is based on Seymour's formula (L00 = 24.7 Log e (20 Y +1), where 0< Y < 1), which was first presented at TAGA 2015, Working Toward A Color Space Built On CIEDE2000. The height of the curve at the end shows that there are 76 shades of gray, based on DE2000. The Munsell Value has been scaled to that.

Is this perceptually linear? That depends on how gracious you want to be. On the one hand, the linearity is not lousy. Given the tools at hand, Munsell did a fairly decent job of making kinda linear.

On the ungracious side, Munsell merely took what he had handy (the size of the opening of his aperture) and used that. Lazy bum! Surely he would have known about the work of Ernst Weber (1834) and Gustav Fechner (1860) which postulated that all our perception is logartihmically based! 

Really pedantic note: There is some confusion about how the gray scale was set up. My description is based on Munsell's description [1905], as well as comments by Tyler and Hardy [1940], Bond and Nickerson [1940], and Gibson and Nickerson [1940], all of which were based on Munsell's words and measured samples. But in a paper from 1912, Munsell described his assignment of Value as being logarithmic, following the Weber-Fechner law.

Linear in hue?

Munsell started this exercise by selecting five paints with vibrant colors: red (Venetian red), Yellow (raw sienna), green (emerald green), blue (cobalt), and purple (madder and cobalt). He then created paints that were opposite hues for each of these. The opposite hues were adjusted so that the balanced out to gray on the Maxwell disks. Thus, he had a set of ten colors with Value of 5 and Chroma of 5.

What's to say that these paints are equally spaced in hue? I am sure that Munsell selected them with that in the back of his mind, but four of the five are just commonly available, single pigment paints.
From the literature that I reviewed in the bibliography below, I could find no evidence that he put much time into psychophysical testing.

I'm gonna say that the hue spacing in the original Munsell color system is only somewhat perceptually linear.

Linear in Chroma?

Munsell's assignment of Chroma values is all based on simple ratios of areas on the Maxwell disks. Thus, in his original system, chroma is linear with reflectance. I did a bit of testing, comparing Munsell's proposition against DE2000. I will smugly state that our perception is not linear with reflectance.

But Munsell begs to differ with me. He performed some tests of this, and summarized his results in 1909:

These experiments show clearly that chroma sensation and chroma intensity (physical saturation) vary not according to the law of Weber and Fechner, but nearly or quite proportionately, and in accordance with the system employed in my color notation.

This paper seems to have been largely ignored by other color researchers. Deane Judd looked at the question of equal steps in chroma in 1932. His bibliography included Munsell's 1909 paper, but he made no mention of it in the text. The same with several of the papers from 1940 listed below.

My brief test suggests this is not true, and the people who were genuinely interested in the question who were aware of Munsell's suggestion ignored it. The graphs from the 1943 paper (Newhall, et al.) are decidedly non-linear in steps of chroma. Barring further evidence, I would say that the original Munsell Color System was not perceptually linear in chroma.

All in all, I'm gonna rate the claim that the original Munsell system was perceptually linear as "Mostly False".

What happened after Albert Munsell

Albert Munsell passed on in 1918, but a lot of work was done on the Munsell Color System by others after his death.

In 1919 and again in 1926, Munsell's son, A. E. O. Munsell submitted samples to the National Bureau of Standards. These were measured spectrophotometrically. The 1919 data was analyzed by Priest et al., and came along with some suggestions for improvement. They suggested that the Value scale be changed. 

This challenge was taken up by Albert's his own son. In 1933, A. E. O. published a paper describing a modification of the function from which Value was computed. This brought value much closer into line with the predictions of CIEDE2000.

The Munsell Color System was largely ignored in the literature until 1940. At that time, seemingly everyone jumped on the bandwagon. A subcommittee of the Optical Society of America was formed, and the December 1940 issue of the Journal of the Optical Society in America published five papers on the Munsell Color System.

Why the sudden effort? Spectrophotometers were expensive and cumbersome, but were becoming available. The 1931 tristimulus curves were available to turn spectral data into human units. Several of the papers noted a desire to create a system which translated physical measurements into something that made intuitive sense.

The Munsell Color System seemed to be best template to shoot for, since it was "[l]ong recognized as the outstanding practical device for color specification by pigmented surface standards."  (Newhall, 1940)

The efforts of the OSA subcommittee culminated in what has become known as the Munsell Renotation Data, introduced in the 1943 paper by Newhall et al. Inconsistencies of the original data were smoothed out, a new Value scaled was introduced, and a huge experiment (3 million observations) was done to nudge the colors into a system that looked perceptually linear.  The final result is a color system that can indeed be said to be perceptually linear.

Oh what a tangled web we weave, from Newhall (1943)

I'm not gonna take up the rest of the story, from the Renotation Data to CIELAB. That's another long and interesting story, I'm sure. But I am running out of gas!

Conclusion

Here is the firmest entirely factual statement that I can make about this paternity suit involving Albert Munsell and the child named Color Science.

Munsell had a passion for teaching color, especially to children. He sought to bring order and remove ambiguity from communication of color. This passion brought him to create the Munsell Color System. This was not the first three-dimensional arrangement of color, nor was it all that close to being perceptually linear. But it had two great features going for it: It was built on the intuitive concepts of hue, chroma, and lightness, and it came with a recipe for building a physical rendition of the color space. As a result, the Munsell Color Space is both a concept for understanding color, and a physical standard to be used in practical communication of color.

The Munsell Color System saw a number of improvements after his death, resulting in the Munsell Renotation Data. This later became the framework for future development of a magic formula to go from measured specrta to three numbers that define a color. The CIELAB formula is the one that stuck.

I realize that my work over the past 25 years has given me a bias toward the importance of measurement of surface colors, and hence a bias toward thinking that CIELAB is important. The next statement is subjective, and based on my admitted biases.

I think that Albert Munsell deserves to be called The Father of Color Science.

Albert Munsell proudly showing off his very attractive John the Math Guy Award

On the other hand...

I would be remiss if I failed to mention a few other individuals, who might reasonably be on the podium with Munsell.

Isaac Newton - He invented the rainbow, right? Well, actually, he did some experiments with light and came up with the theory of the spectrum. Spectrophotmeters are designed to measure this.

Thomas Young - He first proposed the theory that the eye has three different sensors (red, green, and blue) in 1802. Hermann von Helmholtz built on this in 1894.

Ewald Hering - He proposed the color opponent theory in 1878. Light cannot be both red and green; nor can it be both blue and yellow. His three photoreceptors were white versus black, red vs green, and yellow vs blue. This is explicitly built into CIELAB.

It turns out that all of these are correct, but they are looking at different stages in our perception. Newton's spectrum is a real physical thing. The retina does have three Young-Helmholtz sensors. The cones are not exactly RGB, but kinda. And the neural stuff after the cones in the retina creates signals that follow Hering's theory.

So, maybe one of these gents should get the crown? I dunno... maybe I'll make a few more John the Math Guy awards?

Bibliography

Munsell's papers

Munsell, Albert H., A Color Notation, Munsell Color Company, 5th Edition, 1905, Chap V

Munsell, Albert H., On the Relation of the Intensity of Chromatic Stimulus (Physical Saturation) to Chromatic Sensation, Psychological Bulletin, 6(7), 238-239 (1909)

Munsell, Albert H, A Pigment Color System and Notation, Amer. Journal of Psych, Vol 23, no. 2, (April 1912)

Post Munsell, pre-1940

Priest, Irwin, K. S. Gibson, and H. J. McNicholas, An examination of the Munsell color system. I. Spectral and total reflection and the Munsell scale of value, Tech. Papers of the Bureau of Standards, No. 167 (September 1920)

Judd, Deane, Chromatic Sensibility to Stimulus Differences, JOSA 22 (February 1932)

Munsell, A. E. O., L. L. Sloan, and I. H. Godlove, Neutral Sclaes. I. Munsell Neutral Value Scale, JOSA (November, 1933)

Glenn, J. J. and J. T. Killian, Trichromatic analysis of the Munsell Book of Color, MIT Thesis (1935), also in JOSA (December 1940)

The 1940's flurry

Gibson, Kasson S and Dorothy Nickerson, An Analysis of the Munsell Color System Based on Measurements Made in 1919 and 1926, JOSA, December 1940

Newhall, Sidney, Preliminary Report on the O.S.A. Subcommittee on the Spacing of the Munsell Colors, JOSA, December 1940

Tyler, John E. and Arthur C. Hardy, An Analysis of the Original Munsell Color System, JOSA December 1940

Nickerson, Dorothy, History of the Munsell Color System, Company, and Foundation. II. Its Scientific Application, JOSA, December 1940

Bond, Milton E., and Nickerson, Dorothy, Color-Order Systems, Munsell and Ostwald, JOSA, 1942

Newhall, Sidney M., Dorothy Nickerson, and Deane B. Judd, Final Report of the O.S.A. Subcommittee on the Spacing of the Munsell Colors, JOSA July 1943

More recent

Hunter, Richard S., The Measurement of Appearance, John Wiley, 1975, pps. 106 - 119