Editing Munsell color system (section)

== Explanation ==
{{multiple image
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| image1    = Munsell color sphere.png
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| caption1  = Munsell’s color sphere, 1900. Later, Munsell discovered that if hue, value, and chroma were to be kept perceptually uniform, achievable surface colors could not be forced into a regular geometric shape.
| image2    = Munsell 1943 color solid cylindrical coordinates gray.png
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| caption2  = Three-dimensional representation of the 1943 Munsell renotations (with portion cut away). Notice the irregularity of the shape when compared with Munsell's earlier color sphere.
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The system consists of three independent properties of color which can be represented [[cylindrical coordinate system|cylindrically]] in three dimensions as an irregular [[color solid]]:
* ''hue'', measured by degrees around horizontal circles
* ''chroma'', measured radially outward from the neutral (gray) vertical axis
* ''value'', measured vertically on the core cylinder from 0 (black) to 10 (white)
Munsell determined the spacing of colors along these dimensions by taking measurements of human visual responses. In each dimension, Munsell colors are as close to perceptually uniform as he could make them, which makes the resulting shape quite irregular. As Munsell explains:

{{quote|Desire to fit a chosen contour, such as the pyramid, cone, cylinder or cube, coupled with a lack of proper tests, has led to many distorted statements of color relations, and it becomes evident, when physical measurement of pigment values and chromas is studied, that no regular contour will serve.|Albert H. Munsell, “A Pigment Color System and Notation”<ref>Munsell (1912), [https://books.google.com/books?id=FdQLAAAAIAAJ&pg=PA239 p. 239]</ref>}}

=== Hue ===

[[File:Munsell hue wheel equal value and chroma.png|thumb|Gradient Munsell hue wheel at value 5 and constant chroma (6.24)|250px]]

Since the first rendition of the Munsell color system, each horizontal circle is divided into five principal ''hues'': '''R'''ed, '''Y'''ellow, '''G'''reen, '''B'''lue, and '''P'''urple, along with 5 intermediate hues between adjacent principal hues: '''YR''', '''GY''', '''BG''', '''PB''', and '''RP'''.<ref>{{harv|Munsell|1905}}, [https://archive.org/details/acolornotation00munsgoog/page/n16 ch.2, pg. 18]</ref> Despite [[trichromatic]] color being best described with 3 [[primary color]]s or 4 [[unique hues]], Munsell chose to define his color space with 5 principal hues to keep it [[Decimalisation|decimalized]].<ref>{{cite web |last1=Munsell |first1=Albert |title=How Is the Munsell Hue Circle Relevant to Your Work |url=https://munsell.com/color-blog/primary-hue-circle-colorchecker/ |website=Munsell Color System; Color Matching from Munsell Color Company |publisher=X-Rite inc. |access-date=18 February 2025 |language=en |date=10 October 2012 |quote=He wanted to use a decimal system making each step divisible by 5 or 10—a system which is easy to understand and easy for computer programming and cataloging. That’s why Munsell used 5 principal hues—red, yellow, green, blue and purple—instead of the seven colors in the visible spectrum (ROY G BIV).}}</ref><!-- The website states the author as Albert Munsell, despite him being dead since 1918 and not describing him in first person --> Munsell describes the intermediate hues as orange, grass green, peacock blue, violet and plum,<ref>{{harv|Munsell|1905}}, [https://archive.org/details/acolornotation00munsgoog/page/n16 ch.3, pg. 28]</ref> but opts to simplify the notation with the portmanteaus of the principal colors for the sake of intuition.<ref>{{harv|Munsell|1905}}, [https://archive.org/details/acolornotation00munsgoog/page/n16 ch.4, pg. 41]</ref>

In later renditions, these 10 principal and intermediate colors were further subdivided into 10 steps each, so that at least 100 hues are definable. The sub-steps are numbered 1 to 10, which prepends the hue letter(s), e.g. ''8GY''. However, further subdivisions are possible through interpolation, e.g. ''8.7GY''. In practice, color charts conventionally specify 40 hues, in increments of 2.5, progressing as for example 10R to 2.5YR.

Two colors of equal value and chroma, on opposite sides of a hue circle, are [[complementary color]]s, and mix [[additive color|additively]] to the neutral gray of the same value. The diagram below shows 40 evenly spaced Munsell hues, with complements vertically aligned. The [[sRGB]] values of the samples are provided inside each one of the squares.

{{Munsell-hues}}

=== Value ===
''Value'', or [[Lightness (color)|lightness]], varies vertically along the color solid, from black (value 0) at the bottom, to white (value 10) at the top.<ref>Cleland (1921), [http://www.applepainter.com/Chap02/ Ch. 2]</ref> Neutral grays lie along the vertical axis between black and white.

Several color solids before Munsell's plotted luminosity from black on the bottom to white on the top, with a gray gradient between them, but these systems neglected to keep perceptual lightness constant across horizontal slices. Instead, they plotted fully saturated yellow (light), and fully saturated blue and purple (dark) along the equator.

=== Chroma ===
''Chroma'', measured radially from the center of each slice, represents the “purity” of a color (related to [[Colorfulness#Saturation|saturation]]), with lower chroma being less pure (more washed out, as in [[pastel (color)|pastels]]).<ref>Cleland (1921), [http://www.applepainter.com/Chap03/ Ch. 3]</ref> Note that there is no intrinsic upper limit to chroma. Different areas of the color space have different maximal chroma coordinates. For instance light yellow colors have considerably more potential chroma than light purples, due to the [[Color_vision#Physiology_of_color_perception|nature of the eye]] and the physics of color stimuli. This led to a wide range of possible chroma levels—up to the high 30s for some hue–value combinations (though it is difficult or impossible to make physical objects in colors of such high chromas, and they cannot be reproduced on current computer displays). Vivid solid colors are in the range of approximately 8.

{{col-begin}}
{{col-break|width=36em}} {{Munsell-5PB-5Y}} {{col-break}}
{| class="toccolours" align="left" style="margin-top:50px; max-width:20em; min-width:15em; background-color:#E4E4FA; padding:5pt;"
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|Note that the ''Munsell Book of Color'' contains more color samples than this chart for both 5PB and 5Y (particularly bright yellows, up to 5Y 8.5/14). However, they are not reproducible in the [[sRGB color space]], which has a limited [[gamut|color gamut]] designed to match that of televisions and computer displays. There are no samples for values 0 (pure black) and 10 (pure white), which are theoretical limits not reachable in pigment, and no printed samples of value 1.
|}
{{col-end}}

=== Specifying a color ===
A color is fully specified by listing the three numbers for hue, value, and chroma in that order. For instance, a purple of medium lightness and fairly saturated would be 5P 5/10 with 5P meaning the color in the middle of the purple hue band, 5/ meaning medium value (lightness), and a chroma of 10 (see swatch). An [[achromatic color]] is specified by the syntax {{code|N V/}}. For example, a medium grey is specified by "N 5/".

In computer processing, the Munsell colors are converted to a set of "HVC" numbers. The V and C are the same as the normal chroma and value. The H (hue) number is converted by mapping the hue rings into numbers between 0 and 100, where both 0 and 100 correspond to 10RP.<ref>ASTM, Standard D 1535-08, "Standard Practice for Specifying Color by the Munsell System," approved January 1, 2008.</ref>

As the Munsell books, including the 1943 renotation, only contains colors for some points in the Munsell space, it is non-trivial to specify an arbitrary color in Munsell space. [[Interpolation]] must be used to assign meanings to non-book colors such as "2.8Y 6.95/2.3", followed by an ''inversion'' of the fitted Munsell-to-xyY transform. The ASTM has defined a method in 2008, but Centore 2012 is known to work better.<ref>{{cite journal |last1=Centore |first1=Paul |title=An open-source inversion algorithm for the Munsell renotation |journal=Color Research & Application |date=December 2012 |volume=37 |issue=6 |pages=455–464 |doi=10.1002/col.20715}}</ref>