Color Theory: How to convert Munsell HVC to RGB/HSB/HSL

Question

I'm looking at at document that describes the standard colors used in dentistry to describe the color of a tooth. They quote hue, value, chroma values, and indicate they are from the 1905 Munsell description of color:

The system of colour notation developed by A. H. Munsell in 1905 identifies colour in terms of three attributes: HUE, VALUE (Brightness) and CHROMA (saturation) [15]

HUE (H): Munsell defined hue as the quality by which we distinguish one colour from another. He selected five principle colours: red, yellow, green, blue, and purple; and five intermediate colours: yellow-red, green-yellow, blue-green, purple-blue, and red-purple. These were placed around a colour circle at equal points and the colours in between these points are a mixture of the two, in favour of the nearer point/colour (see Fig 1.).

VALUE (V): This notation indicates the lightness or darkness of a colour in relation to a neutral grey scale, which extends from absolute black (value symbol 0) to absolute white (value symbol 10). This is essentially how ‘bright’ the colour is.

CHROMA (C): This indicates the degree of divergence of a given hue from a neutral grey of the same value. The scale of chroma extends from 0 for a neutral grey to 10, 12, 14 or farther, depending upon the strength (saturation) of the sample to be evaluated.

There are various systems for categorising colour, the Vita system is most commonly used in Dentistry. This uses the letters A, B, C and D to notate the hue (colour) of the tooth. The chroma and value are both indicated by a value from 1 to 4. A1 being lighter than A4, but A4 being more saturated than A1. If placed in order of value, i.e. brightness, the order from brightest to darkest would be:

A1, B1, B2, A2, A3, D2, C1, B3, D3, D4, A3.5, B4, C2, A4, C3, C4

The exact values of Hue, Value and Chroma for each of the shades is shown below (16)

So my question is, can anyone convert Munsell HVC into RGB, HSB or HSL?

Hue    Value (Brightness) Chroma(Saturation)
===    ================== ==================
4.5    7.80               1.7
2.4    7.45               2.6
1.3    7.40               2.9
1.6    7.05               3.2
1.6    6.70               3.1
5.1    7.75               1.6
4.3    7.50               2.2
2.3    7.25               3.2
2.4    7.00               3.2
4.3    7.30               1.6
2.8    6.90               2.3
2.6    6.70               2.3
1.6    6.30               2.9
3.0    7.35               1.8
1.8    7.10               2.3
3.7    7.05               2.4

They say that Value(Brightness) varies from 0..10, which is fine. So i take 7.05 to mean 70.5%.

But what is Hue measured in? i'm used to hue being measured in degrees (0..360). But the values i see would all be red - when they should be more yellow, or brown.

Finally, it says that Choma/Saturation can range from 0..10 ...or even higher - which makes it sound like an arbitrary scale.

So can anyone convert Munsell HVC to HSB or HSL, or better yet, RGB?

user_10315269 · Answer

Despite this old post, to update Steve's answer, here are "corrected" links to RIT's repositories of Munsell data:

https://www.rit.edu/cos/colorscience/rc_munsell_renotation.php

And a direct link to spreadsheet of the sRGB converted values of the "real" Munsell colors:

http://www.rit-mcsl.org/MunsellRenotation/real_sRGB.xls

It's a spreadsheet which includes a conversion from Munsell HVC notation to xyY, then to XYZ_C, then converted to D65 illuminant, then to floating point sRGB, then quantized to 8bit sRGB values (which they call dRGB).

As for the OP's question: sRGB is (obviously) an RGB additive color model. But the differences to other color models such as subtractive CMYK are complex enough that a "simple" algorithm won't handle the conversion — while color model transformations can be approximated with a matrix, more often a LUT (Look Up Table) is preferred, such as a LUT in an ICC profile or a 3D LUT as used in film production. (Not all ICC profiles are LUT based, but a LUT based conversion IMO is what is needed here).

The Munsell data certainly falls into this category, as not only is it a different color model, it is not only a subtractive model it is based on perception, while sRGB is based on a simple relationship between red green and blue light.

The spreadsheet is the useable look-up-table, so then a program to convert things like your dental chart to sRGB would take in that data and reference the LUT contained in the spreadsheet, and return the sRGB values.

Side Note: I want to mention for clarity that although some color-space or color-model transforms can be done reasonably with an algorithm/matrix, 3D LUTs are preferred particularly when the LUTs are created from measured data of a given color-model/space, which maps the many non-linearities inherent in some models.

An extreme example is an sRGB image on your computer monitor vs how that image is printed onto paper and appears on the cover of a magazine sitting on a newsstand illuminated with florescent light. That requires a 3D LUT for an accurate transformation!

In the feature film industry (where I mostly work) we use 3D LUTs throughout the image pipeline, not just for converting/transforms, but for "viewing" and for applying/emulating "looks." For instance taking an image shot with a digital camera and applying a LUT of a certain film stock to that image to make it appear as film.

user_7077264 · Answer

Since many of the resources of days long past seem to be dwindling away or, as Microsoft puts it, "is not intended for this version of Windows", scraping together as many resources, converters and look-up-tables as possible, is what this Q&A has turned into. Allow me to add one more, then.

Andrew Werth, a US artist, has a tool on their site to convert from Munsell to RGB and HTML hex codes. It's all inside one single .htm file, so I guess it could be downloaded and used on one's desktop as a sort of tool. Actually, that's what I did.

https://www.andrewwerth.com/color/

I like the art, too.

Color Theory: How to convert Munsell HVC to RGB/HSB/HSL

9 Answers