1.5 The Dimensions of Colour: Chroma

• Chroma
• Measuring Chroma
• Measures of Relative Chroma

Chroma

Figure 1.5.1. Four plates displaying opposite hue pages from the 1915 Atlas of the Munsell Color System by Albert Munsell. A. 5BG and 5R plate. B. 5B and 5YR plate. C. 5PB and 5Y plate. D. 5G and 5RP plate.

Chroma is the chromatic strength of an object colour, the perceived amount of difference from a grey of the same lightness (value.) The term chroma (Greek for "colour") was introduced by Albert Munsell as a dimension of his Munsell system, in which it is represented by distance radially from the central value axis along scales intended to be perceptually uniform across all hues (Fig. 1.5.1). Various terms in different languages had of course been used for object colour strength in older colour order systems, but all of these seem to have referred to some form of relative chroma, that is, chroma relative to some amount deemed to be the maximum for that hue, and typically represented as the radius of a circular diagram.

The word saturation is often used loosely to mean chroma or some kind of relative chroma, but is defined as an entirely distinct concept in CIE terminology (Section 1.7).

Figure 1.5.2. Illustration of the CIE definition of chroma as the colourfulness of an area judged as a proportion of the brightness of a similarly illuminated white area. In the image at the top, the coloured squares in each horizontal row (R1-R3 and P1-P3) have the same chroma (and appear to be the same colour) because they each maintain the same colourfulness relative to the brightness of the areas seen as being white (W1-W3). (Note however that when R1-R3 and P1-P3 are judged against the same white (such as the white surface of the graph), they are perceived as different colours having different chromas).

The formal definition of chroma is based on the idea that when a chromatic light-reflecting object is increasingly strongly illuminated, the colourfulness of its appearance increases, but the brightness of a similarly illuminated white object increases proportionately, so its intrinsic strength of colour or chroma can be defined as the colourfulness judged relative to this brightness (Fig. 1.5.2).

Chroma: "colourfulness of an area judged as a proportion of the brightness of a similarly illuminated area that appears white or highly transmitting"(CIE, 2011, 17-139).

Figure 1.5.3. Comparison of spectral reflectance curves of a medium- and a high-chroma orange-yellow paint. Munsell plot and spectral reflectance curves for Gamblin Conservation Colours from drop2color by Zsolt Kovacs Vajna; paint sample photos from dickblick.com.

Chroma is how we perceive the absolute amount of spectral bias of an object's reflectance/ transmission, that is, its efficiency as a spectrally selective reflector/ transmitter of light. For an object to have high chroma it must reflect/ transmit one or two parts of the spectrum very strongly and the rest very weakly. In Fig. 1.5.3, Gamblin Cadmium Yellow Medium oil paint is similar in hue to Gamblin Yellow Ochre, but has higher Munsell chroma (and value) because it more efficiently reflects the wavelengths that contribute to an orange-yellow appearance.

Measuring Chroma

Figure 1.5.5. The forty hue pages of the Munsell Book of Color, Glossy Edition. The peak-chroma value of these pages varies from 8 to 8.5 for the 5Y hue page to 3 to 4 for the 7.5PB hue page.

Figure 1.5.6. RGB colours of forty Munsell hues, after a diagram by Hans Irtel (colours above chroma 18 not shown)

The scales of chroma in the Munsell system and in CIE L*C*h space (below) are perceptually even, open-ended dimensions with no specific upper limit apart from the theoretical limits of object colours. Munsell chroma is measured in Munsell chroma units, judged by reference to these scales of physical chips in the Atlas of the Munsell Color System (Munsell, 1915) and later the Munsell Book of Color (1929-). Using the correlation with CIE colorimetry contained in the 1943 renotation, Munsell chroma can be related to colorimetric measurements such as CIE XYZ tristimulus values.

Figure 1.5.7. A, Chroma limits of each hue page of the Munsell Book of Color, Glossy Edition (cf. Fig. 1.5.4). B, Chroma limits of mixing gamut of a selection of Golden Heavy Body Acrylics, generated using the program drop2color by Zsolt Kovacs-Vajna.

When Albert Munsell produced perceptually-even scales of colour chips for his 1915 Atlas he established that the maximum chroma he could obtain with his paints not only occurred at different values for different hues, but also varied considerably in absolute magnitude (Fig. 1.5.1). In the modern Munsell Book of Color the highest chromas (16 Munsell chroma units) are attained in the hue range from orange-yellow to orange-red, while the lowest maximum chromas (10 Munsell chroma units) are reached in the hue range from cyan to green (Fig. 1.5.7A). A very similar pattern can be seen in the gamut of colours mixed from any reasonably large range of artists' paints (Fig. 1.5.7B). This lopsided gamut of colours available to painters, with a bulge between red and yellow, does not really present a problem because the range of common object colours is restricted in essentially the same way, for the same combination of physical and physiological reasons.

Figure 1.5.8. Comparison of gamut of a range of artists' acrylic paints (in colour) with that of standard sRGB digital colours (in grey). The paints (Golden Heavy Body Acrylics) exceed the gamut of RGB colours in the areas where the latter is relatively poor (around yellow and cyan), thanks to the presence containing a good range of cadmium colours plus two phthalocyanine greens (blue shade and yellow shade) and a phthalocyanine blue.

The chroma limits of digital colours are very different however, far exceeding artists' paints with Munsell chromas of 24 in the violet-blue to magenta range, down to 18 at red (Figure 1.5.9A, below). Artists' paints on the other hand exceed the gamut of standard (sRGB) digital colours where these are relatively poor in the vicinity of yellow and cyan (Fig. 1.5.8).

Figure 1.5.9. Gamut of a standard RGB colour space (sRGB) in (A) Munsell and (B) CIE L*a*b* space, in oblique perspective view (above) and in plan view (below).

In several important colour spaces colours are arranged as in hue-lightness-chroma spaces, but are specified using two chroma dimensions orthogonal (at right angles) to each other, instead of hue and chroma. These orthogonal dimensions, loosely related to the opponent axes red-green and yellow-blue, are more amenable to calculating colour differences than the polar hue-chroma dimensions. Position in the hue-chroma plane, whether specified by orthogonal or polar dimensions, is sometimes called chrominance (Kerr, 2005). In CIE L*a*b* colour space (Fig. 1.5.9B), widely used in colorimetry, and appearing as Lab colour space in graphics programs such as Photoshop, colours are specified by two chroma dimensions, +a/-a and +b/-b that coincide roughly with the Munsell 10RP/10G and 5Y/5PB hue axes respectively. Thus instead of being specified as having an orange hue at a certain chroma, a colour is specified as having so much reddish chroma (a) and so much yellowish chroma (b). When a more intuitive representation is required, CIE L*a*b* coordinates can be converted very simply to measures of hue, lightness and chroma (L*C*h). The measure of chroma (C*) is calculated by applying Pythagoras' theorem to the a and b dimensions (square root of a² + b²), and h is the hue angle measured counterclockwise from the +a axis. A physical colour atlas of hue pages of coloured chips is available that embodies this system (the RAL Design Atlas). Other orthogonal colour spaces known as YCbCr, YUV and YIQ, are used in television and video, and in jpg compression.

Figure 1.5.10. Adobe Photoshop colour picker, showing RGB colours with Lab lightness (L) of 63. NB: many combinations of Lab values within the square are outside the range of real RGB colours; available RGB colours with L=63 are confined to the area here outlined in black.

In Photoshop colours can be selected using the orthogonal dimensions of Lab space using either the sliders in the Color window or the Lab controls in the colour picker. This permits colours to be chosen by lightness, but one trap is that real RGB colours are confined to a vaguely visible zone towards the middle of the square shown in Fig. 1.5.10. Clicking on the brightly coloured areas outside this zone will yield colours that are the closest available, but which don't actually have the Lab coordinates they are shown as having.

Measures of Relative Chroma

Dimensions of relative chroma judged either (1) as a proportion of the chroma deemed to be the maximum possible for a given hue, or (2) as a proportion of the maximum possible for a given hue at the same lightness, etc., were included in several early colour order systems, including the early 19th century systems of Klotz and Gregoire (see Kuehni and Schwartz, 2008). The concept of a "maximum" chroma is potentially problematic as it depends on what comparisons are ruled in or ruled out. If the maximum chroma is based on the highest chroma attainable with dyes and pigments at a given time, this may change with developments in chemistry. If it is based on estimates of colour content by test subjects, it matters what the subjects' were thinking of as the highest chroma colour. For example, did they include fluorescent-looking colours or the even greater chroma of an area emitting bright monochromatic light?

Figure 1.5.11. A. Denman Ross' colour classification from Snow and Froehlich (1904) B. Three of the twelve hue pages, BV (blue-violet), V (violet) and VR (violet red) from Denman Ross' The Painter's Palette (1919). The vertical lines represent four degrees of "intensity", while the oblique lines represent four degrees of "neutralization".

Albert Munsell's friend and rival Denman Ross presented a simple conceptual classification of colours in terms of three attributes: hue ("color"), lightness ("value") and a measure of relative chroma he called intensity, judged relative to the maxmum attainable for the same hue from a specified set of paints (Fig. 1.5.11). Ross also used a measure called neutralization for chroma relative to the maximum possible for a given hue at a given value. Both intensity and neutralization were to be expressed on a scale of 0, 1/4, 1/2, 3/4 and 1. Fig. 1.5.11B shows Ross' hue pages for three of the twelve hues in his system.

Figure 1.5.12. Horizontal (left) and vertical (right) cross sections of the colour sphere described by Johannes Itten in his The Art of Color (1961)

In the color sphere described by Johannes Itten in his The Art of Color (1961) each hue is represented by a tint-shade scale between white and black at the surface and two intermediate degrees of relative neutralization between these scales and the greyscale axis. (Fig. 1.5.12). In the simple digital colour space HLS, so-called "saturation" is a similar dimension of relative neutralization. The difference between these measures and Ross' neutralization dimension is that the latter measures chroma relative to the maximum possible at a given lightness, while the relative neutralization dimensions of Itten and HLS measure chroma relative to the maximum possible a given hue and vertical position in the system, which varies in lightness for different hues.

The Scandinavian NCS system has a dimension called chromaticness representing chromatic content considered as a proportion of the colour as a whole. It is related to both chroma and saturation as defined by the CIE, and is discussed in a later section.

Page added January 23, 2017. A much earlier (2007) account of chroma is located on this site here.

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