The Dimensions of Colour: Saturation

1.7 The Dimensions of Colour: Saturation


Figure 1.7.1. Three series of RGB colours each of uniform saturation: R1-R3 (high saturation), P1-P3 (medium saturation) and W1-W3 (zero saturation). Although R1 and R3 have the same saturation, R3 is seen as having higher chroma than R1 when viewed in the same context, because by emitting a greater amount of equally saturated red light, it is more colourful (but see also Fig. 1.5.2!).

The word saturation is often used loosely for chroma or some form of relative chroma, but is defined by the CIE as a distinct attribute of perceived colour. In Fig. 1.7.1 the series of RGB colours R1-R3 and P1-P3 each increase progressively in brightness and colourfulness from left to right, and judged as object colours they similarly increase in lightness and chroma. Yet each series is also perceived to have a chromatic attribute in common: R1-R3 are perceived to emit relatively pure red light, while P1-P3 are perceived to emit whitish red light in which the white and red components are in a fixed balance. Within each series colourfulness increases in step with brightness, but because of their greater purity of light, R1-R3 are more colourful at a given brightness than P1-P3.

Figure 1.7.2. Illustration of the CIE definition of saturation. R1-R3 have high saturation, P1-P3 have moderate saturation and W1-W3 have zero saturation.

Saturation as defined by the CIE is this attribute of the relative colourfulness of an area independent of its brightness, which in effect is the relative freedom from whitishness of the light it remits. Saturation in this sense is how we perceive the spectral purity or amount of imbalance of the spectral power distribution of a light, as far as this can be detected by our visual system with just three types of cone cells.
Saturation: "colourfulness of an area judged in proportion to its brightness" (CIE, 2011, 17-1136).
As saturation is defined as a function of two attributes of the colour appearance of a single area, it depends entirely on the appearance of the light from that area, and not on "judgements" in relation to other areas of the visual field, as do the object colour attributes lightness and chroma. Since saturation and colourfulness are not independent, we can describe colours of light either in terms of hue, brightness and colourfulness or in terms of hue, brightness and saturation.

Figure 1.7.3. Lines of uniform saturation on Munsell Book of Color, Glossy Edition (A) 5Y and (B) 5PB hue pages. The 5PB chips attains higher saturation but lower chroma than the 5Y chips.

Saturation differs from the other attributes of colours of light, brightness and colourfulness, in that it remains essentially constant for a given object under different levels of illumination unless the brightness is very high (CIE, 2011, 17-1136, note). When a chromatic object is increasingly strongly lit by the same illuminant, the spectral power distribution of the light it reflects should remain the same while its brightness increases, and we would therefore expect the saturation or perceived spectral purity of this light to tend to also remain the same. Object colours can therefore be characterized by the saturation and relative brightness of the light they reflect, as an alternatice to using their chroma and lightness. Because chroma and lightness are colourfulness and brightness judged relative to the same thing, their ratio reduces to the ratio of colourfulness to brightness, that is, to saturation. So on a Munsell hue page, colours of uniform saturation lie along lines that radiate from near the zero point on the value (lightness) scale, in contrast to lines of uniform chroma which are of course vertical (Fig. 1.7.3).

Figure 1.7.4. A. Seven uniform saturation series (including the achromatic surround) perceived as uniformly coloured objects under varying illumination. David Briggs, 2007, Photoshop CS2. B. Colours from A plotted in YCbCr space. C. Colours from A plotted in L*a*b* space. B and C plotted using the program Colorspace by Philippe Colantoni.

Similarly, if the ratio of the RGB components of a series of digital colours is maintained as their brightness increases, we would also expect their spectral power distribution and saturation to stay the same (as each of the series in Fig. 1.7.1). Figure 1.7.4A shows how readily our visual system reads uniform saturation series as an array of uniformly coloured objects under varying illumination, provided that the arrangement permits such a reading. These radiating lines of uniform saturation, in which chroma increases steadily in step with lightness, are sometimes called shadow or shading series, and are very important to painters because they define the paint or digital colours that create the appearance of a uniformly coloured diffusely reflecting object turning under a light source.

Measuring Saturation

Figure 1.7.5. Munsell hue pages of digital colours from Quantified as chroma relative to lightness, the maximum saturation attained by digital colours of different hues varies greatly.

Different colour appearance models use a variety of formulae working from different parameters to quantify saturation (Fairchild, 2013). The simplest formulae quantify saturation as the ratio of chroma to lightness (RLAB, ZLAB). Quantified in this way, different pages of the Munsell Book of Color attain different maximum saturations, as well as different maximum chromas, on different hue pages (Fig. 1.7.3, 1.5.5). Digital colours are even more varied in the maximum absolute saturation they attain at different hues (Fig. 1.7.5, 1.5.6).

Figure 1.7.6. RGB components of digital colour P3 from Fig. 1.7.1. P3 has an HSB saturation of 100 x 0.5 = 50.

The parameter called "saturation" (S) in HSB (or HSV) colour space, used for example in the Adobe Photoshop colour picker, is a rough physical estimate of the saturation of an RGB colour relative to the maximum saturation possible for its Hue angle (H), and is based on a very simple calculation from its r, g and b components (Fig. 1.7.6). For example, R100 G0 B000, R200 G0 B00 and R100 G100 B000 are all fully saturated (S = 100), while R200 G100 B100 has a saturation of 50. Remember however that the maximum saturation attained by RGB colours varies greatly with hue (Fig. 1.7.5), and an RGB cyan of S=100 (G255 B255) is far less saturated (that is, more whitish) than an RGB blue of S=100 (B255).

Figure 1.7.7. RGB colours of HSB Hue angle = 0 projected onto* the Munsell 7.5R hue page, showing lines of uniform HSB saturation and brightness. ( *Although the colours all have the same digital hue angle (H=0), they drift somewhat in Munsell hue).

The parameter called "saturation" in HLS colour space is completely unrelated to the standard definition of saturation, and is a kind of relative chroma (chroma relative to the maximum possible for a given "L"). "Saturation" has yet another two meanings in Photoshop in the definition of the Hue, Saturation, Color and Luminosity layer modes. When these layer modes are used in Lab mode "saturation" means Lab chroma , but when they are used in RGB mode it is a measure of chroma in YCbCr space relative to the maximum possible for RGB colours.

Figure 1.7.8. Is A the same colour as B or D? It all depends on how you look at it, which is why we need to be so careful about terminology. Viewed as objects in a scene, we see two strips, AB and CD, each of uniform lightness and chroma, AB lighter and higher in chroma than CD. Seen as image colours, however, A and B are surfaces of different lightness and chroma, and emit light of different brightness and colourfulness. They have been painted this way in order to represent the brighter and more colourful light that a surface of uniform chroma reflects where it is more strongly lit. Seen as light, all four areas emit light of the same saturation - in each area, only the red phosphors are glowing (right). Areas A and D emit light of the same hue, saturation and brightness - that is, they are identical image colours, even though they are perceived as very different object colours in the scene. Indeed, because these perceived object colours are so visually insistent, it may be very difficult to see these image colours as identical without in some way breaking the representational spell of the image. Image: David Briggs, 2007.

Page added January 30, 2017.
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