Colour coordinates and colour coordinate systems (colour spaces)

Colours can be described by three orthogonal properties: hue, lightness and chroma. A systematic ordering of colours in a three-dimensional space is therefore possible by using hue, lightness and chroma as the three coordinates. Formal and informal definitions of the coordinates, and their relative axial positions in colour space, are given below.

2.3.1.1.Hue

Hue is the ‘attribute of a visual perception according to which an area appears to be similar to one of the colors, red, yellow, green and blue, or to a combination of adjacent pairs of these

colours considered in a closed ring’ ([CIE] International Commission on Illumination, 1987).

Hue forms a circle around the central axis in colour space. In its meaning, hue is akin to the ‘everyday’ descriptions of colour. The mixing of hues forms a continuum of hues commonly known as a colour wheel (Minolta Camera Co. Ltd., 1993) (Figure 2.4).

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2.3.1.2.Lightness

Lightness is defined as the ‘attribute by which a perceived color is judged to be equivalent to

one of a series of grays ranging from black to white’ (ASTM International, 2013). Lightness

forms the central, vertical axis in colour space. Lightness can be evaluated independently of hue; different hues therefore can have the same lightness. The convention is to represent the continuum of colours in colour space in order of decreasing lightness from top to bottom (Figure 2.5). In ‘everyday’ language ‘bright’ and ‘dark’ are terms commonly used to describe degrees of lightness (Minolta Camera Co. Ltd., 1993).

2.3.1.3.Chroma

Chroma is the ‘attribute of color used to indicate the degree of departure of the colour from a

gray of the same lightness (ASTM International, 2013). In colour space chroma is represented

by the radial axis. Colours increase in chroma, or become more chromatic, with increasing distance from the lightness axis; this increase can be otherwise described as moving from weak

or dull colours to strong or vivid colours (Figure 2.5). Chroma is independent of both hue and lightness (Minolta Camera Co. Ltd., 1993; Berns, 2000).

Figure 2.4 A representation of the colour wheel displaying a continuum of hues which have the same level of lightness and the same chroma range (Minolta Camera Co. Ltd., 1993).

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2.3.1.4.Attributes for a more complete specification of colour appearance: Brightness, Colourfulness and Saturation

Two further dimensions, brightness and colourfulness, are actually required in addition to hue, lightness and chroma for a full specification of colour appearance (Fairchild, 2005):

Brightness is ‘the attribute of a visual sensation according to which an area appears to

emit more or less light’ (Fairchild, 2005);

Colourfulness is ‘the attribute of a visual sensation according to which the perceived

colour of an area appears to be more or less chromatic’ (Fairchild, 2005).

Brightness and colourfulness each describe a perception in the absolute sense, whereas lightness and chroma refer to brightness and colourfulness (respectively), relative to a similarly illuminated white, represented as follows (Fairchild, 2005):

Figure 2.5 A representation of the lightness and chroma dimensions of colour space, shown in a plane of constant hue; increasing chroma is displayed for a single level of lightness only. Adapted from Berns (2000).

19 ܮ݄݅݃ݐ݊݁ݏݏ ൌ ܤݎ݄݅݃ݐ݊݁ݏݏ ܤݎ݄݅݃ݐ݊݁ݏݏሺܹ݄݅ݐ݁ሻ Equation 2.1 ܥ݄ݎ݋݉ܽ ൌ ܥ݋݈݋ݑݎ݂ݑ݈݊݁ݏݏ ܤݎ݄݅݃ݐ݊݁ݏݏሺܹ݄݅ݐ݁ሻ Equation 2.2 Therefore, while brightness increases with the level of illumination, lightness is not affected because it is normalised according to the level of brightness under a given set of illumination and viewing conditions (MacDougall, 2002a; Fairchild, 2005). Also, brightness and colourfulness are assessed in isolation from other colours whereas lightness and chroma are assessed in relation to other colours.

Yet another attribute, saturation, can be determined from the combinations of colourfulness- brightness or chroma-lightness (the latter being an alternative used in some appearance models):

ܵܽݐݑݎܽݐ݅݋݊ ൌ ܥ݋݈݋ݑݎ݂ݑ݈݊݁ݏݏ

ܤݎ݄݅݃ݐ݊݁ݏݏ ݋ݎܵܽݐݑݎܽݐ݅݋݊ ൌ

ܥ݄ݎ݋݉ܽ ܮ݄݅݃ݐ݊݁ݏݏ

Equations 2.3 Although similar to chroma in that it is also a relative colourfulness, saturation is relative to its

own brightness, and unlike chroma, is assessed in isolation from other colours. Saturation remains constant as a gradually deepening shadow is cast over a single colour to form a shadow series (Berns, 2000; Fairchild, 2005).

In many situations however, it is only hue, lightness and chroma that are needed to specify colours because these situations involve the judgment of colours in relation to other colours (Fairchild, 2005). Colour spaces defined by hue, lightness and chroma are the subject of the following sections. Further mention will be made of brightness and colourfulness in a later section, with regard to colour appearance spaces for gamut mapping (Section 2.5.3.1).

20 2.3.1.5.Advantages

Colour coordinate systems enable the assignment of numerical values to colours, thereby providing the basis for precise, standardised colour description and communication which would not be possible if relying solely on descriptions by human observers. Observer descriptions will be more subjective and varied, even for the same object, due to differences in the perception (Section 2.2.2), experiences, vocabulary and descriptive ability between individual observers (Minolta Camera Co. Ltd., 1993; McClements, 2005). These, together with the vast number of possible colours that would need describing (McClements, 2005), mean that observers would have enormous difficulty in quantifying colours.

Each colour space is specific to a single set of viewing and illuminating conditions. Colour spaces also differ in the sampling of the space, and in their application (Berns, 2000). The following sections describe a selection of colour systems that are used in visual and in instrumental colour measurement.