What is colour?

1.2 The Dimensions of What, Exactly?

What is a Colour?

What gives a light a particular hue, brightness and saturation? How does an object come to have a particular hue, lightness and chroma? We perceive lights and objects to have particular colours, but what is not clear to introspection alone is whether the attributes of these perceived colours, such as hues like red and green, are (1) physical properties residing in lights and objects that our visual system simply detects, or (2) ways of seeing certain properties of lights and objects, ways our visual system creates. If the latter, we must make a distinction between colour as a psychological perception and the properties of lights and objects that we see as these perceived colours.

Figure 1.2.1. A. Extract from Newton's Opticks (1704, p.90). B. Newton's colour circle from the Opticks (1704, Book I, Part II, Pl. 3), illustrating his "center of gravity" principle for predicting the colour of a mixture of lights. Unequal amounts of red, orange, yellow, green, blue, indigo and violet "rays", indicated by the relative sizes of the small circles (p, q, r, s, t, u, and x), mix to make light of colour z between spectral orange (Y) and white (O), seen as whitish orange.

Scientific consensus is firmly with the second of these two views, which can be traced back to antiquity via Descartes, Locke and Galileo, but which begins in substantial detail with Sir Isaac Newton's researches into the physical basis of colour. In the well-known passage from his Opticks (1704) shown in Fig.1.2.1A Newton explicitly distinguishes between colour as a psychological perception ("sensation") and what we call colours "in the rays" and "in the object". For Newton the colour of a light is its "power" or "disposition" to be seen as this or that perceived colour. Newton demonstrated elsewhere in the Opticks that for an isolated light this power or disposition depends on the relative balance of the component "rays" (we would now say wavelengths) present, represented approximately by their "center of gravity" in his colour circle (Fig. 1.2.1B). But as his diagram implies, most colours of light can be evoked by many different combinations of "rays" having the same centre of gravity. This means that a given colour of light does not correspond to a single physical combination of "rays" but to a whole class of combinations (we would now say spectral power distributions, as in Fig. 1.2.3, left) that are indistinguishable to the human visual system. We now call members of such a class metameric, and we believe that they are indistinguishable to human vision because they evoke the same relative response of the three cone cell types on which our colour vision depends.
Thus in the Newtonian view the particular colour of a light, specified by its position in his circle, is neither a purely physical nor a purely psychological property, but involves a relation between the two, now called psychophysical. The particular colour of an object for Newton depends on the object's "disposition to reflect this or that sort of rays more copiously than the rest" (we would now say its spectral reflectance), but similarly a whole class of spectral reflectances are indistinguishable (metameric) to the human visual system under the same illumination, so the particular colours of objects are also psychophysical.

Perceived Colour and Psychophysical Colour

The International Lighting Vocabulary of the International Commission on Illumination (CIE)1, whose terminology is widely accepted as standard in science and technology, defines two distinct concepts of "colour":

Perceived colour: "characteristic of visual perception that can be described by attributes of hue, brightness (or lightness) and colourfulness (or saturation or chroma)" (CIE, 2011, 17-198).
Psychophysical colour:"specification of a colour stimulus in terms of operationally defined values, such as 3 tristimulus values" (CIE, 2011, 17-197). Tristimulus values are in turn defined as the "amounts of the 3 reference colour stimuli, in a given trichromatic system, required to match the colour of the stimulus considered", for example RGB and XYZ values in the CIE colorimetric system (CIE, 2011, 17-1345).

Figure 1.2.2. Members of a psychophysical colour class match in colour to a "standard observer" when presented in the same context, but can evoke different perceived colour attributes when presented in different contexts, even when they are physically identical. The central squares on either side are physically identical and so have the same psychophysical colour - they have the same RGB specification (R159 G170 B65) and would match the same Munsell chip placed against them - but they differ in perceived colour.

These definitions correspond to two distinct ways in which the word "colour" is used in common speech as well as in science and technology: Demonstrations of contrast phenomena such as Fig. 1.2.2 can been described either by saying that the influence of the surround affects the colour of the squares, or by saying that on different surrounds the squares appear to be different colours, but are actually the same colour. In the first description the word colour is used for an aspect of immediate visual perception (perceived colour), in the second it refers to the shared colour specification (psychophysical colour) of the squares, one of the 16.7 million "colours" available on an RGB screen. Applying CIE terminology, the same psychophysical colour evokes two different perceived colours.
The six attributes mentioned in the definition of perceived colour - hue, brightness, lightness, colourfulness, saturation and chroma - are all defined perceptually, that is, as attributes of visual perception rather than as physical properties of lights or objects. None of the six are alternative names for the same thing: a perceived colour can be described in terms of hue, lightness and chroma if it is seen as belonging to an illuminated object, or in terms of either hue, brightness and saturation or hue, brightness and colourfulness if it is seen as belonging to light. Brightness is how we perceive the amount of light emitted, transmitted or reflected by an area, while lightness is how we perceive an object's diffuse luminous reflectance, that is, its efficiency as a diffuse reflector of light. Hue, comprising perceptions in the cycle of red, yellow, green and blue, is how we perceive the direction of bias of the spectral power distribution of a light relative to daylight, or of the spectral reflectance of an object relative to a spectrally even reflector. Saturation or freedom from whitishness is how we perceive the relative amount of spectral bias of a light, while colourfulness is how we perceive the absolute amount of spectral bias; it is the cumulative effect of the saturation and brightness of a light. Chroma or visual difference from a grey of the same value 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.
We normally perceive the object colour attributes of hue, lightness and chroma instantly and automatically, without the need for conscious judgement or effort. This ability evidently relies on pre-conscious processes that arrive at a probable interpretation of the information from our eyes in terms of object colours and illumination. For example, they arrive at an interpretation of which areas in the visual field remit little light because they are dimly lit, and which remit little light because they are poor reflectors of light. So when we look at most areas of the visual field we automatically see, not a patch of light, but a combination of an object with a seemingly intrinsic colour (the colour the object would appear in daylight) and an illumination with a seemingly intrinsic colour (hue, brightness and saturation). For example, a white sheet of paper viewed clearly under yellow illumination is usually perceived as being white, the yellowishness being automatically and unconsciously attributed to the colour of the illumination, and a white sheet of paper viewed clearly in dim illumination is also usually perceived as being white, the darkness similarly being attributed to the level of illumination.
Whatever their (highly disputed) nature, these pre-conscious processes are effective enough to give the perceived colour of an object a high degree of stability (object-colour constancy) under different levels and (to a point) under different colours of illumination. The combination of effortlessness and stability creates and maintains the illusion that these perceived colour attributes, which are generated at a post-receptoral level, reside in the objects themselves, where we directly and passively "detect" them. Though they seem to be detected directly and passively, these mentally generated object colour attributes are more like elements of a 3D computer model projected by our minds onto the external world.

Figure 1.2.3. Left: Spectral power distributions of three standard white illuminants (light sources). Though very different physically, these three lights have the same chromaticity (psychophysical colour considered separately from brightness/luminance) and appear the same colour (a warm white). Right: CIE xy chromaticity diagram showing chromaticities of the RGB lights of a standard RGB colour space (sRGB) and of the three illuminants shown on the left. The faint horseshoe-shaped line shows the chromaticities of the monochromatic wavelengths of the spectrum. All images derived from the program ColorSpace by Philippe Colantoni.

A psychophysical colour specification identifies a class of physically varied lights or objects that match in colour (are metameric) when viewed in the same context. (Members of a psychophysical colour class can evoke different perceived colours when presented in different contexts, even when they are physically identical, as in Fig. 1.2.2). As "normal" colour vision varies somewhat between individuals, psychophysical specifications use the necessary assumption of a mathematically defined "standard" human observer. Every metameric class of lights can be specified by the quantities of three standard red, geen and violet lights, the CIE RGB "primaries", that match it, though this specification involves a negative quantity whenever one of the primaries must be added to the test stimulus to obtain a match. CIE RGB values are commonly converted to CIE XYZ values in which the Y is equal to the visible energy of light, called luminance and perceived as brightness.
A three-dimensional set of CIE RGB or XYZ values specifies the psychophysical colour of a light including its brightness/ luminance, just as a set of familiar RGB values does for a digital colour. We can also specify the colour of a light considered separately from its brightness/ luminance, called its chromaticity, using the ratio of the CIE X,Y and Z components, and represent this on a two-dimensional triangular CIE xy chromaticity diagram (Fig. 1.2.3). This diagram is a descendant of Newton's colour circle, in which the spectrum now forms a horseshoe shape instead of a circle, but in which the colour of a light still depends on the balance of its spectral components. Despite their very different spectral power distributions the three CIE illuminants (light sources) shown in Fig. 1.2.3 have the same chromaticity, and all appear "warm white" to an observer with normal colour vision. The CIE xy diagram may be familiar to photographers and digital painters from its use to compare the gamuts (colour ranges) of different colour spaces or of the actual RGB lights used in different models of monitor.
Colours of objects can be specified psychophysically by the CIE RGB or XYZ tristimulus values of the light they reflect/ transmit, but as these values depend on the illuminant used it is necessary to also specify the latter, normally a standard "white" daylight illuminant such as CIE D65 or the older CIE Illuminant C.

Figure 1.2.4. Identical range of digital colours (sRGB colour space) represented in two different psychometric colour spaces. A. Munsell colour space, specifying colours in terms of Munsell hue, Munsell value and Munsell chroma. B. CIE L*a*b* colour space, specifying colours in terms of CIE lightness (L*) and two orthogonal chromatic dimensions corresponding to reddish vs greenish chroma (a* vs -a*) and yellowish vs bluish chroma (b* vs -b*). These chromatic dimensions can alternatively be presented as measures of hue H* and total chroma c in the form of CIE L*H*c space. Illustrations prepared using drop2color by Zsolt Kovacs-Vajna (A) and ColorSpace by Phillippe Colantoni. (B)

As attributes of perception, hue, brightness, lightness, colourfulness, saturation and chroma can not be measured directly, but various quantitative psychometric scales have been devised against which these psychological attributes can be judged. These psychometric scales can be presented purely as a collection of coloured samples, as in the 1915 Munsell Atlas and the 1929 Munsell Book of Color, but modern psychometric scales involve either an atlas of samples tied in with a table of colorimetric equivalents, as in the modern Munsell Book of Color and the Scandinavian Natural Colour System (NCS) atlas, or they are based directly on colorimetric data, as in CIE L*a*b* space. The tristimulus values for an object can thus be converted via a table or a formula to a position in a psychometric colour space that prediucts the perceived colour of that object under specified standard conditions. Since all major psychometric object colour spaces specify perceived colour under some form of daylight illuminant, it could be said that these specifications (e.g. Munsell hue, value and chroma) describe the seemingly intrinsic colour of an object that object-colour constancy to varying degrees recovers.
The alternative psychometric measures used in different systems for the same attribute typically agree in general terms in the way they arrange psychophysical colours, but differ in fine detail (Fig. 1.2.4). In the following pages we will introduce each of the six colour attributes defined by the CIE plus some alternative attributes defined in other systems including the Scandinavian NCS, along with various psychometric scales used as quantitative measures of some of these attributes.

Philosophical Theories of Colour

Colour has attracted a great deal of attention from philosophers in recent decades because of its connections with major metaphysical issues including the nature of physical reality, perception and the mind. Unfortunately a substantial part of the discussion has been taken up with what appear to amount to semantic disputes over the one true referent of the word "colour". For example, eliminativism confines the term "colour" to perceived colour, and thus concludes that colours are illusory in the sense that they are perceiver- and species-dependant, while dispositionalism confines "colour" to the (psychophysical) disposition of lights and objects to cause an experience of perceived colour. These theories isolate different parts of the scientific consensus, from Descartes and Newton to the CIE, that we use the word "colour" in two senses, for a psychological perception and for the properties that we see as those perceptions. More eccentrically, colour physicalism equates the colour of an object with its particular spectral reflectance, classifying its "perceived colour" as merely the appearance of this actual, physical colour. Most of the major philosophical positions seem more or less compatible with current science on the process of colour vision, but colour primitivism on the other hand valiantly defends the position that colours are intrinsic, non-relational, and non-reducible properties of lights and objects. For an infinitely fairer and more authoritative outline of the philosophy of colour than I can provide, the reader should consult Barry Maund's entry "Color" in The Stanford Encyclopedia of Philosophy, written from the perspective of one of the leading exponents of the field.
1The International Commission on Illumination or Commission Internationale de L'Eclairage (CIE) was founded in 1913 and is the organization responsible for the international coordination of lighting-related technical standards. In the field of colour the CIE established the fundamental framework of modern colorimetry including the CIE 2o and 10o standard observers, the CIE standard illuminants (Illuminants C, D50, D65 etc), various widely used colour spaces including CIE XYZ, CIE xyY and CIE L*a*b*, colour difference formulae (CIEDE94, CIEDE2000), and colour appearance models including CIECAM02. Its International Lighting Vocabulary lists definitions for 1448 terms and is by far the most comprehensive and authoritative source on the scientific terminology of light and colour.
Page added January 15, 2017

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