What is colour? colour ontology, perceived colour, psyhophysical colour, primary and secondaty qualities,

1.2 The Dimensions of What, Exactly?

What is a Colour? Perceived Colour and Psychophysical Colour

Figure 1.2.1. The central squares on either side are physically identical - they have the same RGB specification (R159 G170 B65) and would match the same Munsell chip placed against them - but they are perceived to differ in colour. Should we say that their colour is changed by the influence of the surround, or should we say that although they appear to be different colours, the squares are actually the same colour? The answer depends on in which sense we are using the word "colour".

We perceive lights and objects as having intrinsic 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 their perceived colours.
Scientific consensus is firmly with this second view. 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 and psychophysical colour. The definitions are:
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).
These definitions correspond to two distinct ways in which the word "colour" is used in common speech. Demonstrations of contrast phenomena such as Fig. 1.2.1 can be characterized 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.

Figure 1.2.2. 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.

The view of the nature of colour (colour ontology) embodied in these definitions may be said to have three elements. First, we must make a distinction between colour perceptions and the properties of lights and objects that we perceive as their colour. Second, attributes of colour like hue, lightness and chroma are attributes of perception. Third, the property of a light or an object that we perceive as its particular colour is a psychophysical property, meaning that it is neither simply physical, nor simply psychological, but involves the perceiver as well as the stimulus in its definition2. Elements of this scientific view of colour can be traced back to antiquity via Descartes and Galileo, but began 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.1.2.2A Newton explicitly distinguishes between colour as a psychological perception ("sensation") and what are called colours "in the rays" and "in the object". For Newton, colour in light is the "power" or "disposition" of the light to be seen as this or that perceived colour, and Newton demonstrated 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. 1.2.2B). As his diagram implies, most "colours" of light can be evoked by many different combinations of rays having the same centre of gravity, which 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. "Colour" in an object for Newton is the object's "disposition to reflect this or that sort of rays more copiously than the rest".
We now call members of a class of visually indistinguishable colour stimuli metameric, and know that these metameric stimuli are indistinguishable to human vision because they all evoke the same response of the three cone cell types on which our colour vision depends. A class of metameric stimuli can be specified by a set of tristimulus values, defined as the "amounts of the 3 reference colour stimuli, in a given trichromatic system, required to match the colour of the stimulus considered" (CIE, 2011, 17-1345). (As "normal" colour vision varies somewhat between individuals, tristimulus values use the necessary assumption of a mathematically defined "standard" human observer). An example of a set of tristimulus values is an RGB triplet (in a given colour space) that uses amounts of particular red, green and blue "primaries" to specify any colour on a screen. CIE XYZ tristimulus values use three mathematically defined virtual "primaries" to psychophysically specify the colour of any light. Lights with the same tristimulus values will match in colour when viewed together in the same context, but their perceived colour will depend on that context (as in Fig. 1.2.1). Tristimulus values can however be correlated with perceived colour if standard viewing conditions are specified.
Indirectly but in essence, the XYZ tristimulus values of a light together specifiy the overall sizes of its long-, middle- and short-wavelength components, irrespective of small scale variations among wavelengths within each component. The psychophysical colour of an object can be specified by the XYZ trismulus values of the light it reflects under a specified (usually white) illuminant, in essence its disposition to reflect long/medium/short wavelengths "more copiously" than each other.

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 light intensity) and thus match in colour, appearing a warm white when viewed in isolation. 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 set of CIE XYZ tristimulus values specifies the psychophysical colour of a light including its intensity, just as a set of RGB values does for a digital colour. We can also specify the colour of a light considered separately from its intensity, using the ratio of the X, Y and Z components. This ratio defines its chromaticity, which can be represented on a two-dimensional chromaticity diagram such as the CIE xy diagram (Fig. 1.2.3, right). The CIE xy diagram is a descendant of Newton's circlular diagram, in which the spectrum 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.

The Attributes of Perceived Colour: Ways of Seeing

None of the six attributes mentioned above in the CIE definition of perceived colour - hue, brightness, lightness, colourfulness, saturation and chroma - are alternative names for the same thing. All are defined perceptually, that is, as attributes of visual perception rather than as physical properties of lights or objects, but each can also be characterized as a way of perceiving an aspect of those physical properties3.
  • Hue is defined perceptually as the "attribute of a visual perception according to which an area appears to be similar to one of the colours: red, yellow, green, and blue, or to a combination of adjacent pairs of these colours considered in a closed ring" (CIE, 2011 17-542). The hue of a light is the way in which we perceive the direction of bias between the long-, middle and short-wavelength components of its spectral power distribution, relative to daylight. The hue of an object is the way in which we perceive the direction of bias between the long-, middle and short-wavelength components of its spectral reflectance, relative to a spectrally neutral reflector.
  • Brightness is defined perceptually as the"attribute of a visual perception according to which an area appears to emit, or reflect, more or less light" (CIE, 2011, 17-111). Brightness is the way in which we perceive visible energy of light or luminance, that is, radiant energy in the range of visible wavelengths, weighted wavelength-by-wavelength by the response of the human visual system.
  • Lightness (= greyscale value) is defined perceptually as the "brightness of an area judged relative to the brightness of a similarly illuminated area that appears to be white or highly transmitting" CIE 2011, 17-680). Lightness is the way in which we perceive an object's efficiency as a diffuse reflector of light (called its diffuse luminous reflectance).
  • Colourfulness is defined perceptually as the "attribute of a visual perception according to which the perceived colour of an area appears to be more or less chromatic" (CIE, 2011, 17-233). Colourfulness is the way in which we perceive the absolute amount of bias between the long-, middle and short-wavelength components of a light, relative to daylight.
  • Saturation is defined perceptually as the "colourfulness of an area judged in proportion to its brightness" (CIE, 2011, 17-1136). Saturation is the way in which we perceive the relative amount of bias between the long-, middle and short-wavelength components of a light, relative to daylight.
  • Chroma is defined perceptually as the"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). Chroma is the way in which we perceive the absolute amount of bias between the long-, middle and short-wavelength components of an object's reflectance/transmission, relative to a spectrally neutral reflector, or essentially its efficiency as a spectrally selective reflector/transmitter of light.

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)

Being 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. However, modern psychometric scales involve either an atlas of samples tied in with a table of tristimulus values, as in the modern Munsell Book of Color and the Scandinavian Natural Colour System (NCS) atlas, or they are based directly on mathematical transformations of tristimulus values, as in the CIE L*a*b* and CIE L*H*c spaces.
Contrast phenomena like Fig. 1.2.1 show that the naive assumption that an object has a unique intrinsic perceived colour or "local colour", independent of surround, is something of an "illusion". What psychometric systems like the Munsell system or the NCS do is specify an object's perceived colour under specified standard conditions of illumination and surround. With a different surround or illumination than the standard the perceived colour may differ. However because colour order systems generally specify some kind of white illuminant and neutral background as standard conditions, it is reasonable in most contexts to say that a specification like a Munsell notation in effect describes the "local colour" of an object. Alternative psychometric systems typically agree in general terms in the way they space the same psychophysical colours perceptually, but differ in detail (Fig. 1.2.4A,B).
The following pages will introduce each of the six colour attributes defined by the CIE plus a seventh attribute, brilliance, not currently defined by the CIE but appearing (as clearness or its converse, blackness) in some other systems, along with various psychometric scales used as quantitative measures of some of these attributes.

Figure 1.2.5. A demonstration that while colour begins with a physical stimulus, there is more to colour perception than mere "detection" of a physical property. By concentrating alternately on the vertical and horizontal elements in the square on the left, or by simply viewing the square for a period of time, the viewer will see changes in the perceived colours. The same is true of the afterimage observed by focussing on the black dot on the left for ten seconds and then immediately focussing on the dot on the right. (Pattern on left after Figure 4A of Anstis, Vergeer and Van Lier, 2012, Luminance contours can gate afterimage colorsand "real" colors, Journal of Vision (2012) 12(10: 2, 1-13).

Primary and Secondary Qualities: An Evolutionary Perspective

It seems natural to assume that we see the physical world directly, and that colours like red and green exist objectively outside us in objects and in the wavelengths of light, but perhaps it should not be too surprising that perception is subtler and more interesting than this. Like all of our sensory systems, our visual system evolved under selective pressure to provide us with information about our environment, but only such information as is worthwhile obtaining considering the evolutionary cost of obtaining it.
So, while it might have been nice for us to have evolved the capacity to perceive wavelength-by-wavelength spectral power distributions of lights and spectral reflectances of objects, this precision would have been very expensive for us to develop and superfluous for these properties. Because the amounts of each wavelength can vary independently, both of these properties have innumerable degrees of freedom, and spaces representing all of their possible states have potentially infinite dimensions, depending on how closely the spectrum is split. Instead, we content ourselves with perceiving these physical properties through our trichromatic visual system as colours having just three dimensions. In a similar way, while it might have been nice for us to have evolved the capacity to analyze the precise chemical composition of any substance we encounter, the evolutionary cost of developing this capacity would be prohibitive, and we make do with the very coarse-grained representations of chemical composition provided by our senses of taste and smell.
Qualities like colour, taste and smell that are coarse-grained perceptions of much more complex physical properties are commonly distinguished as secondary qualities, as opposed to primary qualities like distance, size, shape, motion, number, and solidity. The physical properties we perceive as the primary qualities are much simpler mathematically and less costly for us to develop the ability to perceive precisely and accurately than those we perceive as the secondary qualities. Thus physical distance, size and number have one degree of freedom, shape has two and volume has three, and so do our perceptions of them; these perceptions do not entail representing a physical property having innumerable degrees of freedom with an "icon" having far fewer. Apart from being perceivable with much higher precision, the physical properties we perceive as primary qualities are also particularly important to us selectively: if I perceive that there is a solid branch about a metre below me, it could be of quite pressing importance to my survival that there actually is a solid branch about a metre below me!
Secondary qualities like colour are the ways in which we perceive complex physical properties, ways of perceiving shaped by evolutionary pragmatism. We will see later how evolutionary pressure also shapes which physical properties we tend to notice most, as well as which aspects of physical properties we are able to perceive at all. Colour vision is primarily of value to us for providing information about the properties of objects rather than the properties of the light at each point in the visual field, and our perceptions of the former can be so salient that it is very difficult to attend to precise comparisons of the latter, as colour constancy illusions such as the Adelson checkerboard illusion demonstrate.

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.
2Unfortunately the subtly nuanced scientific view of colour embodied in the CIE definitions is often reduced to the crude simplification that colours "only exist in the mind". This has led to the mistaken perception that the scientific view of colour is inconsistent with everyday experience, which in turn has motivated a great deal of discussion among philosophers, much of it in support of the "common sense" view that in one way or another, colours are "real". Philosophers characterize the view that (perceived) colours "only exist in the mind" as eliminativism and the view that colour is the (psychophysical) disposition of lights and objects to cause an experience of perceived colour as dispositionalism, and in general treat these as conflicting positions rather than just different usages of the word "colour". 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.
3It should not be necessary to add that there is no claim that each perception represents the corresponding physical property precisely and infallibly.
Page added January 15, 2017; last updated October 14, 2017.


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