What is Colour for Science?

Colour is the subject of a vast and impressive body of empirical research and theory, drawing from physics, physiology and psychology, and the study of colour is perhaps the oldest discipline of psychology. For the ancient Greeks, colours took fifth place after the classical elements of fire, air, water, and earth. A lot is known about the physical properties of objects that are responsible for the appearance of colour: photoreceptors in the eye; colour processing in the visual system; the genetics of colour vision; the various defects of colour vision; the variations in colour vocabulary and categories across cultures; colour constancy; the variation in apparent colour with viewing conditions; colour vision in animals; and about the evolution of colour vision. However, it is important to understand the fundamental questions of colour science before being exposed to the various facets of this interdisciplinary field. This article traces the historical viewpoints of colour science with its misconceptions and advances, provides insights on the physical and mathematical explanation of colour along with the main attributes commonly used to describe it, and finally introduces the contemporary definition of colour. (Byrne & Hilbert, 2003)

Colour Through the Years

Prior to the seventeenth century, it was commonly thought that white light represented light in its purest form and that colours were modifications of white light. The beginning of the science of colour was described by Sir Isaac Newton, who conducted an experiment in which he produced a spectrum from white light using a triangular glass prism. When he recombined the spectrum with a second prism, he once again obtained white light. Thus, he demonstrated that “Light is a heterogeneous mixture of differently refrangible rays”. Newton chose to designate the spectrum as containing seven major colours: red, orange, yellow, green, blue, indigo and violet, but was careful to explain that there are many more colours in the spectrum in addition to these seven, which are obtained by combining these colours with each other and with white and ‘black’. He also maintained that there is no coloured light – there is only the sequence of colours of the spectrum and combinations of these. The colour of a natural body is merely its disposition to reflect lights of some refrangibilities more than others. (Shevell, 2003)

One major opposition to Newton arose from an approach involving the concept of primary colours, which was formalized by the German physiologist Karl Ewald Konstantin Hering in his 1878 “opponent” theory of colour perception. The theory stated that red-green, yellow-blue, and light-dark are three kinds of opposites that control vision. (Nassau, 1997)

The other major opposition arose from the concept that just three primary additive colours are adequate to produce essentially all colours, which was formalized in the “trichromacy” theory of Thomas Young. It was expanded by Herman Ludwig Ferdinand von Helmholtz and James Clerk Maxwell. This approach, usually called the Young-Helmholtz theory, postulated three sets of receptors in the eye, sensitive to blue, green and red light, and was confirmed in the late 1960s. (Nassau, 1997)

The Scientific Perspective

Trichromacy is the most fundamental property of human colour vision. The three sets of photoreceptors in the Young-Helmholtz theory are three types of cones, which are light sensitive cells in the retina of the eye. A somewhat simplistic interpretation is that red cones are sensitive to red light, green cones are sensitive to green light, and blue cones are sensitive to blue light. More precisely, the cones produce a stimulus response to long, medium, and short wavelengths (L, M and S) of visible light. 

Scientific colour values were established earlier this century by the CIE (Commission Internationale de l’Eclairage) group, which defined three colour matching functions that are a numerical description of the chromatic response of the average human (standard colorimetric observer). The CIE defines the colour matching functions as (λ), (λ), and (λ) where λ is the wavelength in nanometers, and designates the three stimulus responses as X, Y and Z. The tristimulus values for a colour with spectral power distribution (λ), which is the amount of power a light source emits at each wavelength of the spectrum, are given in terms of the standard colormetric observer as 

A set of the tristimulus values X, Y and Z represents the perception of a single colour on a neutral gray surround of equal lightness. (Wegman & Said, 2011)

Therefore, the perception of colour has at least two major components. On the one hand, colour can be discussed simply as a phenomenon of certain wavelengths of electromagnetic radiation. On the other hand, most electromagnetic radiation cannot be perceived by human beings. It is the very narrow band of electromagnetic radiation that can be perceived by humans that we call light, and which leads to an incredible sensory experience that we call vision. It is this marvelous confluence of, in some sense, absolute wavelengths of electromagnetic radiation and psychophysical decoding of these wavelengths in the human visual system that gives rise to the perception of colour.

Colour Attributes

Visualising a colour, or the relationship among colours, from their tristimulus values is impossible for most people. For this reason, object colour stimuli are characterized by three psychophysical dimensions – luminance, dominant wavelength, and purity, whose corresponding colour attributes are lightness/brightness, hue, and chroma/colourfulness. Dominant wavelength or hue specifies one of the colours of the spectral sequence. Purity or chroma gives a measure of the absence of white, gray or black which may also be present. For a colour having a given hue and saturation, there can be different levels variously designated as brightness, lightness or luminance. Variance of a physical or psychophysical dimension such as wavelength will normally influence the appearance of all three colour attributes unless one dimension (or alternatively one attribute) is held constant. (Pridmore, 2007)

Contemporary Definition of Colour

By and large, the field of colour science commands a broad consensus. In fact, the most popular opinion among colour scientists may well be the view that nothing is coloured – at least not physical objects in the perceiver’s environment. Colour is a psychological property of our visual experiences when we look at objects and lights, not a physical property of those objects or lights. There may be light of different wavelengths independent of an observer, but there is no colour independent of an observer. (Byrne & Hilbert, 2003) It is now recognized that the nervous system, rather than analyze colours, takes what information there is in the external environment, namely, the reflectance of different surfaces for different wavelengths of light, and transforms that information to construct colours, using its own algorithms to do so. In other words, it constructs something which is a property of the brain, not the world outside. However, colour scientists, philosophers, and other cognitive scientists with opinions on the matter strongly disagree about the answers to certain questions such as is redness a physical property of some sort – for example, a certain way of reflecting light? Or is it a disposition to produce certain sensations in certain perceivers? Or is redness a sui generis property about which not much can be said? (Byrne & Hilbert, 2003)

We have all experienced colour to some extent, even those who have limited colour vision. The perception of colour is a central component of primate vision. Colour facilitates object perception and recognition and plays an important role in scene segmentation and visual memory. Moreover, it provides an aesthetic component to visual experiences that is fundamental to our perception of the world. Despite its enormous importance, and the long history of colour vision studies, much has still to be learned about the physiological basis of colour perception. (Gegenfurtner & Kiper, 2003)

References

Byrne, A., & Hilbert, D. R. (2003). Color realism and color science. Behavioral and Brain Sciences, 3-64.

Gegenfurtner, K. R., & Kiper, D. C. (2003). Color Vision. Annual Review of Neuroscience, 181-206.

Nassau, K. (1997). Color for Science, Art and Technology. Elsevier Science.

Pridmore, R. W. (2007). Effects of Luminance, Wavelength and Purity on the Color Attributes: Brief Review with New Data and Perspectives. Color Research and Application, 208-222.

Shevell, S. K. (2003). The Science of Color. Elsevier Science.

Wegman, E., & Said, Y. (2011). Color theory and design. Wiley Interdisciplinary Reviews: Computational Statistics, 104-117.