S Afr Optom 2012 71(3) 133-143 Colour spaces - a review of historic and modern colour models* GD Hastings and A Rubin Department of Optometry, University of Johannesburg, PO Box 524, Auckland Park, 2006 South Africa <[email protected]> <[email protected]> Received 12 April 2012; revised version accepted 31 August 2012 Abstract colour models and a brief history of the advance- ments that have led to our current understanding Although colour is one of the most interest- of the complicated phenomenon of colour. (S Afr ing and integral parts of vision, most models and Optom 2012 71(3) 133-143) methods of colourimetry (the measurement of col- our) available to describe and quantify colour have Key words: Colour space, colour model, col- been developed outside of optometry. This article ourimetry, colour discrimination. presents a summary of some of the most popular Introduction textiles, and computer graphics industries because as these industries became more sophisticated, more The description and quantification of colour, and universal, and more commercial, models and systems the even more complicated matter of colour percep- needed to be evolved to allow an increasingly more tion, have interested philosophers and scientists for precise and comprehensive description of colour and over 2500 years, since at least the times of the ancient colour perception. Greeks1. Colour is a fundamental part of the sense of Some of the earliest attempts at representing colour sight and has a fascinating effect on how people per- seem to have been inspired by the change of night ceive the world - the poet Julian Grenfell even went to day and involve a linear (one-dimensional) colour so far as to say that “Life is Colour”2. scale ranging from black to white with all possible Although the profession of optometry is generally colours in between3. An example is Athanasius Kirch- familiar with the defects and deficiencies of colour er’s 1646 five-member colour scale, which illustrated vision and the tests available to detect such problems, a change from black, through blue, red, and yellow, to many of the greatest advancements in the understand- white. (This model was adapted from the similar 1613 ing, quantification, and reproduction of colour have model of Francois d’Aguilons3.) Later, radiometry come from developments elsewhere, in fields such as showed that different wavelengths of light could pro- physics, engineering and psychology. Much of the im- duce different perceptions of colour and thus estab- petus for these developments came from the printing, lished an association of certain colours with specific *This paper is based on research for the postgraduate degree of the first author with the supervision of ProfessorA Rubin of the University of Johannesburg 133 The South African Optometrist ISSN 0378-9411 S Afr Optom 2012 71(3) 133-143 GD Hastings and A Rubin-Colour spaces - a review of historic and modern colour models wavelengths. It quickly became apparent, however, therefore, formalized Mariotte’s 1717 suggestion. that the experience of colour perception could not be The sensitivities of the three types of receptor cells quantified fully by using the specification of wave- were quite specifically determined through separate length alone1, 4. Thus the subjective nature of colour colour-matching experiments early in the 20th cen- perception gradually became acknowledged and ac- tury by Wright and Guild1, 12. Although some of the cepted, and more advanced colour models such as assumptions made during those experiments have Newton’s circle evolved that, for apparently the first been questioned, the spectral sensitivity functions time, began regarding colour as a two-dimensional that were determined (for an average eye) from those (2-D) quantity1, 3. empirical data have remained an accepted and stand- The idea of colour being a multivariate quality was ard part of colourimetry9, 10. Linear transformations taken further when Mariotte suggested in 1717 that of the three functions (involving short-, medium-, and three primary colours could produce any desired col- long-wavelengths respectively) are known as colour our when used in the proper combination1, and the matching functions and have formed the basis of first known system that regarded colour as a three- some of the most popular colour systems to date. dimensional (3-D) element was the 1758 double-tri- The second theory, namely Hering’s opponent- angle-pyramid proposed by Tobias Mayer3. Although pairs theory, proposes that colour perception results many subsequent 3-D theories became popular in the from three complex and antagonistic processes each 18th and 19th centuries, most were neither accurate involving a pair of opposing colour sensations. The nor realistic (however, some such as Maxwell’s tri- three pairs are defined as black-white, red-green, and angle, which will be elaborated on later, formed the yellow-blue. A specific colour is thought to be per- foundation of our current understanding of colour) ceived only when its stimulation of the visual system and before further improvements to the modeling is greater than that of its paired colour (otherwise, and representation of colour could be made, a better when the two colours are equally balanced an achro- understanding of the perceptive processes involved matic grey is perceived)1, 5. This theory also incorpo- needed to be realised. rates the three colour-matching functions mentioned Two of the currently best-known and fundamental above, by defining one function as the luminance colour theories that emerged, propose different ways (brightness) of the perceived colour (thus relating to that colour perception could result from three com- the black-white process), and the other two functions ponents or processes. More detail on these theories as representing the chromatic content of the colour3. can be found in references 1, and 5 to 11; only a brief Each theory has, however, subsequently been found overview of each is included here. inadequate to fully describe human colour perception The first theory is known as thetrichromatic theory. on its own. Even from the time of Maxwell’s work Young, Helmholtz, and Maxwell contributed signifi- it could be shown that not all real colours could be cantly to formalizing this theory, which proposes the produced from a combination of three real primaries existence of three sets of sensory mechanisms (cone as the trichromatic theory suggests5, 7. And, although cells) whose unique and independent sensitivity pro- Hurvich and Jameson13 gave the opponent-pairs the- vide a basis for colour discrimination5, 6. Trichromatic ory some mathematical rationalization, it is founded theory incorporated the idea of colour-mixing (when upon largely subjective processes and is difficult to two or more colours coincide spatially and temporally experiment on objectively. Thus, more recently it has so as to be seen as a single colour) and proposed that, been suggested that each theory could be valid and similar to the idea of the Maxwell colour-triangle (a applicable to a separate part in the complex overall triangle whose three corners represent the three pri- process of colour perception5, 11. mary colours being mixed (commonly, spectrally Despite that the exact neural mechanisms and pro- pure red, green, and blue) and whose sides enclose cesses involved are still not yet fully understood, the the range, or gamut, of all possible combinations of most popular systems of colourimetry use three vari- those three primaries), all colours can be formed by ables to specify colour. But, subsequent to the above- the mixing of three primary colours7. This theory, mentioned theories, a more accurate requirement of a 134 The South African Optometrist ISSN 0378-9411 S Afr Optom 2012 71(3) 133-143 GD Hastings and A Rubin-Colour spaces - a review of historic and modern colour models colour space was realised and better models could be image, the illuminant, a reference white, and areas of evolved such as those that will be discussed below. the surrounding field), and are so cumbersome to use, that they have not yet achieved popularity in any in- Colour Spaces dustry4 and will not be included in this article. Any colour point within a colour space can, there- A Colour Space, Defined fore, be specified either by its co-ordinates (specific to A colour space serves as a means of uniquely speci- that colour space) or by a vector that similarly illus- fying, creating, and visualizing colours4, 14. Because trates the position of the colour point relative to a ref- the process of colour perception is a subjective and erence point within the space (such as either the ori- inherently variable process, colour spaces aim to de- gin of the space or another reference colour within the scribe and standardize colours either between differ- space)14, 17. Mathematical operations such as addition ent people, or for machines, or for both4. Colour spac- and subtraction are defined within colour spaces but es have been divided into two broad groups, namely because some of the colour spaces discussed below psychological spaces and geometric spaces. While the are not true vector or mathematical spaces, interesting former are constructed based on experimental data complications can occur near or across the boundaries from subjects (with normal colour vision) and arrange or limits of the spaces concerned. These issues have colours according to how they are perceived (with the important implications regarding statistical and other distance between colours in the space attempting to mathematical analyses that sometimes involve such approximate their observed difference), the latter is boundary colours or situations. a mathematical layout of colours, arranged accord- Some of the historically and currently most popu- ing to a measured property, defined within a speci- lar, influential, and useful colour spaces will be dis- fied range3, 4, 15. Some early colour spaces, such as the cussed in the following sections.