PART 7. HUE

THE DIMENSION OF HUE

We have seen that the continuous, circular character of the range of hue results from the 360^o range of possible combinations of the redness vs greenness and yellowness vs blueness signals in the opponent model. The Commission Internationale de L’Éclairage (CIE) acknowledges this model in defining hue as "the attribute of a visual sensation according to which an area appears to be similar to one of the perceived colours, red, yellow, green and blue, or a combination of two of them". Orthogonal systems such as the a and b dimensions of CIE Lab space can be used in place of the dimension of hue, and are essential for example for calculation of colour differences. The angular coordinate of hue provides however a far more intuitive conceptual framework for painters.

The step of representing hue as a circular dimension was first taken by Sir Isaac Newton in his Optics of 1704 (Figure 7.1). Newton arranged his red to violet sequence of spectral colours in clockwise order, along with the nonspectral colour "purple" ("in or near" the line OD), in an asymmetrically spaced arrangement modelled on a diagram from music theory. Some earlier circular colour diagrams are known (e.g .Forsius, 1611, Robert Fludd, c. 1630; for a medieval example see Gage, 1993, pp. 162), but all of these included black and white among the colours on the circle. So colour circles had existed previously, but the circular dimension of hue begins with Newton.

Figure 7.1. Colour circle from Newton's Opticks (1704).
Source:http://posner.library.cmu.edu/Posner/books/book.cgi?call=535_N56O_1704

Newton offered his diagram as a practical, approximate guide to the results of additive mixing of monochromatic lights. He explained that the colour resulting from such mixing would be expected to lie at the "center of gravity" of the component lights, weighted according to their brightnesses. In that he explicitly stated that colour mixtures on the circumference would be "intense and florid in the highest degree", and that other colours would be intense in proportion to their distance from the centre, the diagram is the first graphical representation of the dimension of saturation, as well as hue. It is also, latently, the first representation of the relationship of complementarity, since the circular diagram implies that lights having opposite colours on the circle should mix to make neutral (white) light. Newton stopped short of this assertion himself, however, as he had not succeeded in achieving this result experimentally.

It is difficult to overstate the importance of Newton's discoveries. A huge part of our objective conceptual framework of colour, in science, technology and art, arose directly or indirectly from them. Newton's circular conception of hue was quickly applied to artists' paints to produce the familiar artist's colour wheel. This colour wheel, or its various refinements such as the Munsell hue circle, have been used as a frame of reference by artists ever since. As John Gage has observed, the concept of complementarity, latent though undeveloped in Newton, was a particularly important development for artists. Even the concept of "warm" and "cool" colours does not seem to have occurred to artists until after they first saw their hues laid out in a circle. The removal of white and black from the list of artists' "simple" colours, a step anticipated by some earlier writers and rejected by others, was established definitively by Newton's demonstration of the compound nature of white light. The apparent conflict exposed by this development, between the three artists' primaries and the essentially infinite gradations of monochromatic colours, led to more than a century of confusion, but eventually inspired the proposal of the trichromatic model of human vision by Palmer,Young, and Maxwell, allowing in turn the development of colour photography and digital imagery in all their forms. Most fundamentally of all, once hue and saturation had been represented as independent dimensions in a diagram, it was a relatively small step (first explicitly taken by Brook Taylor in 1719) to add these to the dimension of brightness, and arrive at a three-dimensional conception of colour.

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