Our visual system does more than provide us with a visual image of the world*. A visual field may contain an array of light and dark areas, but which of these areas represent shadows, and which represent dark-coloured objects? Does a red area in the visual image represent a red object in white light, or a white object in red light? Is a particularly bright point in the visual image a light valued surface, or a light source? To separate the effects of lighting from object colour, the visual image must be interpreted. Since we normally perceive object colours without having to make a conscious effort of interpretation, it follows that this interpretation must be occurring unconsciously as part of the processing of the visual image by our visual system. The ability of our visual system to automatically assign a more or less consistent local colour to an object under illumination of differing hue, brightness, and saturation is known as colour constancy.

Visual adaptation to the brightness and colour of the illumination, through adjustment of the sensitivities of the three cone types, should not be confused with this process of interpreting visual data into illuminant and object colours, although adaptation does assist colour constancy processing by improving the data provided by the eye on colour differences, provided that the illumination is not too strongly coloured or too dim. Under monochromatic light chromatic adaptation is ineffective, so that objects can convey no information on differences in hue and chroma to the eye, and differ only in value (see Figure 10.8, slider at right end of scale), and of course under very dim illumination, where only the rods function, vision is also monochromatic.

Colour constancy is never complete. Under weakly coloured illuminants, an array of surface colours may seem to approximately maintain their perceived hues, but changes in value in relation to each other are inevitable (see Figure 10.8, slider near middle of scale). Colours close to the hue of the illuminant tend to appear relatively lighter; while dissimilar hues tend to appear darker. In addition, two surfaces that match under a white light may not match under a coloured light, or even under another white light with a different spectral power distribution, a phenomenon known as metameric failure. These factors naturally play havoc with the tonal scheme of a painting, which is why it is recommended that paintings should be executed under similar lighting conditions to those under which they will be viewed.

Given the fact that it automatically interprets object colours for us, we can surmise that our visual system must in some way be coming up with a normally satisfactory solution to the "inverse problem" of separating the effects of illumination and object colour throughout the visual field. A surface giving off light whose brightness is consistent with being a reflection of this inferred illumination is seen as having an object colour that may be bright or more or less greyed (having a perceived content of black). An object that seems too bright to be the result of reflection of the inferred illuminant is seen either as a fluorent (fluorescent-looking) object, an independent light, or a specular reflection of an independent light. Evans (1974) proposed the term brilliance for this scale of appearance, and used the term zero grey point for bright colours at the point where they exhibit neither "greyness" nor fluorence.

Several well known "optical illusions" dramatically demonstrate colour constancy in action. In the checkerboard illusion by Edward Adelson (Figure 3.6A), the two areas marked A and B are actually identical in value on the image (i.e. they are the same grey), but our visual system calculates that in a shadow area this grey must represent a white surface, while in the lit area the same grey must represent a dark surface, and that is how we see them. In the same way, in the cube illusion by R. Beau Lotto (Figure 3.6B), our visual system sees the same image colour as being a dark brown object colour in the context of strong lighting, and a fluorent orange in a deeply shaded context. In the cross-piece illusion , also by Lotto (Figure 3.6C), the image colour at the intersection of the two rods is actually an identical middle grey in both cases, but in the apparent context of a yellow translucent filter on the left and a blue translucent filter on the right, this is judged, and seen, to be the reflectance of a blue-grey object and a yellow object respectively.

Figure 3.6. Three optical illusions demonstrating colour constancy in action (follow links for larger images). A. The checkerboard illusion of Edward Adelson. B. The cube illusion of R. Beau Lotto. C. The cross-piece illusion of R . Beau Lotto

In each case these comparisons are made unconsciously, and what we see in our normal way of looking is the inferred local colour. Tonal painters have to learn above all to look at their subjects with a different attitude to normal viewing, in order to judge objectively the hue, "colorfulness", and brightness of the light coming to from each point in the subject. That is, we have to switch off one kind of processing - one that is built into our visual system, wherein each colour is compared with an inferred white - and learn a completely different kind of processing, where each colour is compared with the full range of colours in subject as a whole. With practice we can learn to switch at will between our normal mode of vision and this painter's way of seeing. But we always need to be on guard against the tendency to slip into judging colours in constancy mode, that is, to paint their perceived local colour, instead of the colour that we need to create the illusion of that colour. The problem is analogous to the difficulties encountered in foreshortening in drawing, where we also need to learn to see and draw what is actually in front of our eyes, and not what our brain works out for us.

At this point the beginning painter might ask: "well, if that's the way it looks to my eyes, shouldn't I paint it that way?" The answer to this is a definite no - if we can recreate the stimulus that created the appearance, we will create the effect the we see in our subject; if we instead chase the appearance, we will create something different.

Certain tricks or devices that are sometimes recommended for observing colour can be workable, though many of these have serious limitations. For example, the idea that you can hold up paint on a brush, palette knife or other device and match it with your subject is in general workable only if you have some way of turning up the illumination on your brush until you can match the brightest highlight on your subject with the tone of your paint, and can keep the illumination at the same level while you compare the other colours. (One teacher currently advertising such a method on the internet seems to get around this problem by having his students paint only dimly lit subjects). These methods of course eliminate the option of translating the tonal range of the subject into a your own choice of tonal level and range in your painting. Devices involving an aperture in a card that bears a greyscale or colour chips for comparison suffer from the same difficulties and limitations, and in addition run the risk of giving an excessive impression of the brightness and "colorfulness" of colours seen in isolation, though this can be avoided if the colours are continually compared with the brightest colours in the subject. The latter comparison can be made very effectively by using a blank card with two apertures, which can be moved towards or away from the observer in order to compare more and less separated points.

Other methods involve "squinting", "unfocussing the eyes" or using peripheral vision, or observing the darkened and reduced image of the subject in a flat or convex black mirror. Squinting is best understood as closing the eyes and then just opening them enough to allow a dim impression of the subject. It is generally said to work by eliminating details, allowing the artist to concentrate the relationships of the big masses of the visual field, although for myself at least, another important factor is its effect of flattening the visual field, helping me to view the subject in what psychologist David Katz called film mode, as opposed to surface mode.

*I am of course ignoring here the fact that for most of us it creates two visual images that we combine stereoscopically. Colour constancy processing is presumably aided by stereoscopic cues, but still works effectively in distant scenes or 2D images, where stereoscopic cues are lacking.

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