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CASE STUDY: Does colour constancy exist?

Colour constancy concerns our ability to detect the colour of an object despite changes in illumination. For example, when looking at the sea on your morning stroll along the beach, you see it as blue, and it seems the same at sunset. The light reflected from the sea that enters the eye has a different wavelength at sunset, yet we perceive the colour to be the same. Examples like this lead us to believe that a mechanism in the visual system carries out calculations on the surface colour of objects so that we are able to perceive an object’s single colour in different lighting conditions. However, Foster (2003) questions the evidence for this assertion, arguing that it remains unproven that such a mechanism exists.

Many psychologists assume that colour constancy is a basic feature of visual processing systems, especially since it is associated with biological importance, such as being able to detect ripe from unripe fruit. Colour constancy allows for a stable visual world.

Colour perception depends on the reception of the wavelengths of light being reflected from an object. However, the wavelength entering the eye is a combination of the colour of the light and the properties of the object itself – thus, “a red paper in white light can look the same as a white paper in red light” (Foster, 2003, p. 1). The task of the visual system is to calculate the colour of the object by controlling for, or eliminating, the colour of the light. When several different coloured objects are present this task may be easier since it is easier to eliminate the colour of the light, given cues from more than one object. This is the basis of Land’s retinex colour-constancy schemes. However, measuring perceived colour may not be without its problems.

One method is colour naming, in which participants are required to select a colour name to refer to the colour perceived. It is then possible to record responses to a number of objects in different illuminations. One problem is that this is not very accurate as a method of measurement: participants often use the same colour name (e.g., blue) for different shades (of blue), even when the illumination is constant. It has been estimated that there are more than 2 million distinctly perceivable colours, and this far exceeds the number of colour names that people are willing to use.

A second method is to get participants to match two colours by adjusting the red, green and blue of the second colour using colour controls. So, the same coloured surface is presented under different illuminations and the participant adjusts the colour of the second surface to match the first “as if it were ‘cut from the same piece of paper’” (Foster, 2003, p. 2). Under this method and with computer-simulated scenes, colour matching can be as accurate as 79% to 87%. The problem with this method is that it only guarantees matching two items as equivalent, it does not measure perceived colour. This is because participants need merely to judge how the colour of one surface relates to the colour of one or more other surfaces, and hence this method does not measure colour constancy.

A third method involves participants adjusting the colour of a surface within a scene so that it appears white (achromatic adjustment). The method actually records their perception of the colour of the illuminant, since that is what they are attempting to adjust for. Performance can reach up to 83% of the real unbiased colour (white). However, although this has been used to measure colour constancy, Foster argues that it cannot do so, because in order to complete the task participants can make use of the hue of surrounding colours in the scene. In order to achieve colour constancy the illuminant needs to be identified – however, judgements based on relative brightness say nothing about the nature of the illuminant.

Judgements of colour can be based on numerous sources of information. One common assumption is that the object with the brightest surface is white, and once this is made other colours are perceived relative to that surface. However, there are many other ways, and the fact that many other ways are used implies that there may not be a single mechanism that computes colour constancy.

Because of the limitations in measuring perceived colour, psychologists have no way of knowing whether true colour constancy exists.

Reference

Foster, D.H. (2003). Does colour constancy exist? Trends in Cognitive Sciences, 7(10): 439–43.

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