Lightness and Brightness and Other Confusions

humans to discount the illuminant. (See footnote 94 below). Let us return
once more to the problem of determining the ‘real’ colour of the green apple.
It was said that the faithful translation, from a given point in space, of the
apple’s colours onto a flat surface would leave us with several contenders for
the ‘real’, colour of the apple. Indeed, a digital photograph and its millions of
pixels would leave us with myriads of possibilities, but on closer inspection
this analogy is false. Firstly, we do not fixate objects or scenes at any one time
from one viewpoint, but two. We have stereovision; our two eyes provide us
with two slightly different sets of proximal stimuli that are compared and
synthesized in the brain. Secondly, we do not (voluntarily and naturally) view
the world even from these two viewpoints statically. Either the object moves,
or we move our eyes or bodies in relation to it. 92 This action is central to the
process of extracting relevant information about the world – also its colour.
We perceive through our stereovision and our movement in relation to the
green apple that the apple is round; that the shadows and highlights are transient
qualities and that there is a ‘permanent’ colour to the apple that exists
independent of the variations of highlights, reflections or shadows involved in
the totality. The transient qualities of shadow, half-shadow, highlight, etc.,
are the object’s primary visual spatial attributes that exist in relation to light
and motion. If these are subtracted from the spatial equation, what is left is
what could be called the constancy colour. But there is no way we can perform
the subtraction without destroying the real experience of the object’s
colour. This constancy colour is apprehended in the totality by our experience
of space and movement, but can never be perceived directly. Constancy
colour cannot be measured and has precision only within the limits of ecological
necessity, that is: for the efficient and reliable identification of objects,
scenes and spaces in relation to the subject’s ecology. 93
Colours are to us what they are in their context. They belong to objects, surfaces,
spaces and situations, in which contexts they gain their functional and
emotional significances. We need constants in order to make sense of the
world. To achieve this our mind-body has adapted to the stream of contingencies,
the ever-changing illuminations and angles of view, by developing a
preference for the features that are typical, recurrent and identity-giving. One
of these is the colour of objects irrespective of changes in illuminant or changes
in viewing angle and distance. There are numerous theories of how this is
achieved, but all of them admit a certain level of flexibility (sometimes referred
to as inaccuracy) in colour constancy. 94 Looked at in another way, this
For a more elaborate discussion of the significance of motion in perception see Gibson
1986 and Noë 2004. See also my article Seeing and Perceiving, p 34 in this volume.
See also Natural Experience and Physical Abstractions, pp 24 and 25 figure 2 in this
volume.
Helmholtz proposed that constancy of colours involved ‘discounting the illuminant’
by a process of ‘unconscious inference’. This has been contested even in his own life-
99