Lightness and Brightness and Other Confusions

The real colours of objects
To see is not so much about looking at waves of light, but about looking
at external objects mediated by these waves; the task of the eye is to enlighten
us, not so much about the intensity or quality of the light reaching
us from some object at a given moment, but about the object itself.
– Ewald Hering
The difficulty of judging accurately the colours of objects, surfaces and materials
becomes evident with changes in the ambient light, the viewing angle
and distance as well as the contrast effect of adjacent colours. A house that
looks neutral grey in direct sunlight may look bluish under an overcast sky
and yellowish at sunset. The effects of distance, partly brought on by the
physical effects of scattering and filtering of light by moisture and dust particles
in the intervening air, are most dramatic in the bluish cast of distant
mountains.
Despite these sometimes dramatic and often aesthetically pleasing transformations
of colour, our vision is primarily wired to keep colours stable. This
property or capability is called colour constancy and is probably common to
most living creatures with colour vision. Colour constancy is reliable only up
to a degree. It breaks down entirely under extreme conditions of lighting,
such as very low levels of illuminance or strong chromatic distortion of lighting;
and under an illuminant of poor colour rendering capacity (with discontinuous
or distorted spectral distribution) object colours are often perceived
as changed from ‘normal’. However, if one considers the enormous variations
in both the intensity and the spectral distribution of both the incident and the
reflected light of surfaces under various lighting conditions, it should not be
possible to recognize the colours of any objects in the different illuminations
of our daily lives.
The wavelength composition of ambient and reflected light changes continually
around us. If our colour sense were attuned more to the spectral distributions
of the light reflected from objects than to the colours of objects themselves,
we would be at a loss in the street, at the fruit counter, everywhere.
But fortunately we have the inexplicable ability to see the ‘real’ colour of the
object – or as Helmholtz would say, to ‘discount the illuminant’. It is still not
known, after about 150 years of investigation, how this is achieved and neither
is it entirely clear how robust or accurate this ability is. The answer depends
on the scale of accuracy that is used and context in which it is used. 85
Obviously, a minute change in colour nuance that is evident in highly demanding
colour discernment tasks can and will go entirely unnoticed in everyday situations.
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