Lightness and Brightness and Other Confusions

elation to the light source or observer. This creates an uneven distribution of
luminances even when the illuminance is even – as for instance in spherical
objects that tend to display deep shadows and bright highlights at the same
time. The same is true, of course, for any shape of three-dimensional object in
normal directional light.
We are able to separate the forms of objects surrounding us from the everchanging
patterns of light and shadow. Furthermore we are able separate the
patterns of illuminance from the patterns of luminance. In all but the most
confusing cases we are able, without effort, to separate the colours of highlight,
shadow and half-shadow in a spherical object from what we experience
as the ‘real colour’ or ´substance colour’ and the ‘real lightness’ of the object
itself. 69 The same goes for a flat surface in dappled or uneven light. We do not
say or think that a white wall lit in this way has different lightnesses, but that
it is lit unevenly. In the language of perceptual science, it has different
brightnesses. The confusion in the use of the terms lightness and brightness
comes from their everyday use. Brightness is in most languages limited to
describing phenomenally self-luminous objects and things, such as the sun,
the sky, artificial light sources, but it can also describe ambient or incident
light: a bright day, a bright room, etc. Brightness is also in everyday speech of
many languages used to describe the vividness of colour (a bright pink dress,
a bright blue flower), which confuses the issue even further. Since our capacity
to apprehend the lightness of objects despite their varying brightness
(or luminances) is very robust and requires no conscious effort (we are indeed
mostly unaware of this separation), there has been little need to develop
separate words or separate ways of using the word brightness to describe a)
the variations of illuminance and b) variations of luminance of our surroundings.
It is only when we pause to take a closer look at the nature of seeing,
that this need arises. 70
The human visual system has also a remarkable capacity for adaptation to
intensities of light, both globally and locally. 71 If this were not so, we would
See also the section The Real colour of objects, pp. 92–100, in this article.
In everyday speech we do not refer to two walls painted in the same colour in the
manner: ”The wall in next room is the same white colour, although less bright”. Neither
do we often say: ”The wall was of a uniform brown, although you could never see it as
such, because of its varying brightness, due to the pattern of shadows falling on it”. We
apprehend the reflectances as identical and uniform, despite the variations in luminance
– even to the point of being unaware of the luminance variation. For further
discussion about modes of attention see Merleau-Ponty (2002), pp 30–59, as well as
the section Identity colour and modes of attention in this article and Natural Experiences
and Physical Abstractions, pp 23–24 in this volume.
The human visual system is capable of producing perceptions over an amazing luminance
range of 1–10 000 000 000 000 (ten trillion) cd/m 2 , representing at one end the
absorption of one photon in a rod receptor (enough to create a tiny flash) and at the
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