Lightness and Brightness and Other Confusions
Lightness and Brightness and Other Confusions
Lightness and Brightness and Other Confusions
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elation to the light source or observer. This creates an uneven distribution of<br />
luminances even when the illuminance is even – as for instance in spherical<br />
objects that tend to display deep shadows <strong>and</strong> bright highlights at the same<br />
time. The same is true, of course, for any shape of three-dimensional object in<br />
normal directional light.<br />
We are able to separate the forms of objects surrounding us from the everchanging<br />
patterns of light <strong>and</strong> shadow. Furthermore we are able separate the<br />
patterns of illuminance from the patterns of luminance. In all but the most<br />
confusing cases we are able, without effort, to separate the colours of highlight,<br />
shadow <strong>and</strong> half-shadow in a spherical object from what we experience<br />
as the ‘real colour’ or ´substance colour’ <strong>and</strong> the ‘real lightness’ of the object<br />
itself. 69 The same goes for a flat surface in dappled or uneven light. We do not<br />
say or think that a white wall lit in this way has different lightnesses, but that<br />
it is lit unevenly. In the language of perceptual science, it has different<br />
brightnesses. The confusion in the use of the terms lightness <strong>and</strong> brightness<br />
comes from their everyday use. <strong>Brightness</strong> is in most languages limited to<br />
describing phenomenally self-luminous objects <strong>and</strong> things, such as the sun,<br />
the sky, artificial light sources, but it can also describe ambient or incident<br />
light: a bright day, a bright room, etc. <strong>Brightness</strong> is also in everyday speech of<br />
many languages used to describe the vividness of colour (a bright pink dress,<br />
a bright blue flower), which confuses the issue even further. Since our capacity<br />
to apprehend the lightness of objects despite their varying brightness<br />
(or luminances) is very robust <strong>and</strong> requires no conscious effort (we are indeed<br />
mostly unaware of this separation), there has been little need to develop<br />
separate words or separate ways of using the word brightness to describe a)<br />
the variations of illuminance <strong>and</strong> b) variations of luminance of our surroundings.<br />
It is only when we pause to take a closer look at the nature of seeing,<br />
that this need arises. 70<br />
The human visual system has also a remarkable capacity for adaptation to<br />
intensities of light, both globally <strong>and</strong> locally. 71 If this were not so, we would<br />
<br />
See also the section The Real colour of objects, pp. 92–100, in this article.<br />
<br />
In everyday speech we do not refer to two walls painted in the same colour in the<br />
manner: ”The wall in next room is the same white colour, although less bright”. Neither<br />
do we often say: ”The wall was of a uniform brown, although you could never see it as<br />
such, because of its varying brightness, due to the pattern of shadows falling on it”. We<br />
apprehend the reflectances as identical <strong>and</strong> uniform, despite the variations in luminance<br />
– even to the point of being unaware of the luminance variation. For further<br />
discussion about modes of attention see Merleau-Ponty (2002), pp 30–59, as well as<br />
the section Identity colour <strong>and</strong> modes of attention in this article <strong>and</strong> Natural Experiences<br />
<strong>and</strong> Physical Abstractions, pp 23–24 in this volume.<br />
<br />
<br />
The human visual system is capable of producing perceptions over an amazing luminance<br />
range of 1–10 000 000 000 000 (ten trillion) cd/m 2 , representing at one end the<br />
absorption of one photon in a rod receptor (enough to create a tiny flash) <strong>and</strong> at the<br />
74