Lightness and Brightness and Other Confusions

and data projection) also represent modes of appearance that can waver between
the two types of experience. 68
Lightness constancy
The great difference in appearance of the moon at night and in the daytime
may also result from the fact that at night the moon is viewed in relative isolation.
In most spatial contexts the lightness of objects is perceived as more or
less unchanging despite changes and variations in illumination and juxtaposition.
In other words, a piece of white paper is experienced (in all but the
most extreme cases) as white in deep shadow, penumbra and bright sunlight.
And a piece of coal will look black both outdoors and in the murky depths of
the cellar – even though outdoors it will reflect hundreds of times more light
(as radiant energy) than, say, the dimly lit white walls of the cellar. In other
words, the ambient or incident light may vary, but the lightness estimation
remains remarkably constant. This is called lightness constancy.
To help separate lightness and brightness from each other and from the concepts
of luminance and illuminance, let us consider what our visual system is
for? One of its most important tasks is to provide information about the typifying
qualities of objects and surfaces. Among these qualities are texture,
gloss and reflectance – all more or less permanent properties of objects and
therefore fairly reliable indicators of the objects’ physical properties. These
can be measured. What we call colour and lightness are the brain’s best estimate
as to the chromatic and non-chromatic reflectance properties of a surface.
Identifying lightnesses correctly helps us to identify and categorize
things, places, etc., and lightnesses are important indicators of the “permanent”
qualities of their surfaces and their materiality. Identifying brightnesses
is also important, because we need this information to be able to separate the
permanent property of surface reflectance (perceived as lightness) from the
ever-changing property of illuminance or incident light, as well as to separate
radiant objects from non-radiant objects (or light sources from illuminated
things). The fact that the correlation of perceived lightness to measured reflectance
is far better than the correlation of perceived brightness to measured
luminance reveals something fundamental about the function and ecology
of our visual system: it has evolved to inform us first and foremost about
constancies.
In nature surfaces and objects are often illuminated unevenly. Tree branches
and leaves or other obstructions can create complicated light-and-shadow
patterns of illuminance. Objects are also rarely flat or their surfaces frontal in
The space and installation artist James Turrell (b. 1943) has in many of his works
exploited the disorienting, often illusory, sensation created by confusing the modes of
lightness and brightness.
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