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