Lightness and Brightness and Other Confusions
Lightness and Brightness and Other Confusions
Lightness and Brightness and Other Confusions
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continually confused with each other or with other concepts. Anders Liljefors<br />
67 has written as follows on lightness:<br />
<strong>Lightness</strong> <strong>and</strong> luminance exemplify a complicated relations arising from<br />
the visual sense’s process of interpretation. How we apprehend the lightness<br />
of surface of a certain luminance depends on the arrangement of<br />
luminances in our entire field of vision. The experience of lightness is<br />
relative. For example, a room with a great variation of luminances creates<br />
a stronger impression of brightness than an evenly lit room, even<br />
when the adaptation luminances are the same in both the rooms. (Liljefors<br />
2005)<br />
But what would be brightness as opposed to lightness? Anders Liljefors’s text<br />
contains the word luminance (Swed. luminans), but that is a term from photometry<br />
<strong>and</strong> does not describe visual experience. In most languages there are<br />
separate words for lightness <strong>and</strong> brightness <strong>and</strong> they are used in a similar<br />
manner to their English counterparts. This is not the case in all languages,<br />
however. For example the English-Swedish dictionary gives brightness as<br />
klarhet, but this is something else than the concept of brightness, which is<br />
probably why Liljefors has used luminans in an attempt to describe the experience<br />
of brightness.<br />
Edward Adelson gives a neat summary of the various terms connected with<br />
the perception of light <strong>and</strong> lightness in his article <strong>Lightness</strong> Perception <strong>and</strong><br />
<strong>Lightness</strong> Illusions (Adelson 2000). Adelson also provides a distinction between<br />
lightness <strong>and</strong> brightness:<br />
Luminance is the amount of visible light that comes to the eye from a surface.<br />
Illuminance is the amount of light incident on a surface.<br />
Reflectance is the proportion of incident light that is reflected from a surface.<br />
Reflectance, also called albedo, varies from 0 to 1 or, equivalently, from 0% to 100%<br />
where 0% is ideal black <strong>and</strong> 100% is ideal white. In practice, typical black paint is about<br />
5% <strong>and</strong> typical white paint about 85%. (To keep things simple, we consider only ideal<br />
matte surfaces, for which a single reflectance value offers a complete description.)<br />
Luminance, illuminance, <strong>and</strong> reflectance, are physical quantities that can be<br />
measured by physical devices. There are also two subjective variables that must be<br />
discussed.<br />
<strong>Lightness</strong> is the perceived reflectance of a surface. It represents the visual system's<br />
attempt to extract reflectance based on the luminances in the scene.<br />
<strong>Brightness</strong> is the perceived intensity of light coming from the image itself, rather<br />
than any property of the portrayed scene. <strong>Brightness</strong> is sometimes defined as perceived<br />
luminance. (Adelson 2000).<br />
67<br />
Anders Liljefors is former professor of architectural lighting at the Royal Technical<br />
University (KTH), Sweden <strong>and</strong> founder of the programme for lighting design at the<br />
Department of Lighting Science, Jönköping University, Sweden.<br />
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