Lightness and Brightness and Other Confusions

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