Lightness and Brightness and Other Confusions

Adelson is talking here about light coming from images and scenes, because
his article discusses mainly two-dimensional lightness illusions. But let us not
be confused by that; his definitions apply just as well to spatial contexts.
The term brightness refers to the human experience of the intensity of light
reflected or emitted by objects and surfaces. The sky, the moon or a candle
flame has brightness, rather than lightness. But – as Adelson points out –
also surfaces have brightness in addition to lightness, and their brightness
depends on how much light they are perceived to reflect at a given moment.
In a spatial or natural context, then, lightness refers to an object’s particular
perceived surface quality, to the surface’s or object’s overall ability to absorb
and reflect light. Brightness refers to the experience of the amount of reflected
or radiated light in relation to the overall, or global, scene.
Whiteness
A distinction must be made also between lightness and whiteness. Whiteness
and blackness are parameters of colour in the NCS system and they are sometimes
confused with the concepts of lightness and darkness. Unlike lightness
and darkness, whiteness and blackness are discreet concepts, rather than
expressions of the presence and lack of something. They are defined as relative
visual similarity to the elementary colours black and white. The geometric
model of the NCS colour space also includes the variables hue and chromaticness.
Unlike all other systems (save the now obsolete Ostwald system)
the NCS does not directly include lightness or brightness as one of its variables.
Whiteness and blackness do contribute to the lightness and darkness
(as well as to the chromaticness) of colours in the NCS system, but they have
an indirect relation to lightness: in order to derive lightness from NCS whiteness
and NCS blackness also hue must be taken into account: a yellow with
five percent of whiteness is far lighter than a blue with the same percentage of
whiteness. The reason is that yellow and blue are of different lightness to start
with. The same goes for blackness in relation to the different hues. So what
are whiteness and blackness, if not lightness and darkness? They are the
visual constituents of a colour’s impurity, its lack of vividness or distance
from the ideal pure chromatic hue. As perceived phenomena they could be
described as either mistiness or milkiness or fogginess in the case of whiteness,
and shadiness in blackness or greyishness when both whiteness and
blackness are present.
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