Lightness and Brightness and Other Confusions

Especially problematic is the situation where one and the same word is given
alternative conceptual definitions, while having also a more or less established
everyday usage. Take for example saturation. Even if each of the definitions
is clear, it is very confusing that one and the same term can have so
many slightly varying definitions.
There are also generic words and terms that have been given very specific
meanings within a given scientific discourse. These can be misunderstood or
confused with their more generic meanings outside that discourse. Examples
of this are the terms inherent colour and identity colour. Within the conceptual
framework of their discourse these terms are well defined (and thus useful),
but considered out of that framework they can be very confusing.
Colour identification through three properties
Irrespective of the starting point, be it physical, psychophysical or perceptual,
any single colour can be described and identified through three independent
properties. This makes it possible to organise and examine all possible
colours within a three-dimensional conceptual structure, called a colour
space. There exists, however, some variation between systems as to which
exact visual properties constitute the three parameters of colour – and even
more disagreement about how these parameters are defined. The end result is
that there are in use today several colour systems exhibiting various ideas of
colour.
It is relatively easy to build a model of colour from three variables such hue,
lightness and saturation. The three dimensions lend themselves easily to a
variety of geometric shapes. These colourful cubes, cylinders, spheres, cones,
double cones, tetra- and octahedrons and their more complex asymmetrical
variations may look convincing and beguilingly attractive as models for the
parameters of colour. However, if colour is to be understood as something
that is neither a property of objects nor entirely of the perceiver – but as
something that happens in the dynamic interaction of the two – then no static
geometric model could ever describe it.
Many, but not all, colour systems include lightness as one of its variables.
Sometimes other words are used to denote basically the same thing. In the
Munsell system, for example, the darkness–lightness variable is called value.
Lightness or value typically forms a scale between maximum blackness and
maximum whiteness. Although it is a basic variable it is not defined the same
way in all systems. One system might define lightness in photometric terms,
another with reference to standard samples. This and the conceptual confusion
between lightness and brightness, is one of the topics of this article.
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