Lightness and Brightness and Other Confusions

As the inks developed the three-primary theory gained more and more acceptance
and was reinforced by the discovery of the three primary lights, red
green and blue, in the late 17 th century and Thomas Young’s trichromacy
theory of colour vision (1802). With the theory of Thomas Young and the
later neurological evidence of Hermann von Helmholtz the idea of the three
primaries not only strengthened, but also shifted from a physical and material
basis more and more towards a neurological and physiological one. It
would be easy to assume from the evidence put forward by Young and Helmholtz
that the matter of the primaries was at last settled. They were three –
and for many of the physicists taking the theory of trichromatic vision as a
starting point, they were specifically red, green and blue. The primacy of
these RGB-primaries would probably still prevail were it not for Ewald Hering,
who in 1872 hypothesized a rival theory involving not three but four
unique hues, red, green, yellow and blue, that became the basis of his theory
of opponent colour vision.
Today there is a wide scientific consensus that both the Young-Helmholtz
theory of trichromatic vision and the Hering theory of opponent colour processes
are fundamentally correct – they merely describe different stages of
the coding of colour signals in the path from the retina to the brain.
Some of the differences in the way various colour systems define primary,
elementary or unique hues are due to the fact they describe different stages of
colour coding in the visual pathway. However this is not the only reason for
the differences. There is also confusion about what one means by the words
primary, elementary, principal etc. in connection with hue and colour. A
widely used definition of primary colour states that it is a colour (substance)
that yields secondaries, tertiaries and all other possible colours when mixed
as pigments or other colorants. Another definition starts with perception, as
does the NCS and as did Itten in his own way. A third definition starts from
psychophysics and mixtures of coloured lights. This was the starting point of
the CIE, when it started to build a system for mathematically designating
colours of lights.
The International Commission on Illumination (CIE) began to develop a
system for mapping all colours of lights visible to humans within a mathematical
model in the 1920s. The fruit of this work was the 1931 Yxy -colour
space that underlies all present-day spectrophotometric and colorimetric
systems. Much of the logic of this system was based on trichromatic theory of
colour vision as laid out by Young and Helmholtz in the previous century.
This theory states that the colour-sensitive photoreceptors of the human eye,
the cone receptors, are sensitive to three wavebands of light – short, medium
and long – with overlapping sensitivities, particularly between the medium
and long wavebands. The peak sensitivities of the cones have been measured
to be at around 420–440 nm, 534–545 nm, and 564–580 nm respectively.
The pioneering work of the CIE has dealt with producing mathematical func-
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