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