Lightness and Brightness and Other Confusions
Lightness and Brightness and Other Confusions
Lightness and Brightness and Other Confusions
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
tions for mapping colours of light, visible to the average human, in a way that<br />
allows their reliable scientific definition <strong>and</strong> comparison. Another starting<br />
point for the work was Hermann Grassmann’s (1809–1877) three laws of<br />
additive colour mixture. 81 This simple psychophysical law laid the ground for<br />
the colour matching experiments involving three ‘primary lights’. As Fred<br />
Billmayer <strong>and</strong> Max Saltzman say in their book Principles of Color Technology,<br />
there is nothing magical about these monochromatic lights. The CIE<br />
could have chosen any other set of colours, provided that they are sufficiently<br />
wide apart in the spectrum. (Billmayer & Saltzman 1981, p 40; Kuehni 1983, p<br />
73). 82 In fact there is no set of three primary colours with which it is possible<br />
to mix all possible colours visible to the human eye. So, in order to derive a<br />
system for describing all colours as perceived by humans that was based on<br />
three primaries, negative values were assigned to some of the colours in the<br />
equation. This led to the designation of the three ‘imaginary primaries’ X, Y<br />
<strong>and</strong> Z. They became imaginary, because in the geometry of the CIE Yxy colour<br />
space, for mathematical reasons, they had to be placed outside the gamut of<br />
human colour vision.<br />
The history of the CIE-colour spaces (there are several) is one of transformations,<br />
technical compromises <strong>and</strong> abstractions. Therefore the so-called RGBprimaries,<br />
which are very much a child of this enterprise, should be treated<br />
with the same coolness as any other technological colour application. There<br />
may be a set of three primary lights that excite the colour sensitive cones<br />
maximally, but four or five would do it even better. The CIE 1931 Yxy colour<br />
space should therefore not be regarded as a model of how the human colour<br />
vision system functions. Neither should it be used as source for drawing conclusions<br />
about how many <strong>and</strong> what are the primary colours – or at least not<br />
outside the logic of the CIE system.<br />
<br />
Grassmann’s First Law: Any colour can be matched by a linear combination of three<br />
other colours, provided that none of those three could be matched by a combination of<br />
the other two. Second Law: A mixture of any two colours can be matched by linearly<br />
adding together the mixtures of any three other colours that individually match the two<br />
source colours. Third Law: Colour matching persists at all luminances. This law fails at<br />
very low light levels, where scotopic vision (rod-receptors) takes over from photopic<br />
vision (cone receptors).<br />
In this sense the primaries underlying the CIE 1931 Yxy space are somewhat like<br />
ordnance survey points in triangulation: one could start anywhere <strong>and</strong> still end up with<br />
an accurate measurement of the terrain.<br />
83