Lightness and Brightness and Other Confusions

pigment, the two mixtures may very well look different. Or a restorer of paintings
may touch up an area and achieve a perfect match in the studio only to
find that when the painting is shown in the lighting of the gallery the touchedup
area is plainly visible.
On the other hand, one who thinks that colours are simply spectral power
distributions may be inclined to say silly things, such as “white light is a mixture
of all colours.” This despite the fact that the original decomposer of white
light, Isaac Newton, cautioned, “the rays are not coloured.” Furthermore,
such a devotee of physical colours would be unable to understand how it is
that a monochromatic yellow light could be visually indistinguishable from a
mixture of two lights that look respectively red and green. In short, he would
be unable to understand how colour television is possible. This brings us back
to our second case, in which we might refer to the yellow as a psychophysical
colour.
Psychophysics is concerned with an organism’s behavioural response to physical
stimuli. In the case of colour, we know that human visual systems heavily
filter spectral energy information in their environment, reducing it, in daylight
conditions, to the response ratios of three photoreceptor classes. In the
first half of the 20th century, information about the actual response characteristics
of the photoreceptors was unobtainable, so the colour matches made
by observers under carefully controlled conditions were averaged and mathematically
manipulated to yield the 1931 CIE Standard Observer. This “Observer”
(actually a lookup table), along with the specification of illuminant
standards enables one to calculate colour mixtures that correspond well with
the colour mixtures that real people make. The CIE system and related
psychophysical colour spaces have been developed and improved upon over
the years. For the appropriate standard viewing and illumination conditions,
the CIE Observer will predict a match between our “television patch” yellow
and our paint chip. They are thus the same psychophysical colour.
Despite the mathematical sophistication and practical success of colorimetry,
we must bear in mind that its proper business is mixing and matching. It is
common to see coloured renditions of the tongue-shaped CIE chromaticity
diagram 1 . What is intended to be an intuitive aid in reading the figure all too
readily makes the unwary suppose that the colour sample represented by a
point in the diagram looks like the colour of the region in which the point
appears, and that a straight line from a spectral locus to the white point will
be a line of constant hue, with only the saturation varying. Typically this is
not the case; in the chromaticity diagram the lines of constant hue are mostly
curved to a greater or lesser extent. Hue shifts with desaturation.
1
See figure 14 on page 61.
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