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