New trends in physics teaching, v.4; The ... - unesdoc - Unesco

New Trends in Physics Teaching IV
TRICOLOUR VISION
‘For the Rays (of light) to speak properly are not coloured. In them there is nothing else than a
certain Power and Disposition to stir up a sensation of this or that Colour’. [Newton, 1 .]
Our hearing process can pick out one frequency mixed in with many others. Not so our eyes.
Whereas a given intensity spectrum of a light, surrounded by the same background, wil always
produce the same visual sensation, that sensation may also be elicited by many different spectra
- for example, the sensation of turquoise may be produced by a band of wavelengths at 490 nm,
or by a rather broad mixture of blue and green light.
In 1806 Young suggested that the sensation of any colour could be produced by a mixture of
only three ‘primary colours’. However, Young was a rather unpopular figure at the time, and it
was not until later when the German physicist Helmholtz championed the theory that it was
accepted. In the 1860s, Maxwell developed his colour triangle to represent the proportions of
such colours necessary, as shown in figure 1. The three primary colours, from which the others
were to be derived, were placed at the apexes of the triangle, and a point here represented 100
per cent of these colours. Any other colour could be matched by a combination of these three,
in different proportions, so that point A in figure 1 is a turquoise which can be matched by 40
per cent blue, 40 per cent green and 20 per cent red.
It should be emphasized that the matches were made by eye - and hence depended entirely
on the observer’s vision - and that it was the illuminants that were added - i.e. the illumination
produced by red, green and blue lights matched the illumination of the test lamp. In practice,
the lights were compared by shining them onto the same sheet of white material. As discussed
later, the scientific justification for the three colour theory was only confirmed quite recently.
Maxwell’s triangle gave only crude-quantitative measurements, so in 193 1, the Conseil International
d’Eclairage (CIE) devised a truly quantitative method for specifying the proportions
green
blue magenta red
Figure 1. Maxwell’s colour triangle.
194