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

Colour
Also shown on the same figure is the outline provided by the cards or ‘chips’ printed with
Munsell coloured inks, which probably provide the greatest colour purity possible for printing.
The maximum excursion of purity must lie within the figures. It is clear inks cannot provide
as great a purity as illuminants such as phosphors, and that least purity lies toward green, where,
fortunately, the eye is least sensitive to purity.
Subtractive Colour - Filters
So far, we have been adding illuminants to provide colours of different dominant wavelength and
purity. White light may be considered as a combination of points running around the edge of
chromaticity diagram, excluding the straight line from red to blue. If we filter out all but a small
segment of the periphery, we get a pure hue, colour of one dominant wavelength, but at the
expense of brightness, since we are using only a small fraction of the total light output. As the
region passed by the filter increases, the light transmitted gets brighter, but less pure since the
centroid of the peripheral points moves inward away from the curve. It is difficult to obtain a
bright but pure green, for example, because the greatest curvature of the periphery occurs here,
and since coloured inks work by filtering the white light reflected by the paper, very pure greens
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Figure 15. The lighter a colour is, that is, the more light it reflects, the more restricted its chromaticity range. Here, the
‘McAdam limits’ of the chromaticity of real colours, viewed in daylight (CIE Source C) is shown for different values of their
luminance Y. It is essentially a contour plot, the ‘altitudes’ being the maximum reflectance.
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