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

Colour Difference
Colour
The ability of the normal eye to assess differences of colour is not uniform over the CIE diagram.
McAdam performed a series of experiments with several observers, leading to the results shown in
figure 12. The ellipses represent the ability of the eye to detect differences. It is clear that the eye
is most sensitive to changes in hue and saturation in the blue, Our eyes are not very sensitive to
changes of saturation in the green, The axes of the ellipses have been multiplied by ten. Colour
differences are notoriously difficult to specify, which is a nuisance since so much depends upon
them in the dye and paint industries. Many paint and dye companies depend upon interpolations
of the Munsell scheme of ordering in terms of hue, value and chroma to specify their pigments.
To avoid the limitations of human observers, recently both analogue and digital computers have
been employed to measure colour differences employing spectra obtained from a spectrophotometer.
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Figure 13. Defective colour vision. Loci of colou~s confused by protanopes (a) and deuteranopes (b).
‘Colour blindness’ - Defective Colour Vision
The CIE chromaticity diagram is based on the fact that the response of normal individuals to
matching colour is closely the same. However, a small percentage of the population has a reduced
ability to perceive colours. The mechanism of the retinal process is so little understood that it is
uncertain whether the causes of abnormalities lies in the malfunction of the retina or the processing
that follows this. All forms seem to be inherited. Abnormalities are divided into monochromatism
(‘total colour blindness’), dichromatism (‘partial colour blindness’) in which only two
distinct hues are sensed, and anomalous trichromatism, where the three colour response differs
from the norm. The colour response of a monochromatic (a better word would be ‘achromatic’)
individual is the same for photopic and scotopic vision i.e. high or low levels of illumination. This
leads to the belief that only the rod cells, which respond to low level illumination, are present in
such eyes. Dichromatism takes several forms, the effects of two of which, deuteranopia and
protanopia, are shown in figure 13. Colours lying along the straight lines on these diagrams are
confused by such individuals. Thus, both would be unable to distinguish red from green, and the
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