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

Colour
Perceptually, if our environment is bathed in light from any one of these sources, we would
gain the feeling of white. This is because our eye is not a photometer but works by comparison
and we have nothing with which to make such a comparison if the whole environment has the
same illumination. This brings up the question asked by photographers why a blue filter is
necessary over the lamp before daylight film can be used with a filament lamp. After all, we use
the same eyes both outside and with such a lamp, why cannot the film also be the same? The
answer lies in the fact that in viewing the film, our colour sense is modified by the environment
surrounding the image and this would make the colour as viewed too yellow should the film be
exposed under a filament lamp.
The Colour of Reflected Light
So far, we have discussed the comparison of illuminants, which has the advantage of allowing
us to match the three primary sources and our test lamp under similar conditions. However, in
practice we are generally interested in the colour of objects illuminated by a particular light
source. We obtain the values for the primary sources illuminating a block of highly reflective
material which matches the colour of the object illuminated by the white light of interest, under
the same geometrical conditions. The material must reflect white light diffusely with large,
constant efficiency. Often MgO is used. Then the ratio of the Y co-ordinate for the opaque
material to that for the MgO block is a measure of its reflectance. This is shown diagrammatically
in figure 2, if we place our test sample over that part of the white screen illuminated by the
single lamp. The eye matches the light reflected from the test sample illuminated with the
particular light source, generally one of the CIE standards for white light, with light from the
three sources providing the primary colours, reflected from a magnesium oxide block. If the
luminance of the primary sources is (r), (g) and (b), we may obtain X, Y and 2 from these, using
the equations given before. However, in practice this is rarely done. Instead, a spectrophotometer
is employed. This device compares the-spectrum reflected from the sample with that reflected
from the magnesium oxide block, wavelength by wavelength, giving the reflectance of the sample,
which is the ratio of the intensity of the light reflected from the sample at a specific wavelength,
to that reflected from the magnesium oxide, taken as 1. Figure 10 shows diagrammatically how
we obtain the x and y co-ordinates from the reflectance. We take the intensity spectrum of the
illuminant we desire, multiply this, wavelength by wavelength, with the sample reflectance to
obtain the spectrum seen by the observer. This is multiplied by the three spectral tristimulus
plots, sometimes called the ‘colour matching functions’ (figure 8) wavelength by wavelength
again. The area under the three curves obtained (the integrals) is equal to the X, Y and 2 values,
and from the ratios of these we obtain x, y and z as before. Hence, we obtain the values without
interposing a live observer, by assuming the spectral tristimulus plots.
There are various mathematical techniques for simplifying the rather tedious calculations, but
the employment of computers has rendered most of these obsolete. If we plot points on the
trichromaticity diagram, the magnesium oxide should lie at the same point as its white light
illuminant, if it reflects uniformly. Under these circumstances, Y for the object is the reflectance
normalized against the magnesium oxide.
A1 terna tive Co-ordinates
The trichromaticity co-ordinates are convenient for the physical representation of colour. However,
most people have little feel for their significance, so three other parameters are often
employed which are simpler conceptually.
At this point, we must separate the psychephysically defined quantities of luminance,
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