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

New Trends in Physics Teaching IV
X
X = 14.13
Y = 1 4.20
z = 51.11
Wavelength / n m Wavelength / n m Wavelength / nrn
CIE Standard Source Object CIE Colour-matching CIE Tristtmcllus
functions for the
values
eq u a I - e ne r g y spectrum
Figure 10. Method of calculating the CIE tristimulus values X, Y and 2, for light reflected by an object, from the energy spectrum
of the source, multiplied by the reflection coefficient R of the object, multiplied by the spectral tristimulus values for the equal
energy spectrum. The area under the resultant curve gives us X, Y and Z.
dominant wavelength and purity, with the perceptual quantities of brightness, hue and saturation.
The psycho-physically defined quantities are numbers based on the spectrum of the source, and
obtained from the CIE diagram. The perceptual quantities are the psychological responses associated
with these respective quantities, because they depend on things other than the spectrum of
the source. The study of the physical quantities connected with light and colour, such as the
power of light in watts per square metre at a given wavelength falling on a surface, is called
radiometry. The study of the psycho-physical quantities, such as lumens per square metre, is
photometry. Quantities such as the lumen are specified in terms of a defined average over many
observers to obtain lumens per watt. However, the psychological response, such as saturation,
corresponding to the photometric quantity purity, is not defined, and wil depend on the
observer. For example, change in luminance wil not only alter the perceived brightness of a
colour there wil also be changes in hue (the Bezold-Brucke effect) and in saturation (the Purdy
effect). A similar effect in sound is heard when the psychological pitch of a note rises as the loudness
of the note dies away, though the physical quantity of frequency remains constant.
Luminance is given by the Y co-ordinate as before. (Reflectance for a reflecting sample.)
Draw a straight line on the chromaticity diagram from the point representing the white used as
the illuminant through the point representing our sample, to the periphery of the plot. Dominant
wavelength is then the spectral wavelength at which this line intersects the periphery. We can
match the colour by adding a quantity f of light of the pure spectral value involved, to one unit
of the white light. Purity is the ratio off to the total light (1 + f), i.e. the fraction of pure hue
in the mixture.
As an example, look at the point A in figure 7, which is for an orange paint sample. We draw
a line from the white point, through A until it intersects the periphery at B. From point B, the
value of the dominant wavelength is found to be 592 nm. The purity is given by WA/WB, which
is 0.18/0.30 or 60 per cent.
There is no dominant wavelength for the region directly between red and blue, where colours
such as purple and magenta lie. It is usual to specify the dominant wavelength of the comple-
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