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

New Trends in Physics Teaching IV
Dyes, paints and pigments
The perceived colour of an object depends on the combination of the spectral energy distribution
of the light source, the spectral transmittance or reflectance of the object on which the light falls,
the spectral response curve of the eye - and the psycho-physical process of comparison which
the eye uses to determine colour. We wish to study the way light is modified by seen objects,
when the observer and light source remain fixed. It is the colourants, dyes and pigments added to
fabrics, coated on wood or metal with the aid of a binder, or medium, or embedded in plastic,
which do this. They selectively scatter and absorb light, changing the incident spectral energy
distribution. Dyes were originally defined as water-soluble substances used to colour material
from aqueous solution, while a pigment was an insoluble material, generally a ground-up inorganic
powder, dispersed in the medium to be coloured, which was then applied to a painting. Modern
technology has made such distinctions more complex with the introduction of organic dyes and
acrylic paints.
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Wavelength In3
Figure 21. The change in colour with concentration of a blue dye, whose transmittance spectrum is shown. The colour moves
away from the white point C as the concentration increases. (After EVANS, R.M. [6] .)
Dyes, being transparent, work basically by absorption, in much the same way as the filters
discussed earlier, so in principle the calculation of the colour produced by different concentrations
and mixtures of dyes is straightforward. Because it is necessary to calculate the effect of
the mixture at each wavelength, the advent of the digital computer has greatly speeded up
the calculations. According to Beer's Law, the logarithm of the transmittance T (the fraction of
light transmitted) is given by multiplying the concentration C of the colourant by its absorption
coefficient A (the fraction of the light absorbed at unit concentration)
log T= - CA
There is a simple logic behind this. If a single filter cuts the light to one half, then it is clear two
thicknesses will reduce the light to half as much again, or to 1/4, and three thicknesses to 1/8.
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