As we pointed out in Chapter 5, the mechanism for the production of color by materials is the selective removal of certain wavelengths (or energies) of light from the electromagnetic spectrum. The light penetrates into the material and encounters light-absorbing pigment particles for this selective removal to occur. A photon transfers all of its energy to the absorbing pigment and is "lost," absorbed, that is, from the light beam. The nonabsorbed photons are scattered (or reflected) back from the pigment particle, producing the sensation of color specific to the pigment The absorption of light is illustrated by a beam of light of given intensity (or flux of photons) that penetrates into a material containing a given density of pigment particles that absorb or transmit the photons. The photons penetrate into the material and are absorbed at different depths (Figure 6.11). We also show in Figure 6.11 that some pigments (shaded) will not absorb certain photons. For example, red pigment particles will not absorb photons of light with wavelengths of around 700 nm, but will absorb photons of light with wavelengths of 400500 nm (blue).
The amount of light that is absorbed is not dependent on the intensity of the incident light, but is determined by the density of pigment particles. If the concentration of pigment particles is increased, the amount of absorption is increased and the amount of transmitted light is decreased. The intensity of light passing through a layer of paint will decrease with depth.
Paints with high concentrations of pigment and relatively small amounts of binder (casein, for
Fig. 6.11. Flux of photons incident on a transparent medium containing pigment particles, which either absorb the photons (unshaded particles) or transmit the photons (shaded particles).
example) rely mostly on surface absorption of light to produce color. The color of these paints changes dramatically when they are covered with varnish (Color Plate 21). The simplest change in appearance is the change from a rough matte surface (diffuse reflection when light is scattered in all directions from an irregular surface) to a smooth, glossy surface (specular reflection, where light is reflected in one direction). The varnished paint surface is glossy, and more light is reflected from the varnish surface than from the uncovered paint, leaving less light to be transmitted to and absorbed by the paint layer. This is the source of the darkening effect of a varnished surface. Darkening results from the lower light intensity incident on the paint layer under the varnish.
When the varnish layer is applied, we change the index of refraction relations as shown in Figure 6.12. The reflection of light at the paint layer under a coat of varnish is less than that for the unvarnished paint layer because the difference in refractive indices is smaller at the paint-varnish interface than at the paint-air interface. Once inside the varnish layer, a greater percentage of the light is transmitted into the paint. As a result, more light can be absorbed by the pigment particles, and this will lead to a deeper and richer color.
This concept is applied by painters through the use of a glaze, which is a colored, translucent layer of paint applied over another layer of color. The underlying color is typically opaque, although multiple layers of glaze are sometimes used. The light reflected from the opaque under-layer is filtered by the translucent glaze, resulting in a desired change in color. The colors produced by glazing techniques follow the subtractive color relationships (described in Chapter 5) arising from the absorption and scattering of light by pigment particles in the glaze. Glazes, like varnishes, darken the underlying color slightly and increase the reflective properties of the paint film.
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