The appearance of a material is determined by a combination of factors, including the characteristics of the light source, detector, and material (Judd and Wyszecki, 1963; Billmayer and Saltzman, 1981; Wyszecki and Stiles, 1982; Joshi, 2000). A change in the characteristics of any of these three factors alters the perceived appearance of the material. The light source generates the electromagnetic radiation that the material interacts with before the radiation reaches the human eye. A change in the spectral power distribution of a light source (i.e., the intensity vs. wavelength spectrum of the electromagnetic radiation it generates) will therefore cause a change in the perceived color of a material. Thus, a material appears differently when it is illuminated by a white light than when it is illuminated by a red light, even though the material and the observer have not changed. The most common light source is the “daylight” produced by the sun, which contains radiation with energies ranging across the whole visible spectrum (around 400-700 nm). At night or inside a building, artificial light is a more important light source. Materials reflect, absorb, transmit, and scatter the electromagnetic radiation that impinges on them depending on their geometry, composition, and microstructure. Certain wavelengths of
light are more strongly affected by these processes than others, which lead to the observed differences in color, opacity, and gloss of materials. The sensitivity and specificity of the detector (human eye or instrumental device) to different wavelengths and intensities of light also influence the appearance of a material. Finally, the relative location of the light source, material, and detector may also influence the final appearance of a material. In this chapter we will focus primarily on the influence of the composition and microstructure of emulsions on their overall appearance. Even so, it must be stressed that the same emulsion will appear differently when observed using different light sources or detectors.