ABSTRACT
Anyone who works with optical coatings knows that they can present exceedingly attractive colors. These colors originate in interference effects that enhance reflectance or transmittance in certain parts of the visible spectrum and inhibit it in others. Although colors occur with both transmitted and reflected light, it has long been observed that the most vivid effects are usually found in reflection. In the same way that coatings can be designed to have desired spectral properties, they can also be designed to present desired colors. This is a little more complicated than the usual design processes, because of the subjective nature of color itself. Color is strictly a human response to a luminous stimulus. The response varies with the individual observer. Since the coating is not luminous in itself, a source of light is also required before the colors can be observed. We avoid the obvious difficulties inherent in the variations between individual observers and light sources by using theoretical standards that represent more or less well the average properties, and permit us to remove the subjective nature of the problem. The design goals can then be presented in unambiguous terms but there remain some complications. To observe the color there must be an acceptable level of reflected or transmitted light. This, in turn, has a major influence on what can be achieved. A frequent further requirement is that the coatings should be sufficiently simple for large-scale production at reasonable cost. This chapter is a quite abbreviated account of certain aspects of color that are particularly relevant to optical coatings.
