ABSTRACT
When we design and calculate the performance of optical coatings, we normally assume ideal conditions. Light is collimated and monochromatic, optical thicknesses are exact, and the optical constants of the layers are precisely known. In the real world, none of these conditions is fulfilled exactly. Coatings depart from perfection in terms of thickness, optical constants, coating uniformity, scattering, and so on. Measurements use light with finite aperture and bandwidth. Illumination bandwidth, once the filter is installed in an actual system, can be still less well controlled, as well as the environment. Although the effects may sometimes be surprising, they are nevertheless understood and predictable. Here we list some of the more important effects. We simply reference a few because they are dealt with in greater detail elsewhere in this book.
