Scatter from fluid patches in optical thin-film coatings

Fluids adsorbed within the layers of optical coatings cause gradients of refractive index which scatter light from the reflected and transmitted beams. A general equation which relates the sensitivity to scatter to the strength of the electric field at a point in a coating is derived and used to obtain approximate expressions for the magnitude of scatter expected from antireflection films, multilayered reflecting coatings, and interference filters. Examples of scatter from all-dielectric and metal-dielectric-metal filters are given, and the timedependence and tuning characteristics of the scatter are discussed.