The refractive index of an optical film composed of a mixture of more than one constituent is a function of the micro-structure of the film. Even films that are nominally of a single material have refractive index values that require interpretation on the basis of mixed component models because these values differ significantly from the values in equivalent bulk materials. In the latter case, the film is usually a mixture of bulk material and voids, or bulk material, voids, and adsorbed water. Many models have been used to explain the refractive indices of mixed component systems. The Lorentz-Lorenz model, the Drude model, and the effective media approximation (EMA or Bruggeman model) are the most common models used to estimate isotropic refractive indices of mixtures of isotropic materials. Films composed of anisotropic micro-structures, however, require other models such as the Bragg and Pippard model. One use of mixed component models would be to predict the porosity of optical films. Another would be to predict the refractive index of coevaporated films. However, no one model is applicable to all situations. Therefore, the prediction of refractive index becomes difficult. Usually, the model is chosen after the set of measurements has been done.