Substrate optical theory
The bulk optical properties of crystalline substrate materials are represented by the complex refractive index as a function of wavelength and temperature. Materials for the visible spectrum change comparatively little with temperature or wavelength. However, in the infrared every material is strongly influenced by the effects of lattice absorption and dispersion, with additional effects caused by pronounced changes with temperature. The use of predictive modelling of these properties can therefore be of considerable benefit in understanding the limitations, or advantages, in the filter performance, which can be achieved from knowledge of the materials prior to the multilayer design.