Natural SciencesMaterials EngineeringMaterials ScienceOptical MaterialsOpticsSurface EngineeringChemical Vapor DepositionCoating TechnologyCoatingsOptical Thin FilmsRefractive IndexThin Film Optical PropertiesOptical CoatingsOptical DesignOptical Thin Film CoatingsRefractive Index Engineering
The two main non‑numerical design methods for optical interference coatings—equivalent layers and polynomial synthesis—are compared, noting that equivalent layers perform well with few fixed refractive indices (as in evaporation) while polynomial synthesis offers better designs but cannot predetermine indices. For evaporation‑based coatings, designs must be translated from many refractive indices to a few, whereas for sputtering or CVD, intermediate indices can be generated by mixing or flip‑flopping, enabling direct implementation of the designs. Translating designs from many to few refractive indices for evaporation generally degrades performance, whereas using sputtering or CVD to generate intermediate indices allows superior designs to be implemented.
The current two major non-numerical design methods, equivalent layers and polynomial synthesis, are reviewed and compared. The equivalent layer method works well when only a small number of fixed refractive indices is available. This is the case when the coating is manufactured by evaporation. Polynomial synthesis generates a priori better designs but does not allow predetermination of refractive indices. So, for evaporation, the designs have to be translated from many refractive indices to a few. This process generally downgrades the design. This translation is not necessary when sputtering or chemical vapor deposition is being used. Here, in-between refractive indices can easily be generated by mixing or flip-flopping. As a consequence, superior designs can be implemented.