Ronald R. Willey, Willey Optical, Consultants, Melbourne, FL
The average reflectance (Rave) of a coating over a bandwidth (B) depends on the overall thickness (C), the index of refraction of the last layer (L), and the difference in index between the high and low index materials used in the design (D). Composite experience from a myriad of designs over three decades has been empirically fit to an equation in Excel. The four factors can be entered into the program and the estimated Rave will be calculated along with the estimated minimum number of layers required for the design. This program makes it possible to know what Rave can reasonably be expected beforehand without spending time and effort designing the coating. This can avoid trying to design to an impossible specification and allows minimizing the number of layers in achievable designs. This also allows the designer to decide between a minimum overall thickness of the design versus adding up to three times that minimum thickness for some further reduction of the Rave. This Excel program is available on request from the author.