Metal/dielectric transmission interference filters with low reflectance. 1. Design.

For many applications the high reflectance of conventional multilayer interference filters is undesirable. It is possible to design and construct interference filters in which the transmittance and reflectance are independently controlled and in which the reflectance is very low. However, to do this it is necessary to employ absorbing coating materials and, as a result, a reduction in the peak transmittance must be accepted. We describe three different methods for the design of such coatings. Examples are given of a number of bandpass filters, neutral density filters, cutoff filters, and of an x¯(λ) tristimulus filter, all having a low reflectance. This reflectance is frequently 15 to 50 times lower than that of conventional filters.