Towards optimum multilayer filter design

The turning point method was applied to the design of optical and infrared multilayer filters. Both indirect and direct designs were investigated. In the indirect approach, optimum equiripple filters were first designed for operation between a hypothetical incident medium and the substrate. Conversion for operation between air and the substrate was accomplished by matching at the air-multilayer interface. The direct approach optimises the filter performance operating under practical terminal conditions. Depending on design strategies, the two approaches have their relative merits. Between them they yield highpass and lowpass multilayers with performance approaching the best possible.