Coating design using very thin high- and low-index layers.

Any arbitrary generalized gradient-index interference coating (including homogeneous and inhomogeneous layers) possesses a digital configuration (sequence of thin high- or low-index layers), which is spectrally equivalent at all wavelengths. Such digital configurations are found directly from arbitrary-index profiles by using a prescribed two-layer high-low equivalent to a thin layer of arbitrary index. Digital configurations may be designed directly from given spectral requirements by using a flip-flop optimization scheme.