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The reflectance spectra of iridescent hummingbird feathers were measured spectrophotometrically for many species, and the structure of the iridescent parts of such feathers investigated by electron microscopy. In the direction normal to the iridescent feather surfaces the structure is found to be periodic in optical dimensions. It consists of stacks of 7–15 elliptical platelets of a material with refractive index ~2, each containing a structurally reinforced air gap. To interpret the reflectance spectra in terms of the surface structures, a theory is given of reflection coefficients of stratified media with analytic refractive-index variations. The theory is applied to the iridescence of hummingbird feathers and accounts quantitatively for the iridescent colors as interference colors by reflection from the observed feather structures.
Greenewalt et al. (Sat,) studied this question.