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Predicted values of hue under normal incidence by thin-film optical models considering different values for the refractive index (n; solid lines) and the extinction coefficient (k; dashed lines) of (a) keratin and (b) melanin. Vertical dashed lines indicate the mean value of hue and its 95% CI (grey area), as estimated from the reflectance spectra obtained at normal incidence from feathers of male blue-black grassquits.
