Fig. 8. Reflectance spectra of the turquoise patch on an Indian roller tail. This had diffuse coloration, which looked most `colourful' when viewed obliquely with the light behind the observer. Data are for a feather aligned vertically (see Fig. 1B), but rotating the feather to a horizontal alignment had no effect on reflectance. (A) Reflectance with O=70°: (i) E=5°; (ii) E=45°; (iii) E=70°. (B) Reflectance with E=5°: (i) O=0°; (ii) O=45°; (iii) O=70°. Spectrum iii is the same as spectrum I in A. (C) The spectral location of the reflectance peak (_max) versus E. Values of O (in degrees) are given beside the cross to which they refer. There is a consistent shift to longer wavelengths with increasingly oblique viewing angles. (D) Reflectance under diffuse lighting: (i) O=0°; (ii) O=30°; (iii) O=60°. O, surface orientation; E, illumination elevation.

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