Fig. 1. Viewing geometry. (A) Reflectance spectra were recorded from a 1 mm diameter spot on the feather surface, with the vane perpendicular to the line of sight. This spot was on the horizontal axis of rotation of the specimen holder, which lay perpendicular to the line of sight. The orientation of the feather surface and the position of the light source are specified relative to the line of sight. Surface orientation, O, is the rotation of the surface relative to the perpendicular to the line of sight and is measured clockwise in this figure (i.e. at O=90°, the surface faced upwards). The light source moved in azimuth and elevation, but we refer mainly to effects of varying elevation, with the angle E being given by clockwise rotation from the line of sight (i.e. at E=90°, the light source was directly above the specimen). (B) Feathers were mounted in two alignments. At O=0°, when vertically mounted, the apex was vertically above the base; when horizontally mounted, the vane was horizontal.

Return to article