Fig. 10. Fourier analysis of a concavity from a structurally coloured scale of Papilio ulysses. (A) Transmission electron micrograph of a single scale surface concavity from Papilio ulysses. (B) 2D Fourier power spectrum of A showing an arc-shaped distribution of Fourier power peaks (red pixels) above and below the origin that is created by the concave distortion of the fundamentally laminar array of air bubbles in the scale. The result is a broadening of the range angles over which the light back-scattered to the observer will maintain the same peak hue.