Figure 3

Photographs, scatterograms, and reflectance spectra of single scales, glued to a thin glass micropipette, with modeled spectra. A-D A cream (c), orange (o), black (k), and blue (b) scale, respectively. In each panel, photographs (column I) and scatterograms (column II) of the upper and lower side (abwing: ab, and adwing: ad; see inset C, column III) of the scales are shown above and below, respectively. The red circles in the scatterograms indicate scattering angles of 5°, 30°, 60°, and 90°. The reflectance spectra were measured with respect to a diffusely reflecting white reference, and thus yielded too high values for directionally reflecting media. A-C, column IV, present reflectance spectra modeled with optical thin film and multilayer theory for five thin films consisting of butterfly chitin in air with different thicknesses (dotted curves) and their average (dashed bold curve), for the cream (c), orange (o), and black (k) scales, respectively. The averaged spectra, when multiplied with a spectrum representative for the transmittance spectrum of the scale’s pigment (derived from Figure 1D) yielded the continuous bold curves. The thickness (in nm) of the five scales was: 210 + 10i (c), 115 + 5i (o), 170 + 10i (k), with i =1-5. D column IV, presents reflectance spectra of three chitinous thin films in air with thickness 190, 200, and 210 nm, and reflectance spectra of a stack of three parallel layers in air, with thicknesses 60, 80 and 60 nm, where the upper and lower layer had the refractive index of chitin. The refractive index of the middle layer was taken to be a weighted average of the refractive indices of chitin and air with ratio 1:2 (200a) and ratio 1:1 (200b).