Visible light reflection spectra from cuticle layered materials
The brilliant metallic appearance of two kinds of beetles, the Chrysina aurigans and the Chrysina limbata, is displayed visually as well as from normalized reflection measurements of non polarized visible light under normal incidence. The C. limbata is reflective over the visible spectral wavelength range, with a silver-like aspect, while the golden-like C. aurigans is reflective for wavelengths larger than 525 nm, and with a well defined sequence of minima and maxima depicted in the reflection spectra. Both specimens show selective reflection of circular polarized light with the effect being significantly weaker for the C. limbata. A transfer matrix formalism is applied to approach the main features displayed in the measured reflection spectra, by assuming a chirped multilayer structure with decreasing thicknesses of successive layers through the cuticle of the beetles.