Bright-White Beetle Scales optimise Multiple Scattering of Light

Whiteness arises from diffuse and broadband reflection of light typically achieved through optical scattering in randomly structured media. In contrast to structural colour due to coherent scattering, white appearance generally requires a relatively thick system comprising randomly positioned high refractive-index scattering centres such as from the pigment titanium dioxide. Research has just been published that shows that the exceptionally bright white appearance of Cyphochilus and Lepidiota stigma beetles arises from a remarkably optimum anisotropy of intra-scale chitin networks, which act as a dense scattering media. Using time-resolved measurements, it has been shown that light propagating in the scales of the beetles undergoes pronounced multiple scattering that is associated with the lowest transport mean free path reported to date for low-refractive-index systems. A light transport investigation unveiled a high level of optimisation that achieves high-brightness white in a thin low-mass-per-unit-area anisotropic disordered nano-structure. These findings will likely be relevant for many applications, enabling objects such as paper, plastics, paints, as well as white-light reflectors inside new-generation displays to be made whiter, while at the same time using a smaller amount of material. The research was funded by the European Research Council and the Biotechnology and Biological Sciences Research Council (BBSRC).