Light extraction from the bioluminescent organs of fireflies

A variety of animals emit light, either for intraspecific signalling, for predator repulsion or for using their own vision system in total darkness. As the design of light-emitting diodes has revealed, light extraction from a high refractive index medium is difficult because transmission is limited by total internal reflection. Surface roughness is needed to attempt avoiding this limitation. The optical structure of the bioluminescent organs of fireflies is investigated and the possible role of inhomogeneities for improving the efficiency of the radiative emission is considered. This analysis shows that the light extraction in this complex structure is essentially doubled, compared to the extraction in a reference system consisting of an homogeneous chitinous medium terminated by a flat surface. The inequal fitting of the scales and the lowering of the average refractive index in the photocytes accounts for most of the improvement.