Spectral imaging, reflectivity measurements, and modeling of iridescent butterfly scale structures

Abstract

An innovative device for spectral imaging and reflectivity measurements is developed to study iridescent butterfly scales. Of par- ticular interest is the bright blue Morpho menelaus butterfly. The device called the microscale reflectance spectrometer (MicroRS) is used to measure spectral reflectivity for the M. menelaus between 400 and 900 nm. Different optical effects such as thin-film interference, scattering, and diffraction are examined in an attempt to understand the microstructure of an M. menelaus iridescent butterfly scale. The MicroRS apparatus uses a modified microscope and CCD camera to examine spectral re- flectivity of areas as small as 0.348 30.348 mm 2 . Some individual fea- tures of an M. menelaus scale are found to be of the same order of magnitude or smaller than the resolution of the MicroRS device. As a result, individual pixel analysis is used to determine only that the scale structure produced high reflectivity, 40 to 70%, between 450 and 550 nm and very low reflectivity, less than 8%, above 600 nm. An average reflec- tivity over a larger area is taken, resulting in a peak of 68% at 500 nm. This explains the bright blue iridescence seen from the M. menelaus wing. Results are used in conjunction with a thin-film numerical program to develop a complex model of the butterfly scale structure. The structure includes 24 alternating layers of chitin and air. A complex combination of an effective index of refraction and effective area model is used to match the reflectivity of an M. menelaus butterfly scale. © 2001 Society of Photo-