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

An innovative device for spectral imaging and reflectivity measurements is developed to study iridescent butterfly scales. Of particular interest is t he 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 butte rfly scale. The MicroRS apparatus uses a modified microscope and CCD camera to examine spectral reflectivity of areas as small as 0.348x0.348 mum(2). Some individual features of an M. menelaus scale are found to be of the sam e 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 s cale structure produced high reflectivity, 40 to 70%, between 450 and 550 n m and very low reflectivity, less than 8%, above 600 nm. An average reflect ivity over a larger area is taken, resulting in a peak of 68% at 500 nm. Th is explains the bright blue iridescence seen from the M. menelaus wing. Res ults 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 effectiv e index of refraction and effective area model is used to match the reflect ivity of an M. menelaus butterfly scale. (C) 2001 Society of Photo-Optical Instrumentation Engineers.