In the past, researchers have tried to model butterfly wings as simple, planar thin-film structures to gain insight about their radiative effects, but they have produced results that fail to explain the blue iridescence of the Morpho menelaus. The problem, according to a team at Tufts University in Medford, Mass., is that the models only scratch the surface of the wing's scale structure, which can include 24 alternating layers of chitin and air to produce a thin-film structure as complex as many semiconductor chips.

The Tufts researchers constructed their own model for spectral imaging and reflectivity measurements of small areas over the 400- to 900-nm range. They used a microscale reflectance spectrometer that incorporated a monochromator from Instruments SA Inc., now Jobin Yvon Inc. of Edison, N.J., and a Photometrics CCD camera from Roper Scientific in Tucson, Ariz., to image the spectral reflectivity of areas as small as 0.348 x 0.348 µm. As reported in the October issue of Optical Engineering, the model predicts the peak reflectivity to be at the high end of the blue range, and correlates with a very slight tint of green on the wings that can be viewed under normal incidence.