Thermal analysis of butterfly wings

Nancy D. Lamontagne

Scientists study the way in which biological structures produce various optical phenomena because this information can reveal more about how some organisms communicate and can offer new structures for possible use in man-made materials.

One such organism under study is the Helena butterfly, Morpho rhetenor (family Morphidae), whose blue iridescence results from light scattering from multilayer diffraction elements in intricate periodic nanostructures in its wing-scale flaps. Scientists have found that the structures reflect some wavelengths while absorbing others. However, little is known about the thermal qualities of these layered biological structures. Thus, scientists from the Britannia Royal Naval College in Dartmouth, UK, led by Chris Lavers, turned to infrared light to learn more about the detail within butterfly wings.

A high-resolution passive image of a zebra wing butterfly (640 x 512 pixels) taken in the mid-IR reveals weak internal detail.

It is important that the method used not harm the delicate structure of the butterfly wing. Because passive ambient IR imagery does not reveal details (Figure 1), the researchers tried to get more structural information from thermal images of a butterfly wing using a different method. To do so, they collaborated with Thermal Wave Imaging Inc. of Ferndale, Mich., in using nondestructive IR testing. This method is used for inspection of the leading edges of the space shuttle, for example.

Flash thermography reveals more details, as shown in this image taken approximately 100 ms after heating with an optical flash pulse.

They used a flash thermography instrument from the company. In this technique, a brief pulse of light heats the sample surface. Changes in surface temperature as it cools are recorded by an IR camera and displayed with false colors (Figure 2). As the sample cools, the surface temperature may be affected by internal structural flaws that obstruct heat flow in the sample, revealing details in structure.

The researchers combined this technique with the company’s Thermographic Signal Reconstruction method to increase sensitivity and resolution. A montage of M. rhetenor images shows the thermal temporal progression after optical stimulation, with the onset of diffusion-dominated processes (Figure 3).

These M. rhetenor images show a reconstructed flash sequence. Increasing time correlates with images taken at a greater depth within the specimen.

The researchers believe that this technique will be valuable for use in shallow depth analysis of delicate biological specimens.