Publications on butterfly thin films as solar collectors

The [unproven] notion that the iridescent scales of some butterfly and moth species behave as solar collectors comes from a group of engineers at Tufts University in Massachusetts who have repeated this claim throughout the popular media (See below). Their conclusion is inspired by their own work with semiconductor wafers which show anomalous heating during manufacture, apparently due to the thin films covering them. The wafers have an altered emissivity which can decrease the rate at which they emit their excess heat to their surroundings by a few percent, but which can produce a significant temperature rise in the wafer.

The researchers then looked for examples in nature where such heating might be taking place and 'discovered' iridescent butterflies.

The iridescence of butterfly scales has long been understood as being the result of optical interference -- in this case as a result of alternating layers of air and chitin (the material that makes up insect exoskeletons). While such interference can lead to 'destructive interference' of incident light, that 'destruction' only means that the light is redirected -- transmitted light is reflected  or reflected light is transmitted. Optical interference itself cannot cause absorption. In fact, if butterfly scales had no iridescent coverings on them at all, one might expect there to be less, not more light energy available to the rest of the wing for absorption.

To observe this for yourself, simply place a drop of oil on the wing of an iridescent butterfly. (Vendors can be easily found on the Internet.) The oil has a refractive index similar to the chitin, and replaces the air between the chitin layers. The wing covered by the chitin turns a dull, dark color, most likely brown or black. Some 'cover scales' are, in fact, transparent when their irridescence is removed. Clearly, more light would be absorbed by the underlying dark tissue if the butterfly were not irridescent.

Irridescence does seem to play a major role in evading predators, as the butterfly can flash the bright top sides of its wings, and then fold its wings back up displaying the dull brown undersides of its wings. The confused predator then searches in vain for the bright butterfly of which it caught a fleeting glance.

Literature supportive of thin films as solar collectors:

Technical articles:

Other periodicals: The Internet: (Warning: some of these items may have since been corrected or withdrawn.)

Literature skeptical of thin films as solar collectors:

Technical literature:

Nontechnical literature:

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