What do a TV set, a Georges-Pierre Seurat painting and an emerald swallowtail butterfly have in common? The cathode-ray tube, the pointillist painting and the insect all mix tiny points of color to produce other colors through the perceptual phenomenon known as spatial-averaging color-stimulus synthesis. A research team from the thin-film photonics group at the University of Exeter's School of Physics and the UK Defence Evaluation and Research Agency in Farnborough, however, suggests that the butterfly's approach is unique and could lead to advances in the production of pigment-free colors. A concavity on a wing of the emerald swallowtail, Papilio palinurus, doubly reflects light incident to its sides, producing polarized blue light. The flat bottom reflects unpolarized yellow light; together, they are mixed and appear green. Image courtesy of the University of Exeter. "Color mixing is used in nature quite frequently," said Peter Vukusic, a research fellow at the university and co-author of the team's report on the work in the March 30 issue of Nature. "Usually this color mixing is part structural and part pigmentary." Some birds, he explained, appear green because their yellow pigmentation is combined with blue light generated by Tyndall or Raleigh scattering on their feathers. The emerald swallowtail, classified as Papilio palinurus, is special because this native of Indonesia mixes its brilliant greens from a palette that it produces in a wholly structural way, which the researchers call orthogonal-surface retroreflection. The modulation of a single multilayer structure across the scales of the wing has formed retroreflecting concavities, each approximately 4 to 6 µm in diameter and from 0.5 to 3 µm deep. The cavities reflect yellow light from their flat bottoms but doubly reflect polarized blue light from their orthogonal sides. The result is that the butterfly appears green to the human eye by color-stimulus synthesis, but, to photoreceptors that are sensitive to polarized light, coloration by retroreflection may carry additional information such as species type and wing orientation. "We are currently extending our reading to see if there is a vision story to be told that ties in completely with the structural coloration that our studies are revealing," Vukusic said. Butterflies to banknotes The discovery came about by serendipity, the researcher said. "P. palinurus was one of the species that looked interesting and pretty stunning when I visited a butterfly farm one weekend." Vukusic's team had just completed a study of a butterfly in the genus Morpho, and he noted that the iridescence on this insect seemed different. "I ordered some samples out of curiosity's sake," he said. The defense agency announced in 1998 that it is attempting to develop color-controllable, pigment-free finishes from such studies of butterfly wings. Vukusic said that the researchers have filed a patent for anticounterfeiting measures based on butterfly scales, which they hope will be an alternative to the optically variable pigments used on credit cards, banknotes and traveler's checks.