Biomedical Engineering Reference
In-Depth Information
significance. It is thus not surprising that the study of the mechanisms of color
production in the biological world attracted the attention of scientific giants like
Newton, Michelson, and Lord Rayleigh.
There are several ways to produce coloration in the biological world. The most
general cause of coloration is the presence of pigments. Pigments can selectively
absorb certain wavelengths of light and reflect or scatter others. Fireflies and many
marine organisms can produce coloration by bioluminescence. In bioluminescence,
energy will be released in the form of “cold light” emission via chemical reactions.
Fluorescence can also generate coloration for materials containing fluorescent
molecules when illuminated by ultraviolet (UV) light.
Another important category of color production is structural coloration. Dis-
tinctly different from other categories, structural colors are of structural origin,
namely, from the interaction of natural light with photonic structures whose feature
sizes are comparable to the visible wavelengths. Structural coloration is thus purely
produced by physical means via optical phenomena such as interference, diffraction,
scattering, or their combination.
Structural colors possess many interesting features that are distinct from pig-
mentary colors since they are produced structurally. Generally, structural colors
are bright and highly saturated, and are hereby called “metallic colors” sometimes.
They may display iridescence, 1 i.e., a color change with perspective. Variations in
structures or contrast of refractive indices may alter or even destroy structural colors,
for instance, by applying pressure or infiltrating liquids into air voids. In contrast to
pigmentary colors, structural colors would not fade provided that the associated
photonic structures retain unchanged.
In recent years, structural colors and associated photonic structures have been
subjected to extensive studies because of their scientific and practical importance
[ 1 - 9 ]. Studies of structural colors could offer important information related to their
evolution, biological functions, structural formation, and strategies of light steering.
On the other hand, structural colors may have potential applications in a variety
of industries such as photonics, display, painting, and textile. Natural photonic
structures and their ingenious ways of light steering could be a great source of
inspiration in our design and fabrication of new optical materials and devices for
future technological applications.
8.2
Historical Survey
Historically, Hooke and Newton might be the first ones to give a scientific
description of structural colors. In his topic Micrographia published in 1665, Hooke
described his microscopic observations on the feathers of peacock and duck, and
1 The terms structural color and iridescent color are sometimes used indiscriminately to name
colors produced structurally. An iridescent color is certainly a structural color. But a structural
color may not display iridescence.
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