Biomedical Engineering Reference
In-Depth Information
other feathers of changeable colors. Newton described in his topic
Opticks
published
in 1704 that the color production mechanism for the finely colored feathers of birds
and particularly those of peacock tails is after the same manner of thin films. Both
Hooke and Newton attributed feather colors to the thin films of the transparent parts
of feathers, but the true physical mechanism was still unanswered.
We now know that the correct mechanisms of structural coloration rely on
the wave nature of light, resulting from the interaction of natural light with pho-
tonic structures. But quantitative descriptions of reflection, refraction, interference,
diffraction, and scattering had to wait until the establishment of the electromagnetic
theory by Maxwell in 1873, known as Maxwell's equations.
Ever since the distinction of pigmentary and structural colors, there existed a big
debate on the cause of structural coloration in the early twentieth century. Michelson
[
10
] supported strongly the claim proposed by Walter [
11
] that structural colors are
surface colors
caused from a thin layer of pigments by selective reflection, similar
to those from metallic surfaces. The conclusion was based on the similar behavior
of polarized light when reflected from iridescent structures and from thin films
of aniline dyes. So satisfied with his opinion, he even wrote [
10
] “it is somewhat
surprising to find that the contrary view is still hold by eminent naturalists, and it is
hoped that the further evidence here presented may serve to emphasis the distinction
between 'metallic' or 'surface' colours and the remaining classes of colours (due to
pigment, interference, and diffraction).”
On the opposite side, Lord Rayleigh [
12
] believed that iridescent colors such
as those of peacocks and insects are interference colors from thin films. Many
experimental results defied Michelson's suggestion. For example, in
Micrographia
Hooke described the observation that the iridescent colors of peacock feathers
would be destroyed by water wetting, and “the colours again appear in their former
lustre” by continuous evaporations. Biedermann [
13
] also noticed that the colors
of all iridescent scales of beetles would change after their immersion in liquids,
and if the refractive index of a liquid approaches that of chitin, the colors would
vanish completely. Besides color changes by liquid infiltrations, Mollock [
14
]even
found that the structural colors of scales or feathers could be destroyed by applying
pressure. Merritt [
15
] might be the first to use spectroscopy to characterize structural
colors optically. He measured reflection spectra of pigeon feathers and butterfly
wings under different incident angles, and found that reflection minima shifted
to a short wavelength with increasing incident angles, which could be explained
by thin-film interference. Onslow [
16
]andMason[
17
-
21
] conducted a multitude
of microscopic observations on insect scales and bird feathers, and supported the
interference origin of iridescent colors. With convincing arguments, the opinion that
iridescent colors are of structural origin prevails eventually.
It is known that the spatial resolution of optical microscopes should be smaller
than a half of the visible wavelengths owing to the diffraction limit. As a result,
the relations between structural colors and detailed structural features could be
determined after the invention of electron microscopy in 1930s since many photonic
structures for coloration have submicron feature sizes. With electron microscopy,
Frank and Ruska [
22
] made the first structural observation for the blue feathers of
