Biomedical Engineering Reference
In-Depth Information
colors and associated microstructures, structural characterizations at the submicron
level are needed. To explore the ultimate mechanisms of structural coloration in a
quantitative way, numerical simulations are of great importance.
8.4.1
Optical Observations
With a specimen, the first thing is to determine whether a color is a structural color
or not. If iridescence is observed, it should be a structural color. But this method
cannot be applied to non-iridescent structural colors. Another way is to immerse
the specimen into a liquid aiming to observe the change in color. Commonly used
liquids include water, ethanol, and glycerin. If a color change occurs, the color
should be a structural color. This is because structural coloration is determined by
the spatial arrangement of refractive index. Liquid infiltrations into air voids may
change this arrangement, leading to a change in color. On the contrary, pigmentary
colors do not alter their hue by liquid infiltrations. Structural coloration has an
interesting feature that the reflected and transmitted colors are
complementary
provided that the constituents are transparent. This feature can also be adopted to
distinguish structural and pigmentary coloration.
Optical characterizations of structural colors are conventionally conducted by
optical microscopy under different magnifications. This is because the colored
bodies are sometimes too small to be resolved by our naked eye. In some cases,
a color perceived by us is a mixed color, resulting from differently colored small
bodies which may be resolved with optical microscopy.
For conventional observations, unpolarized light is used. We may also use
polarized light to obtain information on polarization effects. For a photonic structure
covered with an uneven cortex, glares caused by surface reflection and scattering
may hinder our observations. To reduce the glare disturbances, an oil immersion
objective can be used. This is because the oil immersion can reduce or even
eliminate the refractive-index contrast between oil and specimen, as well as
unwanted glares. For certain samples, dark-field microscopy can be used to enhance
the topographic contrast of samples.
8.4.2
Spectral Measurements
Spectral characteristics of natural photonic structures can offer important optical
information on their structural colors. For spectral measurements, angle-resolved
optical spectroscopy is commonly used to obtain angle-dependent reflection, scat-
tering, and even transmission spectra. A typical angle-resolved spectrometer is
schematically shown in Fig.
8.3
.
