Biomedical Engineering Reference
In-Depth Information
Fig. 8.20
Measured relative emergence intensity for an elytron of the male beetle
C. obscuripennis
as a function of wavelength and emergence angle at normal incidence. The emergence angle is
defined as the angle of reflected light with respect to the elytral surface normal. The intensity
of non-specular reflection is smaller than that of specular reflection. For normal incidence, non-
specular reflection occurs mainly at emergence angles that are smaller than 20
ı
. The intensity of
non-specular reflection decreases with increasing emergence angle. At small emergence angles,
non-specular reflection is dominant at wavelengths around 520 nm, but at large emergence angels
it is dominant at slightly lower wavelengths (Reproduced from [
96
])
the elytral structural coloration. The layer with low electron density is chitin. By
chemical treatments [
93
], the layer with high electron density was determined to be
melanoprotein. The average thickness of both chitin and melanoprotein layers shows
distinct differences in different regions. The average thickness of the chitin layer is
about 90, 85, and 78 nm at the ridge, the inclined side, and the basin, respectively.
For the melanoprotein layer its average thickness in different regions is nearly the
same, about 66 nm.
Sculpted multilayers can produce optical effects different from flat ones. In flat
multilayers, only specular reflection is expected, leading to conspicuous structural
coloration. In contrast, both specular and non-specular reflections exist in sculpted
multilayers. Owing to the surface profile, the effective area for specular reflection
in a sculpted multilayer is much smaller than that in a flat multilayer. This will lead
to inconspicuous structural coloration. In addition to specular reflection, a sculpted
multilayer can also render non-specular reflection, giving rise to light emergence at
directions of non-specular reflection, as shown in Fig.
8.20
. A resulting consequence
is that structural coloration can be perceived in a much wider range of viewing
angles. This feature may be of significance in signal communication and conspecific
recognition.
