Photonic Structures in the Animal Kingdom: Valuable Inspirations for Biomimetic Applications - Biomimetic Architectures by Plasma Processing - page 11

The BRDF is a four-dimensional function, which furthermore
depends on the light wavelength

− 1

l

; its dimension is sr , that is, the

unit of an inverse spatial angle.
Animals are macro-sized objects with large surfaces and
different topologies and thus, the task of characterising the
photonic structure via a direct and complete measurement of the
BRDF along the entirety of its surface and all spatial directions is
extremely challenging and burdensome. Realistically, experimental
approaches must be appropriately simplified while still generating
sufficient data to characterise the light scattering properties of the
investigated sample.
Recently, an imaging scatterometer (ISM) was developed that
allows rapid collection of BRDF data by imaging the hemispherical
far-field distribution of light scattered from small photonic objects
[19,27,31,32] (Fig. 1.5a). With the ISM, the reflected light flux, that
is, the hemispherical light distribution scattered from the sample
as a function of the reflected angle, L (

w

) = L (

j

,

q

), can be

r out r out out

quantitatively documented with a digital camera.
The core of the ISM is an ellipsoidal mirror, in the first focal
point F of which the sample is placed. The proper positioning of the

sample in the first focal point can be observed through a small, axial
hole, a half mirror and camera C

1

(Fig. 1.5a). White light from a light

1

, Fig. 1.5a) is focused on the sample. The mirror
reflects hemispherically scattered light into the second focal point of
the ellipse F

source (either S or S

1 2

and thus effectively folds the 180

°

reflected light into

2

a narrow cone of approx. 20

°

that is imaged onto the digital camera

C . The resulting images are converted into polar plots with a Matlab

routine that also corrects for optical distortions of the setup [32].
The ISM is a very powerful device in that it allows imaging of
the light scattering properties of photonic samples for any given
geometry (see Fig. 1.5). Figures 1.5d,e shows the iridescence
of the chitin-melanin multilayer of the Japanese Jewel Beetle,
C. fulgidissima

2

(Fig. 1.3c). The iridescence is uniform in all spatial
direction and shows a blue-shift towards larger scattering angles
[27].