Chemistry Reference
In-Depth Information
A multilayer with a high
Q-
factor reflects light only in a narrow
wavelength range.
1.3.3 Iridescence
As noted before, a key feature of photonic structures is iridescence,
that is, the colour of the photonic structure changes with the
viewing direction. Mathematically speaking, a photonic structure is
thus defined by a function that describes the local light scattering
properties of the structure, given the angle of light incidence and
light detection. The function connecting the incident irradiance
E
,
i
incoming from the spatial angle
w
, with the radiance
L
, reflected
in
r
into the spatial angle,
(Fig. 1.4) is the bidirectional reflectance
distribution function (BRDF) [30-32].
w
in
dL
dE
( )
()
w
w
ww
wq
dL
(
)
r
out
outr out
f
l
ww
(,
)
=
=
(1.4)
in
out
w
d
cos
E
(
in
)
i
in
in
in i
Figure 1.4
The BRDF. The BRDF relates the reflected light
L
(
w
) into
r
out
w
w
the spatial angle
to the light flux
E
(
) incident from the
out
i
in
w
w
spatial angle
. The spatial angles,
are parameterised
in
in/out
q
j
in polar coordinates
with indices i and r for the
directions of incidence and reflection. Adapted from Ref. [31].
and
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