Biomedical Engineering Reference
In-Depth Information
a
c
b
d
Fig. 8.10
Measured and predicted reflection spectra of iridescent neck feathers of domestic
pigeons for unpolarized light at different incident angles. (
a
)and(
b
) Normalized measured results
for green and purple feathers, respectively. (
c
)and(
d
) Predicted results for a thin film with a
thickness of 595 and 530 nm mimicking green and purple barbules, respectively. The converted
RGB colors from the reflection spectra at different incident angles are shown in insets (Reproduced
from [
41
])
plays a role of a poor mirror.
4
As a result, the observed reflectance
R
should result
from the interference in the dorsal cortex layer combined with the reflection from
the poor mirror, namely,
R D R
1
C .1 R
1
/R
2
.1 R
1
/;
(8.6)
where
R
1
is the reflectance of the dorsal cortex alone and
R
2
is the reflectance of
the poor mirror. From experiments, we can determine
R
2
5
% by measuring the
region in a barbule where the dorsal cortex layer is removed.
For a thin film in air, its reflectance is given by [
23
]
4r
2
sin
2
2d
p
n
2
sin
2
=
R
1
D
C 4r
2
sin
2
2d
p
n
2
sin
2
=
;
(8.7)
.1 r
2
/
2
4
The central layer may not completely absorb impinging light, and thus acts as a poor mirror with
a very low reflectivity.