Biomedical Engineering Reference
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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.
 
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