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80
60
Metallic
40
20
Semiconducting
(a)
0
Semiconducting
80
60
40
20
Metallic
(b)
0
1
Solar irradiance
1. 5
1
0.5
Relative luminous
efficiency of the eye
0.5
(c)
0
0
0
500
1000 1500
Wavelength (nm)
2000
2500
11.6 Spectral refl ectance (upper panel) and transmittance (middle
panel) for a 0.05-
￿ ￿ ￿ ￿ ￿ ￿
μ
m-thick thermochromic VO 2 fi lm in semiconducting
c ) states. The lower panel illustrates typical
spectra for the luminous effi ciency of the human eye and for solar
irradiance. After Mlyuka et al. (2009a).
(
τ
<
τ
c ) and metallic (
τ
>
τ
() =
() (
)
()
T
τ
d
λ
ϕ
λ
T
λ τ
,
d
λ
ϕ
λ
.
[11.1]
lum sol
,
lum sol
,
lum sol
,
This equation was used to compute data, shown in Fig. 11.7, on T lum (
τ
, t )
and T sol (
, t ) where t is the thickness of the VO 2 fi lm (Li et al. , 2010). These
computations used standard formulas for thin-fi lm optics (Born and Wolf,
1999) together with optical constants n and k of VO 2 as evaluated before
(Mlyuka et al. , 2009b). The discussion below will also make use of the cor-
responding dielectric constant, denoted
τ
ε
=
ε 1
+
i
ε 2 , which is related to the
n 2 - k 2 and
optical constants via
ε 1
=
ε 2
=
2 nk .
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