µ¦¦³»
¤ª·
µµ¦Â¨³Á°¨µª·
´
¥ ¤®µª·
¥µ¨´
¥´
¬·
¦´Ê
¸É
19 ¦³Î
µe
2552
6
Table 2 Optical constants (
n
and
k
) of Ag-alloy and TiO
2
films at different wavelengths evaluated from TFCalc
Wavelength (nm)
Ag-alloy thin films
TiO
2
thin films
n
k
n
k
250
0.100
7.00
2.90
8.00
300
0.100
4.40
2.80
2.40
350
0.100
1.65
2.73
0.28
400
0.075
2.10
2.55
0.00
450
0.055
2.53
2.47
0.00
500
0.050
2.92
2.38
0.00
550
0.055
3.30
2.34
0.00
600
0.060
3.65
2.32
0.00
650
0.070
3.99
2.32
0.00
750
0.080
4.72
2.38
0.00
The optical constants (
n
and
k
) of Ag-alloy thin films and TiO
2
thin films in the wavelength from 250 to
750 nm were evaluate from TFCalc show in table 2. The optical constant of Ag-alloy thin film at 500 nm are
n
= 0.05 and
k
= 2.92 which close to the value of pure-Ag (Johnson and Christy, 1972), while TiO
2
thin film at
500 nm are
n
= 2.38 and
k
= 0.
Feasibility study of Ag-alloy based heat mirror
An ideal heat mirror would be such that it allows visible energy to be transmitted through the window,
while the majority of all the IR including that of the sun is reflected away from the window. It means that it has
transmittance (
T
) and reflectance (
R
) to be given by
T
=1 and
R
=0 for wavelength = 400 to 700 nm (visible region),
and
T
=0 and
R
=1 for wavelength >700 nm (IR region).
D/M/D or D/A/D films on a glass substrate were use as a spectrally selective filter that reflects IR
radiation (due to the properties of the metal or alloy layer) and transmits most of the visible spectrum. The highly
reflective metal or alloy thin film, that otherwise transmits very little energy in the visible, was sandwiched
between the two dielectric layers that act as antireflective coatings such as to enhance the energy transmitted in the
visible region. Thus the visible transmittance was chosen to be the criteria parameter for a given metal or alloy
thickness, which in turn controls the IR reflectance.
The heat mirror thin film arrangement was taken to be: air/D/A/D/glass (Fig. 2). The two dielectric
layers are taken as to be of the same material; in this work, we use TiO
2
as dielectric film. The simulations were
performed for normal incidence using the TFCalc software. The effects of Ag-alloy layer’s thickness on
transmittance were simulated at a design wavelength of 550 nm and the substrate was taken to be glass (
n
= 1.5).
The optical constants of Ag-alloy and TiO
2
thin film for use in simulation are taking from table 2
