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Figure 3.41.: Orientation dependence of Reflection coefficients R s and R p , n =1.2.
Figure 3.42.: Orientation dependence of Reflection coefficient, n =1.2.
For angle of incidence smaller than the Brewster angle, the reflection coefficient
is roughly 0.8% which induces error of 0.5 K, assuming the surrounding has the
same temperature as the water drop. A surrounding of a lower temperature helps
on further suppressing the error.
The reflection coefficient is wavelength and temperature dependent, because of
the refractive index. The wavelength dependency of refractive index of water at
room temperature is given by Downing and Williams in 1975 [38]. The data for
water at different temperatures, including the supercooled water, are given by Zasetsky
et al. [174] in 2005, as the diagrams in Figure 3.43 show.
The covered wavelength ranges from 2.5 µm to 25 µm as the spectroscopic
wavenumber ˜ν = 1/λ indicates. The refractive index has an irregular dependency
3.4. Measurement of the Water Drop Temperature 87