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Figure 3.38.: The orientation dependency of emissivity of water, data from VDI
Wärmeatlas [58].
It should be noted that the thermal radiation is filtered by the glass window, and
further selected by the infrared objective. The infrared objective has a spectrum of
3 µm to 5 µm. These bias on wavelength however does not influence the estimation
of the emissivity, because it is taken into account in the calibration.
Reflection
The incident light ray on the air/water interface is partially reflected by the water
surface and partially transmitted into water and eventually absorbed, providing
that the water layer has an infinite depth.
The law of reflection states that the angle of incidence equals the angle of reflection.
θ i = θ f , (3.36)
where i denotes incidence and r denotes reflection.
Snell’s law states that for a given pair of media and a light ray with a single wavelength,
the ratio of the sines of the angle of incidence θ i and angle of refraction θ t is
equivalent to the ratio of phase velocities (v i /v t ) in the two media, or equivalently,
to the opposite ratio of the indices of refraction (n t /n i ):
sinθ i
sinθ t
= n t
n i
, (3.37)
where t denotes transmission. Since the refraction index of air is nearly 1, n is
used here to designate the refractive index of water.
84 3. Experimental Setup