Measurement of the reflection–polarization pattern of the flat water surface under a clear sky at sunset

Using 180° field of view imaging polarimetry, we measured the polarization characteristics (degree and angle of linear polarization) of the entire clear sunset sky and the reflection–polarization pattern of the flat water surface under this sky in order to test the validity of the earlier theoretical predictions made by Horváth [J Theor Biol 175 (1995) 27.] and Schwind and Horváth [Naturwissenschaften 80 (1993) 82.] on the fine structure of this reflection–polarization pattern. We compared the measured reflection–polarization pattern with the theoretical reflection–polarization patterns computed for single-scattering Rayleigh skylight and measured real skylight reflected from the flat water surface, the repolarization characteristics of which are described by the Fresnel theory. Analysing and comparing the theoretical and measured reflection–polarization patterns, we could establish that the earlier predictions of Horváth (1995) and Schwind and Horváth (1993) were correct. Contrary to other full-sky polarimeters, our 180° field of view rotating-analyzer imaging photopolarimeter is useful not only for sky measurements, but also for downward viewing polarimetry. It can be used in atmospheric optics as well as in biological investigations involving animal polarization vision and orientation on the basis of skylight polarization or water-surface-reflected polarized light.