Reflection–Polarization Characteristics of Greenhouses Studied by Drone-Polarimetry Focusing on Polarized Light Pollution of Glass Surfaces

Drone-based imaging polarimetry is a valuable new tool for the remote sensing of the polarization characteristics of the Earth’s surface. After briefly reviewing two earlier drone-polarimetric studies, we present here the results of our drone-polarimetric campaigns, in which we measured the reflection–polarization patterns of greenhouses. From the measured patterns of the degree and angle of linear polarization of reflected light, we calculated the measure (plp) of polarized light pollution of glass surfaces. The knowledge of polarized light pollution is important for aquatic insect ecology, since polarotactic aquatic insects are the endangered victims of artificial horizontally polarized light sources. We found that the so-called Palm House of a botanical garden has only a low polarized light pollution, 3.6% ≤ plp ≤ 13.7%, while the greenhouses with tilted roofs are strongly polarized-light-polluting, with 24.8% ≤ plp ≤ 40.4%. Similarly, other tilted-roofed greenhouses contain very high polarized light pollution, plp ≤ 76.7%. Under overcast skies, the polarization patterns and plp values of greenhouses practically only depend on the direction of view relative to the glass surfaces, as the rotationally invariant diffuse cloud light is the only light source. However, under cloudless skies, the polarization patterns of glass surfaces significantly depend on the azimuth direction of view and its angle relative to the solar meridian because, in this case, sunlight is the dominant light source, rather than the sky. In the case of a given direction of view, those glass surfaces are the strongest polarized-light-polluting, from which sunlight and/or skylight is reflected at or near Brewster’s angle in a nearly vertical plane, i.e., with directions of polarization close to horizontal. Therefore, the plp value is usually greatest when the sun shines directly or from behind. The plp value of greenhouses is always the smallest in the green spectral range due to the green plants under the glass.