Reflection-Polarization Characteristics of Water Surfaces

During dawn and dusk, the sun is near the horizon, and when looking at the water perpendicular to the solar vertical near the Brewster’s angle 53° from the nadir (mirror point of the zenith), the Brewster’s dark patch (BDP) occurs on the water surface. The BDP exists only if the solar elevation angle θS above the horizon is smaller than 58°. Its existence is due to the fact that in the plane perpendicular to the solar vertical, the skylight incident onto the water surface at low solar elevations is vertically polarized, thus leading to a negligible amount of water-reflected radiation when looking at the Brewster’s angle. On certain paintings, the dark region seen on the water surface can be interpreted as a BDP. In the Hungarian Lake Balaton, dark water patches form between every autumn and spring due to the inflow of black organic matter into the bright lake-water. Drone-based imaging polarimetry revealed that these dark lake-patches reflect light with very high (60% ≤ d ≤ 80%) degrees of horizontal polarization at the Brewster’s angle, while the bright lake-water is only weakly (d < 20%) horizontally polarizing. These dark water patches attract water-seeking polarotactic insects, which oviposit more frequently in them than in the brighter lake-water. With larval samplings, it was showed that both the density and the average size of chironomid larvae were significantly larger in a dark lake patch than in the surrounding bright lake-water. The main drawback of sequential imaging polarimetry (taking the necessary 3–4 polarization pictures through polarizers in succession) is that during exposure the target/illumination must not move/change, otherwise motion/change artefacts are caused after evaluation of the polarization pictures. However, taking the polarization pictures with a long enough exposure, the changes of the moving/changing target can be averaged, and thus motion/change artefacts are reduced when the motion has a stable mean. Although simultaneous polarimeters (taking the polarization pictures at once) do not suffer from motion/change artefacts and are available on the market, long enough exposures can efficiently minimize polarization blurs of sequential imaging polarimetry while in use.