Imaging polarimetry of forest canopies: how the azimuth direction of the sun, occluded by vegetation, can be assessed from the polarization pattern of the sunlit foliage

Abstract

Radiance, color, and polarization of the light in forests combine to create complex optical patterns. Earlier sporadic polarimetric studies in forests were limited by the narrow fields of view of the polarimeters used in such studies. Since polarization patterns in the entire upper hemisphere of the visual environment of forests could be important for forest-inhabiting animals that make use of linearly polarized light for orientation, we measured 180 degrees field-of-view polarization distributions in Finnish forests. From a hot air balloon we also measured the polarization patterns of Hungarian grasslands lit by the rising sun. We found that the pattern of the angle of polarization alpha of sunlit grasslands and sunlit tree canopies was qualitatively the same as that of the sky. We show here that contrary to an earlier assumption, the alpha-pattern characteristic of the sky always remains visible underneath overhead vegetation, independently of the solar elevation and the sky conditions (clear or partly cloudy with visible sun's disc), provided the foliage is sunlit and not only when large patches of the clear sky are visible through the vegetation. Since the mirror symmetry axis of the alpha-pattern of the sunlit foliage is the solar-antisolar meridian, the azimuth direction of the sun, occluded by vegetation, can be assessed in forests from this polarization pattern. Possible consequences of this robust polarization feature of the optical environment in forests are briefly discussed with regard to polarization-based animal navigation.

Bio-Inspired Polarized Skylight-Based Navigation Sensors: A Review

“…The qualitatively robust pattern of polarized skylight direction could be obtained under any condition and even in situations when the sun was not visible [114], such as under canopy and foliage [115], and during overcasts and heavy haze. This is because only a small section of clear sky is sufficient for the animals to obtain a compass bearing for accurate navigation [116].…”

Polarization Characteristics of Forest Canopies with Biological Implications

“…Using imaging polarimetry, Shashar et al (1998) studied the linear polarization of light in a tropical rain forest. Since polarization patterns in the entire upper hemisphere of the visual environment of forests could be important for forest-inhabiting animals that make use of polarization patterns for orientation, Hegedüs et al (2007a) measured the 180 field-of-view polarization patterns of the overhead foliage in a variety of forest types by full-sky imaging polarimetry. They characterized some distinct light environments in the forest, each having a typical linearly polarized light field.…”

Light-Trap Catch of Turnip Moth (Agrotis segetum Denis et Schiffermüller, 1775) in Connection with the Night Sky Polarization Phenomena

“…The degree of polarization also depends on the atmospheric conditions. In cloudy and foggy skies, as well as under canopies the degree of polarisation are much smaller compared to clear skies [6][7][8]. However, the direction of polarization pattern is very robust, even under forest canopies, where light scattering occur only in a very thin air layer under the foliage, the typical 8-shaped pattern as well as the axis of symmetry is well recognizable.…”