József Gál

Polarization patterns of the summer sky and its neutral points measured by full–sky imaging polarimetry in Finnish Lapland north of the Arctic Circle

Using 180° field–of–view (full–sky) imaging polarimetry, the patterns of the degree and angle of polarization of the entire summer sky were measured on 25 June 1999 at a location north of the Arctic Circle in Finnish Lapland as a function of the angular solar zenith distance. A detailed description of the used full–sky imaging polarimeter and its calibration is given. A series of the degree and angle of polarization pattern of the full sky is presented in the form of high–resolution circular maps measured in the blue (450 nm) spectral range as a function of the solar zenith distance. Graphs of the spectral dependence of the degree and angle of polarization of skylight at 90° from the Sun along the antisolar meridian are shown. The celestial regions of negative polarization and the consequence of the existence of this anomalous polarization, the neutral points, are visualized. The measured values of the angular zenith distance of the Arago and Babinet neutral points are presented as a function of the zenith distance of the Sun for the red (650 nm), green (550 nm) and blue (450 nm) ranges of the spectrum. The major aim of this work is to give a clear and comprehensive picture, with the help of full–sky imaging polarimetry, of what is going on in the entire polarized skydome. We demonstrate how variable the degree of polarization of skylight and the position of the neutral points can be within 24 h on a sunny, almost cloudless, visually clear day.

Polarization of the moonlit clear night sky measured by full-sky imaging polarimetry at full Moon: Comparison of the polarization of moonlit and sunlit skies

Using 180° field of view (full-sky) imaging polarimetry, the patterns of the degree and angle of polarization of the moonlit clear night sky were measured in every half an hour throughout the night at full Moon. The patterns were compared with those of the sunlit sky. The observed patterns including the positions of the Arago and Babinet neutral points of the moonlit night sky and sunlit day sky are practically identical if the zenith angle of the Moon is the same as that of the sun. The possible biological relevance of the polarization pattern of the moonlit night sky in the polarization vision and orientation of night-active insects is briefly discussed.

Why is it worth flying at dusk for aquatic insects? Polarotactic water detection is easiest at low solar elevations

SUMMARY Using 180° field-of-view imaging polarimetry, we measured the reflection-polarization patterns of two artificial surfaces (water-dummies) in the red, green and blue spectral ranges under clear and partly cloudy skies at different solar elevations. The dummies consisted of a horizontal glass pane with a matt black or matt light grey cloth underneath, imitating a dark or bright water body, respectively. Assuming that polarotactic water insects interpret a surface as representing water if the degree of linear polarization of reflected light is higher than a threshold and the deviation of the direction of polarization from the horizontal is lower than a threshold, we calculated the proportion, P, of the artificial surfaces detected polarotactically as water. We found that at sunrise and sunset P is maximal for both water-dummies and their reflection-polarizational characteristics are most similar. From this, we conclude that polarotactic water detection is easiest at low solar elevations, because the risk that a polarotactic insect will be unable to recognize the surface of a dark or bright water body is minimal. This partly explains why many aquatic insect species usually fly en masse at dusk. The daily change in the reflection-polarization pattern of water surfaces is an important visual ecological factor that may contribute to the preference of the twilight period for habitat searching by polarotactic water insects. Air temperature at sunrise is generally low, so dusk is the optimal period for polarotactic aquatic insects to seek new habitats.

Polarization Portrait of the Arago Point: Video-polarimetric Imaging of the Neutral Points of Skylight Polarization

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Image formation by bifocal lenses in a trilobite eye

In this work we report on a unique and ancient type of eye, in which the lower surface of the upper calcite lens units possessed an enigmatic central bulge making the dioptric apparatus similar to a bifocal lens. This eye belonged to the trilobite Dalmanitina socialis, which became extinct several hundred million years ago. As far as we know, image formation by bifocal lenses of this kind did/does not occur in any other ancient or modern animal visual system. We suggest that the function of these bifocal lenses may be to enable the trilobite to see simultaneously both very near (e.g. floating food particles and tiny preys) and far (e.g. sea floor, conspecifics, or approaching enemies) in the optical environment through the central and peripheral lens region, respectively. This was the only reasonable function we could find to explain the puzzling lens shape. We admit that it is not clear whether bifocality was necessary for the animal studied. We show that the misleading and accidental resemblance of an erroneous correcting lens surface (designed by René DesCartes in 1637 [DesCartes, R. (1637). Oeuvres de DesCartes. La Géometrie. Livre 2. pp. 134. J. Maire, Leyden] to the correcting interface in the compound Dalmanitina lens may be the reason why the earlier students of the Dalmanitina lens did not recognize its possible bifocality.

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

Abstract 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 Horvath [J Theor Biol 175 (1995) 27.] and Schwind and Horvath [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 Horvath (1995) and Schwind and Horvath (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.

Why Are Water-Seeking Insects Not Attracted by Mirages? The Polarization Pattern of Mirages

Naturwissenschaften 85 (1998)

Polarized light and oviposition site selection in the yellow fever mosquito: no evidence for positive polarotaxis in Aedes aegypti.

Aquatic insects and insects associated with water use horizontally polarized light (i.e., positive polarotaxis) to detect potential aquatic or moist oviposition sites. Mosquitoes lay their eggs onto wet substrata, in water, water-filled tree/rock holes, or man-made small containers/bottles/old tyres containing water. Until now it has remained unknown whether mosquitoes are polarotactic or not. The knowledge how mosquitoes locate water would be important to develop new control measures against them. Thus, we studied in dual-choice laboratory experiments the role of horizontally polarized light in the selection of oviposition sites in blood-fed, gravid females of the yellow fever mosquito, Aedes aegypti. On the basis of our results we propose that Ae. aegypti is not polarotactic. Thus the yellow fever mosquito is the first known water-associated insect species that does not detect water by means of the horizontally polarized water-reflected light. This can be explained by the reflection-polarization characteristics of small-volume water-filled cavities/containers preferred by Ae. aegypti as oviposition sites.

Neutral points of skylight polarization observed during the total eclipse on 11 August 1999

We report here on the observation of unpolarized (neutral) points in the sky during the total solar eclipse on 11 August 1999. Near the zenith a neutral point was observed at 450 nm at two different points of time during totality. Around this celestial point the distribution of the angle of polarization was heterogeneous: The electric field vectors on the one side were approximately perpendicular to those on the other side. At another moment of totality, near the zenith a local minimum of the degree of linear polarization occurred at 550 nm. Near the antisolar meridian, at a low elevation another two neutral points occurred at 450 nm at a certain moment during totality. Approximately at the position of these neutral points, at another moment of totality a local minimum of the degree of polarization occurred at 550 nm, whereas at 450 nm a neutral point was observed, around which the angle-of-polarization pattern was homogeneous: The electric field vectors were approximately horizontal on both sides of the neutral point.

Fine structure of the celestial polarization pattern and its temporal change during the total solar eclipse of 11 August 1999

Abstract Using 180° field-of-view (full-sky) imaging polarimetry, we measured the spatiotemporal change of the polarization pattern of the entire celestial hemisphere during the total solar eclipse of 11 August 1999 in Kecel, Hungary. We compared these patterns with the normal celestial polarization patterns measured at the same times on the subsequent day of the eclipse. As a second control sky, the celestial polarization pattern measured on 26 August 1999 in Tunisia was chosen with the same solar zenith distance as that at the Hungarian eclipse. We computed the corresponding theoretical celestial polarization patterns on the basis of the single-scattering Rayleigh model. The spectral characteristics of the polarization pattern in the sky during totality were also measured in the red (650 nm), green (550 nm), and blue (450 nm) ranges of the spectrum. A qualitative explanation was given for the origin of the angle of polarization ( E -vector) pattern and the neutral point of skylight polarization near the zenith observed during totality. The relation of our results to earlier observations on skylight polarization during total eclipses was analyzed. The agreements with previous eclipse observations were discussed. The reasons for some disagreements with previous eclipse observations were explained in connection with the spectral dependence of skylight polarization and the fine structure of the celestial E -vector pattern during totality.