Sky Polarization Pattern Research Articles

Desert ants do not rely on sky compass information for the perception of inclined path segments

On their foraging excursions, desert ants (Cataglyphis fortis) navigate by means of path integration, exhibiting high precision even in undulating terrain. To avoid errors in their home vector the ants must measure the slopes of ascents and descents in their itinerary. This information is necessary to convert the distances actually walked on the slopes into the ground distance, which is the crucial parameter for vector navigation. However, it is as yet unknown how Cataglyphis perceives the slopes of inclines. We asked whether the ants gauge slopes by means of the changes in the sky's polarization pattern or the sun's position imposed by the unusual head posture when walking on inclines. During training on a ramp, we manipulated the sky compass cues available for the ants. If both sky compass cues, polarization pattern and sun position, were excluded during ramp training, the ants behaved in subsequent tests as if they had never encountered a slope. However, the presence of either of the two compass cues was sufficient for the ants to memorize the sloped path segment. In a third experiment, the supposed polarization cue was excluded by covering the training and test channels with a linear polarization filter. In these tests the ants clearly showed that they had perceived and memorized an ascent also without a change in the polarization pattern. Hence, changes in polarization perception can be ruled out as the crucial cue for gauging slopes, and we conclude that slopes are monitored by a, still unknown, proprioceptive mechanism.

Estimating Sunlight Polarization Using a Fish-eye Lens

In outdoor scenes, polarization of the sky provides a significant clue to understanding the environment. The polarized state of light conveys the information for obtaining the orientation of the sun. Robot navigation, sensor planning, and many other application areas benefit from using this navigation mechanism. Unlike previous investigations, we analyze sky polarization patterns when the fish-eye lens is not vertical, since a camera in a general position is effective in analyzing outdoor measurements. We have tilted the measurement system based on a fish-eye lens, a CCD camera, and a linear polarizer, in order to analyze transition of the 180-degree sky polarization patterns while tilting. We also compared our results measured under overcast skies with the corresponding celestial polarization patterns calculated using the single-scattering Rayleigh model.

Ultrastructure and orientation of ommatidia in the dorsal rim area of the locust compound eye

In many insect species, a dorsal rim area (DRA) in the compound eye is adapted to analyze the sky polarization pattern for compass orientation. In the desert locust Schistocerca gregaria, these specializations are particularly striking. The DRA of the locust consists of about 400 ommatidia. The facets have an irregular shape, and pore canals are often present in the corneae. Screening pigment is missing in the region of the dioptric apparatus suggesting large receptive fields. The rhabdoms are shorter, but about four times larger in cross-section than the rhabdoms of ordinary ommatida. Eight retinula cells contribute to the rhabdom. The microvilli of retinula cell 7 and of cells 1, 2, 5, 6, 8 are highly aligned throughout the rhabdom and form two blocks of orthogonal orientation. The microvilli in the minute rhabdomeres of retinula cells 3 and 4, in contrast, show no particular alignment. As in other insect species, microvillar orientations are arranged in a fan-like pattern across the DRA. Photoreceptor axons project to distinct areas in the dorsal lamina and medulla. The morphological specializations in the DRA of the locust eye most likely maximize the polarization sensitivity and suggest that the locust uses this eye region for analysis of the sky polarization pattern.

How the clear-sky angle of polarization pattern continues underneath clouds: full-sky measurements and implications for animal orientation.

One of the biologically most important parameters of the cloudy sky is the proportion P of the celestial polarization pattern available for use in animal navigation. We evaluated this parameter by measuring the polarization patterns of clear and cloudy skies using 180 degrees (full-sky) imaging polarimetry in the red (650 nm), green (550 nm) and blue (450 nm) ranges of the spectrum under clear and partly cloudy conditions. The resulting data were compared with the corresponding celestial polarization patterns calculated using the single-scattering Rayleigh model. We show convincingly that the pattern of the angle of polarization (e-vectors) in a clear sky continues underneath clouds if regions of the clouds and parts of the airspace between the clouds and the earth surface (being shady at the position of the observer) are directly lit by the sun. The scattering and polarization of direct sunlight on the cloud particles and in the air columns underneath the clouds result in the same e-vector pattern as that present in clear sky. This phenomenon can be exploited for animal navigation if the degree of polarization is higher than the perceptual threshold of the visual system, because the angle rather than the degree of polarization is the most important optical cue used in the polarization compass. Hence, the clouds reduce the extent of sky polarization pattern that is useful for animal orientation much less than has hitherto been assumed. We further demonstrate quantitatively that the shorter the wavelength, the greater the proportion of celestial polarization that can be used by animals under cloudy-sky conditions. As has already been suggested by others, this phenomenon may solve the ultraviolet paradox of polarization vision in insects such as hymenopterans and dipterans. The present study extends previous findings by using the technique of 180 degrees imaging polarimetry to measure and analyse celestial polarization patterns.

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.

Reflection-Polarization pattern at water surfaces and correction of a common representation of the polarization pattern of the sky

Many hydrophilous insects recognize water by the polarization of the reflected light [1]. When the water is illuminated by diffuse, nonpolarized sky light, the animals have a relatively simple polarization pattern to identify: all the e vectors in the reflected light are horizontal. But when the sky is clear and visibility good, very complicated polarization patterns can develop due to superposition of the polarization characteristics of the water surface and the partially reflected polarization pattern of the sky light. We have calculated such patterns for an ideal water surface, and present them here. We also present a revised picture of the polarization of sky light. The drawing of the sky polarization pattern published in 1959 by Stockhammer [2], which has been reproduced in many review articles and textbooks (e.g. [3-6]), and the two-dimensional representation of the sky polarization pattern published by Wehner [7,8] contain an error in the directions of the e vectors. Here we correct this error. For the calculations of reflection polarization the sky polarization pattern was slightly idealized; the lines of equal polarization were assumed to form exact circles around the position of the sun, as in Wehner's [9] three-dimensional polarization pattern, and the e vectors were assumed to be exactly aligned with these circles. Departures from this regular pattern in the vicinity of the sun (Babinet and Brewster neutral point) were disregarded. These departures are negligible because here the stronger polarization of the water surface predominates. The degree of polarization was approximated by d = dmax-Sin 2 cd(l+cos2c0 (see [10], p. 288; a is the angular distance from the sun). For sun elevations of 0 ° (sun at the horizon), 30 ° , 60 ° , and 90 ° , dma~ was taken to be 77, 70, 63, and 56%, respectively (see [10], p. 295). To calculate the reflection of the sky polarization pattern the appropriate Fresnel formulas were used ([11], p. 332). With an ideal water surface under diffuse, unpolarized sky light, all the e vectors in the reflection-polarization pattern are arranged concentrically around the center of the pattern; that is, from the viewpoint of an observer they appear horizontal (Fig. la). Polarization is maximal at the Brewster angle 0 = 5 3 ° (dotted line representing a degree of polarization d=100 %). The relative intensity of the reflected light is not shown in the figures. For unpolarized incident light it is 2% at ®=0 ° (in the center of the pattern) and 7.6 % at the Brewster angle; the amount of light reflected rises rapidly for ® > 60 °, reaching 100 % at ®=90 ° (at the edge of the pattern). With a clear, cloudless sky the polarization pattern visible over the water surface is considerably more complicated, because the polarization pattern of the sky itself contributes, to some extent,