Fig. 5. (A) Polarization pattern of the sky for two elevations of the sun (filled circle; left, 55°; right, 5°). In this two-dimensional representation of the polarization pattern, the concentric circles indicate parallels of latitude of the celestial hemisphere with the zenith at the centre. e-vector orientations (indicated by the bars) are plotted with respect to the tangent to the parallel of latitude at the indicated positions (compare Fig.2a in) (Schwind and Horváth, 1993). In this representation of the polarization pattern, e-vector orientations can be read directly from the graph; specifically, e-vectors that appear parallel in the sky also appear parallel in the graph. The degree of polarization is indicated by the length and width of the bars. Note that e-vector orientation in this figure is mapped the same way as _max in Fig.1B. (B–D) Responses of an opto-electronic model POL1-neurone to natural skylight polarization. Three examples for different cloud conditions are shown. Left, 180° fisheye photographs indicating actual sky conditions; (B) a loosely cloud-scattered sky, (C) an asymmetrically clouded sky and (D) a heavily clouded sky. The zenith is at the centre (see Z in B), and the horizon is at the circumference of the photographs. The black disks in the photograph are sun screens that shade the photograph and the model neurone from direct sun light. The small disk indicates the position of the sun, and the thin line extending from below the disk gives the symmetry line of the polarization pattern (compare with A). The circle in B indicates the outline of the 60° visual field of the model. The bright 8-shaped area near the antisolar horizon in C is caused by reflections of sunlight in the camera lens. Right, response curves of the model to the skies on the left obtained by scanning the sky through 360°. Ordinate, response in dimensionless logarithmic units (solid line) (see Labhart, 1999); abscissa, orientation of the model with respect to the polarization pattern. Dotted line in D shows the response on a larger scale (right ordinate) Since the model is tuned to horizontal e-vector orientations, the response maxima are indicators of the solar (0°) and the antisolar (180°) azimuth (compare with A). The response curves show that, even under cloudy skies, the maxima deviate only little from the solar or antisolar azimuth. (E) Performance of the model POL1-neurone with a large (60°) and a small (15° or 7.5°) visual field. The histograms indicate the deviation (error) of the antisolar response maximum from the antisolar azimuth under clear (upper graphs) and cloudy (lower graphs) skies. The ordinate indicates the number of occurrences (measurements). Note the inferior directional performance (larger errors) with small visual fields (compare the left and right lower graphs). The effective degree of polarization was 10% for this comparative study. (B-E were compiled from data in Labhart, 1999)