Fig 1.
Optical characteristics of photovoltaic solar panels.
A) Dark photovoltaic modules coated by a reflecting planar cover layer act as polarization traps for polarotactic insects (left) if the photovoltaic-reflected light is partially or completely horizontally polarized. An appropriate texturing of the cover layer strongly reduces polarized light pollution and improves sunlight-harvesting (right). B-C) Scanning electron microscope images of the rose petal replicated cover layer analyzed herein, and incorporating the microtexture of rose petals into a polymeric material (PMMA). Its measured surface reflectance spectrum is shown for both a blackened rear side (D) and for a Cu(In,Ga)Se2 (CIGS) thin-film solar cell coupled to it (E). An untextured (”planar”) cover layer is used as a reference and both normal and highly oblique angle of incidences are considered. The coloured areas surrounding the (solid or dotted) curves indicate the standard deviation over N = 4 individual measurements.
Fig 2.
Imaging polarimetry of the test surfaces.
Polarization patterns of 10 cm × 10 cm CIGS solar modules equipped with various cover layers under clear sky and for different observer viewing directions (columns): 1) Rose petal, 2) artificial microlens array, 3) planar PMMA, 4) no coating. For the cases of sun shining from in front of the observer, from behind the observer, from the left and from the right, three types of images are displayed: Colour photograph (top row), as well as degree of linear polarization d (middle row) and angle of polarization α (bottom row) at 450 nm (blue). In the middle row, the numerical values are the degrees of polarization averaged for the different test surfaces. The white bars in the bottom row show the local average direction of polarization. The tilt angle of the polarimeter’s optical axis was set to -35° from the horizontal.
Fig 3.
SEM images of the commercial microlens array foils.
We remind the reader that this is the artificial microlens array (MLA) foil that was included in the measurements of reflection-polarization characteristics of the planar and rose petal textured (PMMA) layers (see Fig 2).
Fig 4.
Photographs of the three test surfaces (RP: Rose petal, GRP: Glass-covered rose petal, SBP: Smooth black plastic) used in the field experiments with horseflies, and corresponding patterns of the degree and angle of polarization.
These patterns were measured with imaging polarimetry in the blue (450 nm) spectral range when the sun shone A) from in front of the observer, B) from behind the observer, C) from the left and D) from the right, as light from the clear sky was reflected from the test surfaces. The tilt angle of the optical axis was -35° from the horizontal. In the angle of polarization patterns, the white bars show the average directions of polarization of the test surfaces.
Fig 5.
Arrangements and results of the field experiments with mayflies and horseflies.
The experimental site of the field experiments with mayflies (A) and horseflies (C), including the arrangement of the three test surfaces, and the average daily numbers of landings (B, D) with the results of the Wilcoxon signed-rank test. RP: rose petal, GRP: glass-covered rose petal, SBP: smooth black plastic, N: number of observations per category, n.s.: not significant with p > 0.05, *: 0.001 < p < 0.05, **: 0.0001 < p < 0.001, ***: p < 0.0001.
Fig 6.
Photographs and patterns of the degree and angle of polarization of the three test surfaces (RP: Rose petal, GRP: Glass-covered rose petal, SBP: Smooth black plastic) laid on a dry asphalt road in the field experiments with mayflies measured with imaging polarimetry at 450 nm (blue).
In A), the polarimeter’s optical axis pointed toward East, approximately parallel to the antisolar meridian, when light from the clear sky was reflected off the test surfaces. In B), the polarimeter’s optical axis pointed toward South, approximately perpendicular to the antisolar meridian, when light from the forest vegetation was reflected off the test surfaces. The tilt angle of the optical axis in both cases was -35° from the horizontal. The d-values are given in the degree of polarization patterns averaged for the whole test surface. In the angle of polarization patterns, the white bars show the average directions of polarization of the test surfaces.
Fig 7.
Illustration of a ray-tracing-based simulation approach for studying the properties of light reflected from densely packed and disordered microtextures.
A) Definition of the spherical coordinate system for characterizing the observer’s viewing directions in the farfield. The planar rectangular and transparent light source is marked by an orange rectangle. It emitted 108 parallel and unpolarized rays of identical initial power with random starting positions into the central part of the microtexture models. B) At every observer viewing direction (φ, θ), the incoming rays’ (exemplary ray paths are drawn as faint orange lines) propagation direction k defines the z-axis of a local coordinate system that is used for measuring the angle of polarization (noted AOP in the figure). The y-axis is chosen parallel to the local φ = constant line. The angle of polarization of the electric field vector of light represented by an orange double-headed arrow is then always measured relative to the local x-axis (mathematically positive). The array of parallel orange lines under the orange rectangle represents parallel sun rays arriving from an intermediate elevation near an azimuth of 180°. C)-E) depict exemplary microtexture models (cone’s aspect ratio = 0.6, full tiling of the base) illustrating three extreme cases, namely the disorder-free configuration (C), the maximum degree of height disorder (D) and the maximum disorder in the cones’ horizontal position (E).
Fig 8.
Numerical assessment of the influence of structural disorder on the polarized light pollution caused by microtextured surfaces.
For aspect ratio AR = 0.3 (A), 0.6 (B), and 1 (C), the two characteristic solid angle ratios Sillum/2π and Sattract/Sillum are displayed as a function of the angle of incidence AoI. D) Depicts the solid angle fraction Sattract/2π versus AoI for which horseflies and mayflies would detect the reflecting surface as a water surface with respect to the entire hemisphere of possible observer viewing directions for aspect ratio AR = 0.6. The shaded area in panel D indicates the full range of numerical results that were found when ramping up both height and position disorder.