Pál Barta

Bio: Pál Barta is an academic researcher. The author has contributed to research in topics: Linear polarization & Rayleigh sky model. The author has an hindex of 2, co-authored 2 publications receiving 32 citations.

Polarization transition between sunlit and moonlit skies with possible implications for animal orientation and Viking navigation: anomalous celestial twilight polarization at partial moon.

Abstract: Using full-sky imaging polarimetry, we measured the celestial distribution of polarization during sunset and sunrise at partial (78% and 72%) and full (100%) moon in the red (650 nm), green (550 nm), and blue (450 nm) parts of the spectrum. We investigated the temporal change of the patterns of degree p and angle α of linear polarization of sunlit and moonlit skies at dusk and dawn. We describe here the position change of the neutral points of sky polarization, and present video clips about the celestial polarization transition at moonlit twilight. We found that at partial moon and at a medium latitude (47° 15.481′ N) during this transition there is a relatively short (10–20 min) period when (i) the maximum of p of skylight decreases, and (ii) from the celestial α pattern neither the solar–antisolar nor the lunar–antilunar meridian can be unambiguously determined. These meridians can serve as reference directions of animal orientation and Viking navigation based on sky polarization. The possible influence of these atmospheric optical phenomena during the polarization transition between sunlit and moonlit skies on the orientation of polarization-sensitive crepuscular/nocturnal animals and the hypothesized navigation of sunstone-aided Viking seafarers is discussed.

Testing a polarimetric cloud imager aboard research vessel Polarstern: comparison of color-based and polarimetric cloud detection algorithms

Abstract: Cloud cover estimation is an important part of routine meteorological observations. Cloudiness measurements are used in climate model evaluation, nowcasting solar radiation, parameterizing the fluctuations of sea surface insolation, and building energy transfer models of the atmosphere. Currently, the most widespread ground-based method to measure cloudiness is based on analyzing the unpolarized intensity and color distribution of the sky obtained by digital cameras. As a new approach, we propose that cloud detection can be aided by the additional use of skylight polarization measured by 180° field-of-view imaging polarimetry. In the fall of 2010, we tested such a novel polarimetric cloud detector aboard the research vessel Polarstern during expedition ANT-XXVII/1. One of our goals was to test the durability of the measurement hardware under the extreme conditions of a trans-Atlantic cruise. Here, we describe the instrument and compare the results of several different cloud detection algorithms, some conventional and some newly developed. We also discuss the weaknesses of our design and its possible improvements. The comparison with cloud detection algorithms developed for traditional nonpolarimetric full-sky imagers allowed us to evaluate the added value of polarimetric quantities. We found that (1) neural-network-based algorithms perform the best among the investigated schemes and (2) global information (the mean and variance of intensity), nonoptical information (e.g., sun-view geometry), and polarimetric information (e.g., the degree of polarization) improve the accuracy of cloud detection, albeit slightly.

It's Not a Bug, It's a Feature: Functional Materials in Insects

Abstract: Over the course of their wildly successful proliferation across the earth, the insects as a taxon have evolved enviable adaptations to their diverse habitats, which include adhesives, locomotor systems, hydrophobic surfaces, and sensors and actuators that transduce mechanical, acoustic, optical, thermal, and chemical signals. Insect-inspired designs currently appear in a range of contexts, including antireflective coatings, optical displays, and computing algorithms. However, as over one million distinct and highly specialized species of insects have colonized nearly all habitable regions on the planet, they still provide a largely untapped pool of unique problem-solving strategies. With the intent of providing materials scientists and engineers with a muse for the next generation of bioinspired materials, here, a selection of some of the most spectacular adaptations that insects have evolved is assembled and organized by function. The insects presented display dazzling optical properties as a result of natural photonic crystals, precise hierarchical patterns that span length scales from nanometers to millimeters, and formidable defense mechanisms that deploy an arsenal of chemical weaponry. Successful mimicry of these adaptations may facilitate technological solutions to as wide a range of problems as they solve in the insects that originated them.

Automatic cloud classification of whole sky images

Abstract: Abstract. The recently increasing development of whole sky imagers enables temporal and spatial high-resolution sky observations. One application already performed in most cases is the estimation of fractional sky cover. A distinction between different cloud types, however, is still in progress. Here, an automatic cloud classification algorithm is presented, based on a set of mainly statistical features describing the color as well as the texture of an image. The k-nearest-neighbour classifier is used due to its high performance in solving complex issues, simplicity of implementation and low computational complexity. Seven different sky conditions are distinguished: high thin clouds (cirrus and cirrostratus), high patched cumuliform clouds (cirrocumulus and altocumulus), stratocumulus clouds, low cumuliform clouds, thick clouds (cumulonimbus and nimbostratus), stratiform clouds and clear sky. Based on the Leave-One-Out Cross-Validation the algorithm achieves an accuracy of about 97%. In addition, a test run of random images is presented, still outperforming previous algorithms by yielding a success rate of about 75%, or up to 88% if only "serious" errors with respect to radiation impact are considered. Reasons for the decrement in accuracy are discussed, and ideas to further improve the classification results, especially in problematic cases, are investigated.

Sky light polarization detection with linear polarizer triplet in light field camera inspired by insect vision.

Abstract: Stable information of a sky light polarization pattern can be used for navigation with various advantages such as better performance of anti-interference, no "error cumulative effect," and so on. But the existing method of sky light polarization measurement is weak in real-time performance or with a complex system. Inspired by the navigational capability of a Cataglyphis with its compound eyes, we introduce a new approach to acquire the all-sky image under different polarization directions with one camera and without a rotating polarizer, so as to detect the polarization pattern across the full sky in a single snapshot. Our system is based on a handheld light field camera with a wide-angle lens and a triplet linear polarizer placed over its aperture stop. Experimental results agree with the theoretical predictions. Not only real-time detection but simple and costless architecture demonstrates the superiority of the approach proposed in this paper.

Study on skylight polarization patterns over the ocean for polarized light navigation application.

Abstract: Polarized skylight navigation has excellent navigation performance with no error accumulation over time and low susceptibility to interference The skylight polarization distribution contains rich directional information, such as the solar meridian, the neutral point, and the polarization angle, which plays a key role in the polarization navigation But up to now the polarizations of both sunlit and moonlit skies have been investigated mainly over the land In this work, the polarization distribution patterns of the skylight over the East China Sea and the Yellow Sea were studied The polarization patterns were captured continuously during daytime and nighttime by using a full-sky imaging polarimetry system and then compared with the simulation results using the libRadtran radiative transfer software package The result shows that the skylight polarization distribution over the sea has almost the same pattern as that on the land The accuracy of the angle of polarization and the degree of polarization dropped significantly under the cloudy sky It was found that when the ship sailed on the sea, the direction of the real meridian was close to the solar azimuth during the daytime and close to the lunar azimuth during the nighttime It was also found that the nautical twilight polarization distribution was affected by both the solar polarization and the lunar polarization, but the solar polarization was dominant The experiments show that the skylight polarization distribution pattern over the sea can still be applied in the field of polarization navigation Thus, it is feasible for ships and unmanned aerial vehicles to use the polarized skylight to navigate and orient on the sea