Noise creates polarization artefacts

A B Tibbs,N W Roberts,D R Bull,I M Daly

doi:10.1088/1748-3190/aa9e22

A B Tibbs, N W Roberts + Show 2 more

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https://doi.org/10.1088/1748-3190/aa9e22

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Journal: Bioinspiration & biomimetics	Publication Date: Dec 28, 2017
Citations: 15	License type: cc-by

Affiliation: University of Bristol

Abstract

The accuracy of calculations of both the degree and angle of polarization depend strongly on the noise in the measurements used. The noise in the measurements recorded by both camera based systems and spectrometers can lead to significant artefacts and incorrect conclusions about high degrees of polarization when in fact none exist. Three approaches are taken in this work: firstly, the absolute error introduced as a function of the signal to noise ratio for polarization measurements is quantified in detail. An important finding here is the reason for why several studies incorrectly suggest that black (low reflectivity) objects are highly polarized. The high degree of polarization is only an artefact of the noise in the calculation. Secondly, several simple steps to avoid such errors are suggested. Thirdly, if these points can not be followed, two methods are presented for mitigating the effects of noise: a maximum likelihood estimation method and a new denoising algorithm to best calculate the degree of polarization of natural polarization information.

Highlights

Polarization information is abundant in both terrestrial and aquatic environments, with many animals using this channel of information for a wide range of visual tasks including navigation [1,2,3], communication [4,5,6,7] and visual contrast enhancement [8,9,10]
This paper focuses on the measurement of degree of linear polarization (DoLP) because a current issue in the literature is the incorrect reporting of the absolute values of DoLP
This point is illustrated in figure 1, where the polarization image of an unpolarized scene was simulated by assuming each camera component was identical, and the DoLP was calculated

Summary

Introduction

Polarization information is abundant in both terrestrial and aquatic environments, with many animals using this channel of information for a wide range of visual tasks including navigation [1,2,3], communication [4,5,6,7] and visual contrast enhancement [8,9,10]. Visualising the polarization of light in the natural environment and understanding the ecological and behavioural relevance has proved challenging. Humans are effectively blind to the polarization of light [11], relying on artificial visualisation techniques and human mathematics and language to create a representation of the polarization of light. This is analogous to the behavioural importance of ultraviolet light signals and photoreception which has been difficult to understand [12, 13]. The angle of polarization (AoP); the average angle at which the electric fields of waves of light in a beam oscillate.

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