# Skylight polarization heading sensor using waveplate retardance shift with incidence

Léo Poughon, Vincent Aubry, Jocelyn Monnoyer, Stéphane Viollet, Julien Serres

► To cite this version:

Léo Poughon, Vincent Aubry, Jocelyn Monnoyer, Stéphane Viollet, Julien Serres. Skylight polarization heading sensor using waveplate retardance shift with incidence. Journée des Jeunes Chercheurs en Robotique (JJCR 2023), GDR Robotique; Franck Ruffier; Maan El Badaoui El Najjar, Oct 2023, Moliets et Maâ, France. hal-04521170

HAL Id: hal-04521170

https://hal.science/hal-04521170

Submitted on 26 Mar 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Skylight polarization heading sensor using
waveplate retardance shift with incidence

Léo Poughon¹,², Vincent Aubry², Jocelyn Monnoyer²,
Stéphane Viollet ¹, Julien R Serres¹,³

¹Aix Marseille Univ, CNRS, ISM, 163 Av. de Luminy, 13009 Marseille, France

²Stellantis, Rte de Gisy, 78140 Velizy-Villacoublay, France

³Institut Universitaire de France (IUF), 1 rue Descartes, 72231 Paris cedex 05, France

**Keywords:** Skylight Polarization, Navigation, Deep Learning, Heading, Celestial Com-
pass, Polarized Vision.

In nature, navigating insects such as ants, bees and flies rely on the polarization of light scattered by the sky to estimate their orientation relative to the sun, and thus orient themselves [1]. Numerous artificial sensors have been proposed in an attempt to reproduce their optical compass [2, 3, 4] but none is yet fully satisfactory for automotive use, either in terms of robustness, size, acquisition time or cost (~€2,500 for a polarimetric camera).

This study describes a innovative heading sensor architecture based on polarization
pattern detection via an optical transformation (patent application [5]). This architec-
ture is based on variations in the retardance of a waveplate as a function of the angle
of incidence of the polarized light rays, resulting in the appearance on the image of
iridescent colors, depending on the orientation of the incident rays and the state of
polarization. The outcome is a low-cost, lightweight sensor that would cost about the
same the color camera used (here a Raspberry Pi color camera, i.e. ~€30).

<figure><img src="image_1.png" /><figcaption>Figure 1: Outdoor image obtained by clear sky, in front of a building. Iridescent colors can be seen.</figcaption></figure>

An optical simulation of the light-sensor interaction is presented, including a com-
plete acquisition chain simulation (Rayleigh sky model, waveplate model in regard of
incidence of rays, lens distortion, and color sensor).
Skylight polarization heading sensor using
waveplate retardance shift with incidence

Léo Poughon¹,², Vincent Aubry², Jocelyn Monnoyer²,
Stéphane Viollet ¹, Julien R Serres¹,³

¹Aix Marseille Univ, CNRS, ISM, 163 Av. de Luminy, 13009 Marseille, France

²Stellantis, Rte de Gisy, 78 140 Velizy-Villacoublay, France

³Institut Universitaire de France (IUF), 1 rue Descartes, 72 231 Paris cedex 05, France

**Keywords:** Skylight Polarization, Navigation, Deep Learning, Heading, Celestial Com-
pass, Polarized Vision.

In nature, navigating insects such as ants, bees and flies rely on the polarization of light scattered by the sky to estimate their orientation relative to the sun, and thus orient themselves [1]. Numerous artificial sensors have been proposed in an attempt to reproduce their optical compass [2, 3, 4] but none is yet fully satisfactory for automotive use, either in terms of robustness, size, acquisition time or cost (~€2,500 for a polarimetric camera).

This study describes a innovative heading sensor architecture based on polarization
pattern detection via an optical transformation (patent application [5]). This architec-
ture is based on variations in the retardance of a waveplate as a function of the angle
of incidence of the polarized light rays, resulting in the appearance on the image of
iridescent colors, depending on the orientation of the incident rays and the state of
polarization. The outcome is a low-cost, lightweight sensor that would cost about the
same the color camera used (here a Raspberry Pi color camera, i.e. ~€30).

<figure><img src="image_2.png" /><figcaption>Figure 1: Outdoor image obtained by clear sky, in front of a building. Iridescent colors can be seen.</figcaption></figure>

An optical simulation of the light-sensor interaction is presented, including a com-
plete acquisition chain simulation (Rayleigh sky model, waveplate model in regard of
incidence of rays, lens distortion, and color sensor).
A prototype based on the use of a plastic waveplate was built and fixated on a rotating motorized mount, allowing us to get outdoor images of sky with known sensor orientation regarding to the sun (Fig. 2).

Finally, methods for processing this type of images (Fig. 1) to estimate heading based on a convolutional neural network training will also be discussed.

<figure><img src="image_2.png" /><figcaption>Figure 2: Motorized rotative mount supporting the optical sensor during outdoor experiments.</figcaption></figure>

## References

[1] Gábor Horváth and Dezső Varjú. *Polarized Light in Animal Vision*. Springer Berlin Heidelberg, Berlin, Heidelberg, 2004.

[2] Salmah Karman, S. Diah, and Ille Gebeshuber. Bio-Inspired Polarized Skylight-Based Navigation Sensors: A Review. *Sensors*, 12(11):14232-14261, October 2012.

[3] Julien Dupeyroux, Julien R. Serres, and Stéphane Viollet. AntBot: A six-legged walking robot able to home like desert ants in outdoor environments. *Science Robotics*, 4(27):eaau0307, February 2019.

[4] Fang Kong, Yingjing Guo, Jianhua Zhang, Xiaojing Fan, and Xiaohan Guo. Review on bio-inspired polarized skylight navigation. *Chinese Journal of Aeronautics*, 36(9):14-37, 2023.

[5] Léo Poughon, Stefano Mafrica, Jocelyn Monnoyer, Laetitia Pradere, Julien R Serres, and Stéphane Viollet. Procédé et système pour déterminer des données caractérisant un cap suivi par un véhicule automobile à un instant courant. (FR3128528A1), 2021.
A prototype based on the use of a plastic waveplate was built and fixated on a rotating motorized mount, allowing us to get outdoor images of sky with known sensor orientation regarding to the sun (Fig. 2).

Finally, methods for processing this type of images (Fig. 1) to estimate heading based on a convolutional neural network training will also be discussed.

<figure><img src="image_3.png" /><figcaption>Figure 2: Motorized rotative mount supporting the optical sensor during outdoor experiments.</figcaption></figure>

## References

[1] Gábor Horváth and Dezső Varjú. *Polarized Light in Animal Vision*. Springer Berlin Heidelberg, Berlin, Heidelberg, 2004.

[2] Salmah Karman, S. Diah, and Ille Gebeshuber. Bio-Inspired Polarized Skylight-Based Navigation Sensors: A Review. *Sensors*, 12(11):14232–14261, October 2012.

[3] Julien Dupeyroux, Julien R. Serres, and Stéphane Viollet. AntBot: A six-legged walking robot able to home like desert ants in outdoor environments. *Science Robotics*, 4(27):eaau0307, February 2019.

[4] Fang Kong, Yingjing Guo, Jianhua Zhang, Xiaojing Fan, and Xiaohan Guo. Review on bio-inspired polarized skylight navigation. *Chinese Journal of Aeronautics*, 36(9):14–37, 2023.

[5] Léo Poughon, Stefano Mafrica, Jocelyn Monnoyer, Laetitia Pradere, Julien R Serres, and Stéphane Viollet. Procédé et système pour déterminer des données caractérisant un cap suivi par un véhicule automobile à un instant courant. (FR3128528A1), 2021.