This review article aims to address common research questions in passive polarized vision for robotics. What kind of polarization sensing can we embed into robots? Can we find our geolocation and true north heading by detecting light scattering from the sky as animals do? How should polarization images be related to the physical properties of reflecting surfaces in the context of scene understanding? This review article is divided into three main sections to address these questions, as well as to assist roboticists in identifying future directions in passive polarized vision for robotics. After an introduction, three key interconnected areas will be covered in the following sections: embedded polarization imaging; polarized vision for robotics navigation; and polarized vision for scene understanding. We will then discuss how polarized vision, a type of vision commonly used in the animal kingdom, should be implemented in robotics; this type of vision has not yet been exploited in robotics service. Passive polarized vision could be a supplemental perceptive modality of localization techniques to complement and reinforce more conventional ones.

True north can be determined on Earth by three means: magnetic compasses, stars, and via the global navigation satellite systems (GNSS), each of which has its own drawbacks. GNSS are sensitive to jamming and spoofing, magnetic compasses are vulnerable to magnetic interferences, and the stars can be used only at night with a clear sky. As an alternative to these methods, nature-inspired navigational cues are of particular interest. Celestial polarization, which is used by insects such as Cataglyphis ants, can provide useful directional cues. Migrating birds calibrate their magnetic compasses by observing the celestial rotation at night. By combining these cues, we have developed a bioinspired optical method for finding the celestial pole during the daytime. This method, which we have named SkyPole, is based on the rotation of the skylight polarization pattern. A polarimetric camera was used to measure the degree of skylight polarization rotating with the Sun. Image difference processes were then applied to the time-varying measurements in order to determine the north celestial pole’s position and thus the observer’s latitude and bearing with respect to the true north.

Global navigation satellite systems (GNSS) provide robust and accurate navigation information. However, GNSS can be spoofed, potentially leading to dramatic errors in localization estimation. Inspired by nature, research has been undertaken into skylight polarization as a reliable alternative to the existing navigation systems. Recent studies have used skylight polarization to estimate the observer’s position. Nevertheless, such methods generally require a precise knowledge of time, date, and solar ephemeris. Alternatively, time invariances of the skylight polarization pattern, obtained from at least three polarization snapshots, were used to estimate the position of the north celestial pole (NCP) in the sky, without additional information, providing the true north bearing and the latitude of the observer. Here, we explain how these skylight polarization time invariances are related to the solar declination, and how this relation can be used to find the position of the NCP. Based on this property, we introduce a method to find the position of the NCP requiring only two skylight polarization observations over time and a prior knowledge about the solar declination. We tested our method on real skylight polarization images acquired by a polarimetric camera during a 1 month observation. A comparison between estimated and ground truth coordinates validates the theoretical model. Finally, our method can also be used in reverse to find the solar declination from the skylight polarization time invariances and the position of the NCP, demonstrating how skylight polarization can be used as a celestial calendar.