Fig. 5: Detection of a CSR fiducial marker and removal of background using circular polarization. | Light: Science & Applications

Fig. 5: Detection of a CSR fiducial marker and removal of background using circular polarization.

From: Unclonable human-invisible machine vision markers leveraging the omnidirectional chiral Bragg diffraction of cholesteric spherical reflectors

Fig. 5

a Imaging set-up for background removal based on circular polarization, consisting of two cameras attached to a beam splitter, each camera having a right-handed circular polarizer in front of the lens. b–e Screenshots from software running real-time background subtraction and detection of a fiducial marker pattern (10 cm width) created by right-handed CSR shells (about 5–10% of beads are also present) with green retroreflection. The left and middle columns show the raw video feeds for left- and right-handed polarization, respectively, while the right column shows the subtraction of the left from the right channel, followed by switch to monochrome and post-processing. In panel (b), the marker is placed on a uniform metal plate painted with similar green color to that of the CSR retroreflection. In (c) and (d), the marker is placed on a window, with nature (c) and built environment (d) backgrounds, respectively. In (e) the marker is placed on a mirror, showing a room with windows reflected by the mirror. In all cases a white torch light placed next to the imaging set-up is illuminating the marker along the imaging direction. For clarity, the software screen shots in (b) and (e) have been adjusted digitally for higher exposure (equally over the entire area)

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