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Home Publications York Polarisation Photometric stereo Q-Bot Composites Theory 1 Theory 2 Theory 3 Theory 4 OverviewLight consists of orthogonal electric and magnetic fields. Most natural light is unpolarised and so consists of randomly fluctuating field directions. However, a range of natural phenomena (e.g. scattering and reflection) and human inventions (e.g. polarising filters and liquid crystal displays) cause the light to become polarised. That is, the electric and magnetic fields become confined to specific planes or get constrained in other ways. In the field of Computer Vision, both natural and artificially generated polarised light has been utilised for a range of applications including specularity reduction, shape recovery, reflectance analysis, image enhancement and segmentation and separation of reflectance components. Polarisation and specularitiesThe images of the porcelain bear model below were obtained using a camera with one linear polariser mounted on the lens and another in front of the only light source. For the first image, the two polarisers were oriented parallel to each other, while the for the second image they were at 90°. The second image clearly shows a minimisation of specularity. This is of great benefit to a range of computer vision methods that assume specularities are absent from images such as shape-from-shading and photometric stereo.
The theory behind the specularity reduction is that the polarised incoming light induces electron oscillations in the reflecting medium in only one direction. This means that the specularly reflected light, which is generated by these oscillations, has an electric field that is constrained to a single plane also. If the lens polariser is oriented at right-angles to this plane then all of this light is blocked from the camera. On the other hand, the diffusely reflected light is depolarised by subsurface scattering and surface roughness. This means that much of the diffusely reflected light is transmitted through the lens polariser. |
