Imaging Polarimetry for Industrial Inspection

The use of the polarisation properties of light for the determination of significant aspects of surfaces in an industrial setting is considered. It is concluded that the technique is both general and robust enough to warrant development. An example is presented of the use of the polarisation signature of surfaces to achieve efficient segmentation of metals and dielectrics in the presence both of uneven lighting and of a lacquer coating. The technique is intrinsically simple given a suitable image acquisition scheme and the necessary images could be acquired at video rates using only simple circuitry. The design of polarising optics is considered briefly. 1. Polarimetry as a Diagnostic Tool Scattered light has polarisation properties which differ from those of the incident illumination. For initially unpolarised light the scattered light will be partially plane polarised with the azimuth of the largest component being contained either in, or perpendicular to, the plane of vision defined by the light source, observer and observed surface patch. If / ; and I2 are respectively the intensities of vibration perpendicular and parallel to the plane of vision the 'Degree of Polarisation', P, is defined by P=(IjJ2)/(Ii+l2); P may be either positive or negative and depends critically on both the viewing geometry and the composition and structure of the illuminated surface. These observations are not novel. Polarimetry has been extensively used in astronomy as a tool for exploring inaccessible regions [1]. The application of the technique to industrial inspection does not appear to have been widely investigated. Use of high polarisation as diagnostic of highlights has been proposed but apparently not widely applied. Wolff [2,3] has reported preliminary studies, as have Koshikawa, Shirai and Mercsh [4,5,6]. Beyond these the literature is sparse. BMVC 1991 doi:10.5244/C.5.47