Episcopic coaxial illumination device for the simultaneous recording of the speckle signature in the spectrum and in the image of scattering reflective surfaces

Abstract

Inspection of optically rough surfaces in search of defects or other surface features with deterministic reflectance distributions is a subject well suited to optical techniques. We present a device with episcopic coaxial illumination, specifically developed for such kind of inspection tasks, which simultaneously renders both a coherent image and the spatial spectrum of a portion of the surface, precisely defined by the illuminating laser spot. It is based on the wellknown single-lens coherent image processing system, with beamsplitters added to insert the illuminating laser beam and to allow simultaneous access to the Fourier transform and the image planes. The device allows inspecting the speckle signature of surface features in both planes, thus allowing different defect recognition approaches. By selecting the size of the illuminated area of the object or the lens aperture, different speckle sizes can be obtained. If the speckle size is made large enough, identification of individual features can be made on the basis of their particular speckle signatures. Some envisaged applications are the characterization of defects or structures in rough surfaces, the evaluation of speckle statistics in precisely defined zones of surfaces or the identification of authentication marks.