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

Abstract

The inspection of optically rough surfaces searching for defects or other macroscopic features, or with the aim of characterizing roughness, are tasks well suited to optical techniques. We present the concept, the architecture, the mathematical modeling, the calibration, and some results of a device with episcopic coaxial illumination, specifically developed for surface inspections, which simultaneously renders both a coherent image and the energy spectrum of the complex reflection coefficient of a portion of the surface, precisely delimited by the illuminating laser spot. This concept is based on the well-known, single-lens, coherent image processing setup with beamsplitters added to insert the illuminating beam and to allow simultaneous access to the Fourier transform plane and to the image plane. Information about the resolved macroscopic features and the nonresolved surface microstructure (through the corresponding speckle signature) is obtained in both planes which enables different surface analysis strategies. The speckle sizes in the spectrum, and in the image, can be controlled by selecting the size of the illuminated area of the object and the lens aperture, respectively. Some envisaged applications are the detection and characterization of defects or macroscopic structures in rough surfaces, identification of authentication marks, evaluation of roughness parameters, and of speckle statistics.