Directional consistency of dark-field illumination for defect images

Scratch defects in dark-field imaging exhibit directional characteristics, with variations in incident light angles causing significant differences in images. This poses a challenge for the detection of randomly oriented scratches, as the illumination method directly impacts the missed detection and detection repeatability. The study establishes a dark-field imaging model, which is based on the finite-difference time-domain method. By varying illumination directions and superimposing uni-directional incoherent illumination imaging data, the dark-field images of scratches under both line-scan and annular light sources are obtained. The relationships between imaging intensities and imaging widths of scratches with incident angles are analyzed, which can quantify the directional consistency. Simulation results reveal that increasing the number of illumination beams effectively reduces fluctuations in both imaging intensity and imaging width, thereby improving the dark-field imaging consistency. Notably, annular light sources with odd-numbered beams offer an advantage in imaging consistency over the even-numbered configurations. The trend of the experimental results using dark-field systems is in accord with the simulations. A comparative analysis of defect detection between line-scan and annular light sources confirms that the annular source has superior performance in defect detectability.