Dark field illumination (DFI) is an elegant inspection technique sometimes used to detect particles on a specular surface. However, traditional DFI struggles with repeatability, limiting its applications in automated inspection. We present an improvement to DFI by introducing a modulated dark field illumination (MDFI) that utilizes the phase rather than the intensity in the detection of defects. For modulated dark field illumination (MDFI), the phase-based information is independent from the reflectance of the surface, but has a higher sensitivity to the light scattered from a defect than DFI. As a result, we obtain a robust computational image process method that is insensitive to the environment and provides clearly defined defect information. In order to extend the application to industry, the instantaneous MDFI systems were developed and validated.
Dark-field illumination is a simple yet elegant imaging technique that can be used to detect the presence of particles on a specular surface. However, the sensitivity of dark-field illumination to initial conditions affects its repeatability. This is problematic in cases where automation is desired. We present an improvement to the current method of using a modulation field that relies on phase calculations rather than intensity. As a result, we obtain a computational method that is insensitive to noise and provides clearly defined particle information, allowing a global threshold to be set for autonomous measurement purposes. After introducing the theory behind our method, we present experimental results for various scenarios and compare them to those obtained using the dark-field approach.