Modulated dark-field phasing detection for automatic optical inspection

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.