An endpoint detection method includes processing an outer surface of a substrate, directing an incident light beam through a window in an opaque metal body onto the surface being processed, receiving at a detector a reflected light beam from the substrate and generating a signal from the detector, and generating a signal based on the reflected light beam received at the detector, and detecting a processing endpoint. The signal is a time-varying cyclic signal that varies as the thickness of the layer varies over time, and detecting the processing endpoint includes detecting that a portion of a cycle of the cyclic signal has passed, the portion being less than a full cycle of the cyclic signal.