Abstract: The thickness of a layer on a substrate is measured in-situ during chemical mechanical polishing. A light beam is divided through a window in a polishing pad, and the motion of the polishing pad relative to the substrate causes the light beam to move in a path across the substrate surface. An interference signal produced by the light beam reflecting off the substrate is monitored, and a plurality of intensity measurements are extracted from the interference signal. Each intensity measurement corresponds to a sampling zone in the path across the substrate surface. A radial position is determined for each sampling zone, and the intensity measurements are divided into a plurality of radial ranges according to the radial positions. The layer thickness is computed for each radial range from the intensity measurements associated with that radial range.