A substrate processing apparatus comprises a process chamber capable of processing a first material on the substrate. A radiation source is capable of emitting radiation that is reflected from the substrate during processing. A radiation detector is provided to detect the reflected radiation and generate a signal trace. A controller is adapted to receive the signal trace and evaluate an endpoint of processing the first material from a change in the signal trace that is distinctive of an exposure of a second material having a different reflectivity coefficient than the first material.