Abstract: A process for monitoring and determining the end point of a wet etch process for removing a thin solid film 116 from a substrate by directing a light beam onto the substrate and monitoring the intensity of reflected beams, including the step of selecting a coherence length of the incoming beam 120 so that it is small enough so that no interference occurs in the liquid layer and large enough so that interference can occur in the thin solid film, i.e., light reflected from the interface between the liquid 118 and the top of the thin film, and light reflected from the interface between the bottom of the thin solid film and the substrate interferes. If the liquid layer is about 100 micrometers thick, and the thin film is about 1 micrometer thick, a coherence length of about 10 micrometers is suitable. Such coherence length can be provided with a suitable bandpass filter.