A HeNe laser has been used to fabricate photochemically a photoluminescent porous Si thin film on top of crystalline Si. The porous Si film has Gaussian shaped upper and lower interfaces. When the reflection of a laser beam from this film during, or after, the photochemical process is observed, two distinct, concentric circular interference patterns are observed. A pattern of thick rings is superimposed upon a pattern of fine rings. The reflected beam is far more divergent than the incident beam. The formation of the outer rings is a coherent phenomenon. Analysis of the patterns indicates that reflection from the upper interface is not involved in the ring formation process but that optical interference and Fresnel diffraction of the light reflected from the bottom interface cause the pattern formation. It is shown that the radius of the pattern is linearly proportional to the optical path length through the film. Therefore, measurements of pattern sizes yield information about the depth and index of refraction of the porous Si film. This observation provides us with a novel, in situ technique for measuring the kinetics of formation of the photoluminescent silicon thin films and might be exploited for applications in chemical sensing.