Optical interferometry for measuring instantaneous thickness of transparent solid and liquid films

An optical‐interference method is proposed which enables measurements of instantaneous, uniform thickness of the order of 10 μm–1 mm of transparent (or weakly absorbing) solid materials or liquids. It uses a converging lens set by its focal length separated from the subject film. The lens turns parallel, monochromatic, coherent rays in an expanded laser beam into converging rays propagating as spherical waves toward the subject film. The rays reflected at the front and rear surfaces of the film pass again through the lens to be retransformed from the spherical waves into the plane waves, and form interference fringes on a screen. Knowing the number of fringes provided by the rays which are incident on the subject film at angles between two specified angles, we can readily calculate the thickness of the film. A prototype apparatus based on this method has been constructed and used successfully in measuring the thicknesses of both solid and liquid films.