The thickness of a transparent layer of oil upon the surface of water is measured as the distance between the surface of oil film and the interface of the oil with the water. Two experimental results have demonstrated that the interface can reflect a white-light beam well enough to form an interferogram, even if the light is subjected to oil-film dispersion. When a beam of white light is incident vertically onto the oil-film surface, a scanning white-light interferometer in the Michelson configuration is employed to locate two serial reflections, surface reflection and interface reflection. The thickness of the transparent oil film on water is calculated based on the separation of these two interferograms. A limitation thickness, approximately 250 microm with 1.25 microm resolution, is achieved under the condition that there is 50 nW of optical power incident onto the oil-film surface with a wavelength centered at 1310 nm.

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