A record of the surface wave of optical systems is obtained directly without ambiguity of sign. From the setting proposed by Keilmann (1970) two variants of a same principle have been tested: a method by optical path difference interferometer mechanical modulation, a second, by laser source frequency modulation. The principal disadvantages and advantages of these methods, and some results obtained with the last one, are presented. The measurement range is better than 100 mu m and the accuracy is about 1 mu m.