Continuous recording of the absorption spectra of thin films by an optical guided‐wave technique is demonstrated. In the case of a garnet thin film of compositoin (YSmLuCa)3(FeGe)5O12it is shown that the near‐infrared Sm3+absorption bands are clearly visible in contrast with conventional transmission measurement. Comparison with the absorption spectrum of bulk Sm3Fe5O12garnet allows the determination of an Sm concentration in the film.