Using interferometry and Fourier analysis, the photothermal phase map is obtained from the induced thermal lens effect in a liquid sample with high absorbance at the laser wavelength. The experimental setup consists of an adaptation of two beams thermal lens experiment, where both arms of a Sagnac's interferometer are used as probe beams. The interferograms are recorded with a CCD camera and then processed using Fourier analysis techniques. The results show that the coefficient of thermal diffusivity of a high concentrated water solution of Tartrazine is greater than that of pure water. The technique can be used to study heat transfer in opaque samples to the wavelength of the excitation beam.