The concept of a thermal wave phase shifter for photothermal studies of weakly absorbing samples is described. Both sample and reference are excited simultaneously, but their respective photothermal signals reach the detector subsequently. Depending on the phase shift photothermal absorption or transmission spectra can be recorded readily with the same sample. The versatility of this technique is demonstrated by spectroscopic studies of neodymium oxide doped poly(methyl methacrylate) film. At energy fluences in the order of 10 mJ/cm2 sensitivities of 1014 molecules/cm2 are achieved with conventional, unstabilized light sources. The quantum yield for nonradiative decay is determined to be 90% throughout the entire visible spectrum.