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The methodology for a heterodyned laser--induced transient thermal grating technique for non-contact, non--destructive measurements of thermal transport in opaque material is presented.Phase--controlled heterodyne detection allows us to isolate pure phase or amplitude transient grating signal contributions by varying the relative phase between reference and probe beams.The phase grating signal includes components associated with both transient reflectivity and surface displacement whereas the amplitude grating contribution is governed by transient reflectivity alone. By analyzing the latter with the two--dimensional thermal diffusion model, we extract the in--plane thermal diffusivity of the sample. Measurements on a 5 µm thick single crystal PbTe film yielded excellent agreement with the model over a range of grating periods from 1.6 to 2.8 µm. The measured thermal diffusivity of 1.3 × 10 --6 m 2 /s was found to be slightly lower than the bulk value.