A technique is described for the simultaneous measurement of the difference in the normal components of strain and of the shear strain in luminescent III-V material from the degree of polarization (DOP) of photoluminescence. This technique for the measurement of shear strain and of the difference in the normal components of strain in InP was calibrated by applying known external loads on the bars of InP with V grooves etched into the bars and by fitting the experimental results to two-dimensional finite-element simulations. Fits to the difference in the normal components of strain (as opposed to stress) yielded significantly smaller residues. On this basis we conclude that the DOP of luminescence is proportional to the difference in the normal components of strain.