It is shown that a single light intensity measurement does not allow the determination of the sign of the ratio of strain‐optical constants determined ultrasonically. However, if an experiment is performed in which the ratios are overdetermined, it is possible to determine the signs by applying the criterion that the results of the various measurements must be consistent. The equations which must be satisfied for the criterion of consistency are obtained for nine of the 32 crystal classes. The application of the technique is presented with numerical values for the case of NaCl.