Helmholtz stereopsis guarantees unbiasedness by BRDF of the search for inter-image correspondences. In a practical setup, calibrated pixel sensitivity and corrected light anisotropy are required for the method to work well. In this paper a simple method for joint light-camera radiometric calibration is proposed. Such calibration is shown to be an ill-posed numerical problem. In the worst case of a single image pair, it can be regularized along epipolar lines only. The calibration problem becomes regularizable everywhere in the case of more pairs and cameras in a general position. A general, simple and fast calibration procedure is proposed that includes a regularizer for the single-pair case. It is shown in a ground-truth experiment that the accuracy of reconstructed surface normals improves by an order of magnitude after radiometric calibration.