This paper presents an illumination-free photometric metric for evaluating the goodness of a rigid transformation aligning two overlapping range images, under the assumption of Lambertian surface. Our metric is based on photometric re-projection error but not on feature detection and matching. We synthesize the color of one image using albedo of the other image to compute the photometric re-projection error. The unknown illumination and albedo are estimated from the correspondences induced by the input transformation using the spherical harmonics representation of image formation. This way allows us to derive an illumination-free photometric metric for range image alignment. We use a hypothesize-and-test method to search for the transformation that minimizes our illumination-free photometric function. Transformation candidates are efficiently generated by employing the spherical representation of each image. Experimental results using synthetic and real data show the usefulness of the proposed metric.