Technologies for comparing observed intensities using a probabilistic similarity measure. In the probabilistic similarity measure example, there is no attempt to estimate a true intensity. Rather, the similarity of two observed intensities is defined as the likelihood that they each resulted from the same but unknown true identity while taking into account the noise characteristics of the camera observing the intensities. Since the true intensity is unknown, all possible true intensities are taken into account, rather than using a specific true intensity estimate. The probabilistic similarity measure indicates the degree to which two intensities correspond to the same intensity without estimating a true scene intensity value.