One-, two-, and three-dimensional photon femtoscopy

Femtoscopy with photon pairs is a particularly attractive tool for studying high-energy nuclear collisions. Proposed and extensively discussed in several influential theory articles, it has seen only few applications in experiment because of statistics limitations. With the progress of detectors and electronics, only now it is coming within reach. In this paper we discuss the choice of kinematic variables for two-photon correlation functions. In particular, we argue against $C(Q_{\rm inv})$ and in favor of $C(\Delta E,Q_{\rm inv})$.