A phase measurable optical heterodyne interferometer which is capable for measuring both the modulus r12(0) and the phase ψ12(0)arg{r12(0)} of the spatial coherence function r12(0) without the need of the optically stringent requirement for the equality of the path-lengths in the two arms of the interferometer is established. An extended formulation for the intensity correlation function of optical fields is also presented in relation to the theory of the interferometer. The experimental verifications are presented and are compared with the results obtained from other interferometric systems.