In the 90s of the last century, Klyshko proposed an effective field method, based on the advanced wave concept, to describe the basic physical characteristics of spatial correlation for entangled two-photon pairsgenerated in a laser-pumped nonlinear crystal. As a result, the issue of two-photon optics can be understood by conventional one-photon optics. This is a fundamental way to understand one of the laws of nature for mankind. However, the a priori conditionof the advanced wave interpretation requires a basic relationship for the impulse response function of the optical system. Here a theoretical demonstration of the basic relationship is presented and discussed.