Covariant formulation of light propagation in cosmological models

We present a covariant approach to the problem of light beam propagation in cosmological models within the framework of classical geometric optics in general relativity. Using the concept of screen surface orthogonal to the observer’s world-line and to the bundle of geodesics, we introduce covariant four-dimensional definitions and derive propagation equations for Sachs and Jacobi optical fields and for the area distance.