Gravitational lens optical scalars in terms of energy-momentum   distributions
  This is a general work on gravitational lensing. We present new expressions for the optical scalars and the deflection angle in terms of the energy-momentum tensor components of matter distributions. Our work generalizes standard references in the literature where normally stringent assumptions are made on the sources. The new expressions are manifestly gauge invariant, since they are presented in terms of curvature components. We also present a method of approximation for solving the lens equations, that can be applied to any order. 
Emanuel Gallo and Osvaldo M. Moreschi
gr-qc astro-ph.CO