In this paper, we study the routing and wavelength assignment (RWA) problem of a set of multicast sessions on already dimensioned wavelength routed network, while taking optical power impairments into consideration. Our main objective is to minimize the session blocking probability. We formulate the problem as a mixed-integer linear program (MILP). A greedy algorithm is also introduced, which provides fast, yet efficient, solutions for large problem sizes. The CPLEX linear optimization package is used to solve the MILP formulation, while simulation was used to experiment with the greedy algorithm.