Abstract
An expression for the current density in a tunnel junction caused by an electromagnetic field is derived. The calculations are realized with the Matsubara (1955) method of imaginary times. The first order perturbation theory of Ambegaokar and Baratoff (1963) is generalized in a form, by which all higher order terms can be obtained by an iterative formula. These terms will become important, when the normal conductance of the junction has a magnitude much exceeding 1 Omega -1.