  We propose a method for the approximate computation of the Green function of
a scalar massless field subjected to potential barriers of given size and shape
in spacetime. The potential of the barriers has the form
V(phi)=xi(phi^2-phi_0^2)^2; xi is very large and phi_0 very close to zero, the
product (xi phi_0^2) being finite and small. This is equivalent to the
insertion of a suitable constraint in the functional integral for phi. The
Green function contains a double Fourier transform of the characteristic
function of the region where the potential has support.
