How to write ophbdx.f

Dear experts,
I want to set reflection of a interface(between region A and region B), if photons from region A to region B, all photon will be transported with no reflection, and if photons from region B to region A, no photon will be transported with full reflection. I think it maybe use user routine ophbdx.f, but I do not know how to write it, or any other simple way?
Waiting for your help.