  We study oblique incident light absorption in opaque media with rough
surfaces. An analytical approach with modified boundary conditions taking into
account the surface roughness in metallic or dielectric films has been
discussed. Our approach reveals interference-linked terms that modify the
absorption dependence on different characteristics. We have discussed the
limits of our approach that hold valid from the visible to the microwave
region. Polarization and angular dependences of roughness-induced absorption
are revealed. The existence of an incident angle or a wavelength for which the
absorptance of a rough surface becomes equal to that of a flat surface is
predicted. Based on this phenomenon a method of determining roughness
correlation length is suggested.
