In FILMSCATTER software we use a theoretical framework to calculate the quantities B.R.D.F cos θ and the T.I.S. This is a first-order vector perturbation theory for a multilayer stack with inhomogeneities in the volume of the layers.I highly advise you to consult the references below for the validity of these models before you use the software.

The Bidirectional Reflectance Distribution Function cos θ (B.R.D.F cos θ)

The Bidirectional Reflectance Distribution Function cos θ is calculated versus the scattering angle.

Total Integrated Scattering (T.I.S)

The Total Integrated Scattering is calculated versus the wavelength.

Power Spectral Density (P.S.D)

The Power Spectral Density.

References:

1. P. Bussemer, P, K. Hehl, S. Kassam, "Theory of light scattering from rough surfaces and interfaces and from volume inhomogeneities in an optical layer stack," Waves in Random Media: An Institute of Physics Journal; Oct1991, Vol. 1 Issue 4, p207-221, 15p.