In FILMSCATTER software we use three theoretical framework to calculate the quantities B.R.D.F cos θ and the T.I.S. They are first-order vector perturbation theories for a multilayer stack with random roughness at each interface. The interfaces can be correlated or not. 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. Bousquet, F. Flory, and P. Roche, Scattering from multilayer thin films: theory and experiment,J. Opt. Soc. Am., 71: 1115–1123.
2. J. M. Elson, "Multilayer-coated optics: guided-wave coupling and scattering by means of interface random roughness," J. Opt. Soc. Am. A 12(4), 729 (1995).
3. 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.