A scalar scattering theory is developed that predicts the angular distribution of light scattered and the total integrated scatter from a randomly rough or inhomogeneous optical interference coating. Three types of random variation are considered: uncorrelated roughness, additive roughness, and uncorrelated index inhomogeneity. The scattering calculations are formulated so that the output of any conventional thin film analysis program along with a coating’s surface or index statistics could be used to calculate the scattering distribution of a coating. The scattering calculations are compared to experimental measurements from a sixteen-layer high reflector coating with small additive roughness σ = 2.4 Å and large correlated roughness σ = 93 Å. © 1991 Optical Society of America.