Characterization of surface roughness has generally been limited to rms deviation from the mean and occasionally the autocorrelation length. By considering the surface to be a superposition of many sinusoidal gratings, the surface spectral density function (SDF) has been calculated from measurements of the light scattered out of a reflected He-Ne laser beam. By rotating the sample, the SDF can be found for anisotropic surfaces (such as machined surfaces). The SDF allows examination of roughness as a function of spatial frequency, which is important for many applications. Interferometric and scattering results are compared for a rough (350-Arms) machined surface, and finally a smooth (50-Arms) diamond machined surface is characterized.