The science goals of the Advanced Technology Solar Telescope (ATST) put particularly tight scatter specifications on its primary mirror, a 4m in diameter Zerodur mirror. Given the characteristics of the ATST-M1, it was crucial to find an easy solution for measuring the BRDF (Bi-Reflectance Distribution Function) of optically smooth surfaces. The Rayleigh-Rice vector perturbation is a widely used theory establishing a one-to-one relationship between BRDF and power spectral density of a smooth, clean and front reflective surface. The MicroFinish Topographer (MFT), a phase measuring interferometer, is convenient for micro-roughness measurements of surfaces of almost any shape or material. Thus, this talk presents experimental data supporting that using the PSD calculated from the MFT measurements allows predicting the BRDF of optically smooth surfaces. As spatial frequencies and angular directions are related to each other and that the MFT measurements reliably cover a large range of spatial frequencies ([0.1-200] cycles/mm), the BRDF is predicted over a large range of angular directions, including angles very close to specular direction (θs= 2mrad).