Abstract: Apparatus for measuring surface roughness utilizes diffraction patterns derived from the surface under test. The energies in the different orders are detected and processed utilizing various different algorithms dependent upon a number of factors such as the number of orders in the diffraction pattern and asymmetry in the diffraction pattern. Various filter functions both for the diffraction pattern itself and for values of surface roughness after measurement are disclosed. One algorithm for measuring surface roughness involves comparison of values representing the energy orders with reference values, preferably reference values obtained by an interpolation operation between one set of reference values representative of a diffraction pattern from a symmetrical surface and another set representative of a diffraction pattern from an asymmetrical surface.