An apparatus is disclosed for measuring the roughness of a surface with improved accuracy. The apparatus, several embodiments of which are disclosed, includes a device for illuminating a surface being studied and a photosensitive detector for determining the maximum intensity of light reflected on the surface. Apparatus is also provided for detemining two points at which the intensity of the reflected light has fallen to a selected fraction of its maximum intensity. The distance between these two points is then measured along the intersection of the plane of incidence of the illuminating beam with a plane perpendicular to the direction of specular reflection. This distance provides an accurate quantitative measure of the roughness of the surface being studied.