It is shown that, by modifying the conventional light scattering geometry, it is possible to characterize the root‐mean‐square roughnesswin a scale‐invariant rough surface forwvalues more than one order of magnitude larger than those previously reported. Measurement ofwon the order of the wavelength of light has been demonstrated. A consequence of this development is that one can combine the diffraction theory and the measurement to determine the surface roughness exponent &agr;, which was not possible under conventional light scattering conditions. ©1995 American Institute of Physics.