A new method is presented for the measurement of surface roughness in the range Ra,0·05–10 w m. The data from a strongly defocused low-contrast image of a rough surface is scanned, and the contrast value,C= † I d I ¢, is processed. A relatively low degree of spatial and temporal coherence, combined with a relatively high-image aperture and a detector aperture (which is much wider than the nominal speckle width of the corresponding highly coherent speckle pattern) led to important instrumental advantages, e.g., illumination by an incandescent lamp, simple photodetectors, short measuring times and simple, inexpensive, compact construction. The superposition of uniform intensity to the phase contrast structure, which leads to a strictly monotonic dependence of the measured contrast over a very wide range of the roughness values R a is of great importance. Practical measurements of metallic surface standards are reported and discussed, and the theoretical aspects of the superposition of uniform intensity are pointed out with the help of a new analytic expression for the stochastic contrast.