If a rough surface is illuminated by a coherent lightwave of
wavelength λ1, it is not possible to determine the surface
profile from the phases of the speckle field formed by the
scattered light. If the rough surface is illuminated, however,
by an additional coherent wave of wavelength λ2, the phase
differences between the two speckle fields do contain
information about the macroscopic surface profile even if
subject to a statistical error. It is shown that (1) the
macroscopic surface profile may be determined from the phase
differences if the effective wavelength Λ = λ1λ2/|λ1 - λ2| is
sufficiently larger than the standard deviation of the
microscopic profile of the illuminated surface, and (2) the
statistical error is reasonably small if the phase measurements
are obtained from speckles of sufficient intensity. Using a
heterodyne interferometer we demonstrate the feasibility of this
technique. In the first experiment we determine the radius of
curvature of a rough spherical surface. In the second experiment