The effectiveness of incoherence‐induced optical smoothing techniques is studied experimentally and good agreement with theory is found. Using incoherent laser‐drive radiation, the Rayleigh–Taylor instability is investigated in planar foil targets with imposed sinusoidal modulations. Growth rates of up to 60% of the classical value are inferred from the measurements and the nonlinear behavior of the instability is investigated. Growth is observed at all wavelengths tested between 30 and 100 &mgr;m.