The wavefront is an important characteristic for a dielectric mirror. Its measurement is usually performed with interferometers. We introduce a new method to evaluate only the coating wavefront distortion due to nonuniform thickness errors by using a reflectometer. This method uses some reflectance or transmittance maps at a wavenumber σ(m) for which the reflectance or transmittance factor variation is high. These variations are translated into some central wavenumber σ(c) variations, which enables the determination of a phase map from experimental treatment.