Abstract: A phase conjugate interferometer has a partially reflective conventional mirror placed in front of and in close proximity to a phase conjugate mirror using internally self pumped phase conjugation in barium titanate or other photorefractive material. The reflective surface may be the surface of the phase conjugate mirror. An optical system under test is illuminated with coherent light and the wavefront at the exit pupil thereof is imaged through a beam splitter onto the reflective surface (imaging being unnecessary if the distance between the exit pupil and the reflective surface is sufficiently small that negligible diffraction occurs over that distance). Part of the wavefront is reflected at the reflective surface of the partially reflecting conventional mirror. The transmitted portion of the wavefront is incident on the phase conjugate mirror where a wavefront reversed replica of the incident wave is produced and reflected.