A quantitative magneto-optical imaging method is used to form an image of a target material. An active material is placed close to the target material and produces a Faraday rotation in a polarized light beam. The Faraday rotation of the active material is essentially proportional to the magnetization of the target material when this latter is subjected to an exciting magnetic field. Photodetector means detect the beam reflected after passing through the active material. The light from the reflected beam can then be analyzed for obtaining the amplitude and phase of an interfering magnetic field generated by a defect in the target material.