Magneto-optical imaging with an indicator film is used for investigating the magnetic pattern in ferromagnetic material specimens whose optical properties do not allow the direct visualization of their magnetic pattern. The magneto-optical indicator is a ferromagnetic film with low coercitivity that detects the magnetic field from a magnetic source on which it is placed. The measurement of the indicator film is performed with a polarization microscope. In this way, the magnetic pattern generated by ferromagnets, superconductors, or current carrying conductors has been successfully visualized. By means of epitaxial iron garnet films, we obtained the quantitative imaging of the stray field induced by the magnetic microstructure in ferromagnetic films and crystals. The quantitative measurement of the perpendicular component of the magnetic field, obtained by a suitable nonlinear calibration, is demonstrated for single domain walls in ferromagnetic materials (in this paper for manganite and garnet films).