An ODMR microscope can detect witty magnetic fields, on the order of nanotesla, with high spatial resolution. Electron resonance is optically read out via fluorescence. The sensing element here is a diamond plate with implanted nitrogen atoms that exhibit strong red fluorescence. The intensity of this fluorescence varies upon irradiation of microwaves that interact with the electrons in the diamond's impurity sites. Since the states of the electrons are affected by external magnetic fields, the fluorescence signal provides information about local magnetic fields in the vicinity of the respective impurity in the diamond. A magnetic sample in the immediate vicinity of the diamond can thus be tested for its magnetic properties.