The magneto-optic principle is based on the Faraday effect. It describes the rotation of the polarization plane of linearly polarized light when passing through a transparent medium. With a closer look, linearly polarized light consists of superposition of a left and a right-circularly polarized wave with same frequency and phase. When light is passing a magneto-optical medium on which a magnetic field is applied parallel to the direction of the light, it splits into two oppositely rotating circularly polarized waves.

For the two partial waves, it causes phase shifting, because they have different refractive indices and different speeds. Their frequency remains the same. This shift results in the rotation of the polarization plane. The different angles of rotation depending on the local magnetic field strength result in contrast-differences which can be evaluated visually. Thus, a direct real-time visualization of quasi-static magnetic fields over the entire sensor surface is achieved.

To produce the sensors, Bismuth substituted Yttrium Iron garnet layers are deposited on a substrate by liquid phase epitaxy. To get a best magneto-optical sensor out of the grown garnet layers these layers additional processes are necessary.