The Intensity Effect in Magneto-Optical Filters

We used a laser system for determining the bandpasses of the two vapour cells, the Magneto-Optical Filter (MOF) and the Wing Selector (WS), which are the core of solar narrow-band filters based on the MOF technology. A new result, which we called the Intensity Effect, was found: the MOF and WS bandpasses depend not only on the temperature at which the cell is heated and the external magnetic field in which the cell is embedded, but also on the radiation intensity entering the cell. A theoretical interpretation of the Intensity Effect is proposed in terms of the kinetic equilibrium of the potassium atomic populations inside the vapour cell. We need to take the Intensity Effect into account for setting-up MOF based instruments for solar and stellar observations as well as for modelling the MOF and WS spectral transmissions.