Optical emission spectroscopy (OES) is a non invasive diagnostic tool for the the characterization of a plasma source. In order to interpret the data from these measurements, it is necessary to take advantage of a collisional radiative code that, starting from the light emitted by the plasma source, can translate this information into plasma parameters. The goal of this work is the development of a collisional radiative code, AtoH, for the interpretation of visible spectrum of hydrogen plasma discharges performed at the resonant antenna ion device (RAID). The first step of the project is the realization of a code that, starting from known plasma conditions, is able to estimate the population density of the excited states of atomic hydrogen, and therefore the emissivity.