SPIDER is the full scale prototype for the ITER Heating Neutral Beam (HNB) source. The production of negative ions in the source mostly relies on the conversion of H/D atoms and positive ions on a 1.6 m x 0.8 m converter surface, which is covered with cesium in order to reduce its work function and maximize the conversion efficiency. In SPIDER, optical emission spectroscopy (OES) is used to study the light emitted by the plasma. In particular, the intensities of the cesium emission lines are tracked and used to reconstruct the distribution of its excited states. By coupling these measurements with a collisional radiative (CR) model, estimates of the cesium ground-state density and plasma parameters can be obtained in various parts of the source which are unreachable by other diagnostics. This work describes such a model and its use in conjunction with OES during the first operation with cesium in SPIDER. The electron density at the bottom of the source was found to be much larger than the rest, as a consequence of vertical drifts caused by the horizontal filter field, which would likely indicate a lower negative-ion availability for the beam. The effect of increasing the cesium evaporation rate on the negative-ion and electron densities was also investigated.