The neutral beam injectors of the ITER experiment will be based on negative ion sources for the generation of beams composed by 1MeV H/D particles. The prototype of these sources is currently under testing in the SPIDER experiment, part of the Neutral Beam Test Facility of Consorzio RFX, Padua, Italy. Among the targets of the experimentation in SPIDER, it is of foremost importance to maximize the beam current density produced by the accelerator. The SPIDER operating conditions can be optimized thanks to a cavity ring-down spectroscopy diagnostic, which provides line-integrated measurements of negative ion density in proximity of the accelerator apertures. The specific implementation in SPIDER shows a drift in ring down time measurements, which develops in a time scale of few hours, thus possibly affecting the negative ion density estimates in plasma pulses of 1 h duration, as required by ITER. Possible causes and solutions are discussed. Regarding the source performance, this paper presents how negative ion density is influenced by the RF power used to sustain the plasma, and by the magnetic filter field present in SPIDER to limit the amount of co-extracted electrons. In this study, SPIDER was operated in hydrogen and deuterium, in Cs-free conditions.