Low frequency noise measurements is used tool to characterize and analyse defects in photovoltaic cells, under both darkness and illumination condition. Several studies have attempted to identify the noise fluctuations under light, none of them have raised the question of the physical origin of the noise, as fluctuations can be induced by light stimulated defects or by the light source itself. This issue is investigated in this study through static current and noise measurements for a photoresistor and a np diode. The comparison of the results demonstrates that, depending on the voltage range, the noise measured can be identified as caused either by the light source or by the same mechanism than the device under darkness.