Single photon avalanche diode (SPAD) is the new generation of Geiger-Muller (GM) detectors, developed with semiconductor technology, and able to detect single photons, mainly in visible range. In this work we study the signal generation process and the dead time (DT) mechanisms of the device under a constant light regime. According to our results, it is possible to discriminate low rate signals from afterpulse and noise production and, moreover, to overcome the saturation effect due to the dead time losses. Starting from hybrid DT model [S. H. Lee and R. P. Gardner, Appl. Radiat. Isot. 53, 731 (2000)] we have been able to evaluate the real amount of incident photon rate up to 10(7) cps using a passive quenched device with 0.97 mus total dead time. In this way the passive quenched SPAD achieves the same performance of the active quenched one showing that relatively complex data analysis and complex device implementation are comparable solutions for constant light measurement. We also analyze some effects, lacking in GM counter, which should be introduced in the analysis of semiconductor device, as afterpulse, reduced photon detection efficiency, and noise production.