As silicon photomultiplier (SiPM) technology still has potential for further improvements, we describe a method to map the photon detection probability of a single-photon avalanche diode (SPAD) at the microscale level, in order to evaluate the behavior of the SPAD triggering capabilities as a function of the position of the incident light and to determine its homogeneity. For this purpose, we performed the analysis of the light response for a single active SPAD, with all its neighbors switched off, using a scanning setup, composed of the digital SiPM SPADnet-I data acquisition system, a microscope, a servo positioning system, a spectrograph and a CCD camera. We varied the wavelength of the incoming light to probe the device volume from the P-well to the N-well, with the ultimate goal of understanding and improving overall sensor performance. The comparison of this study to SPAD design and electric field technology computer-aided design (TCAD) simulation enables one to estimate how much the P-well drawn area could be increased to optimize fill factor without causing premature edge breakdown through guard ring failure. © 2017 IOP Publishing Ltd and Sissa Medialab srl.