A single photon avalanche diode based range detecting apparatus includes a reference array of single photon avalanche diodes configured to receive light from an illumination source via an internally coupled path. A return array of single photon avalanche diodes is configured to receive light from the illumination source via an external free space path. A calibration pulse generator is configured to generate a calibration signal pulse. Readout circuitry is configured to receive an output of the reference array via a reference signal path, an output of the return array via a return signal path, and an output of the calibration pulse generator via a calibration signal path. The readout circuitry is configured to determine a delay difference value between the reference signal path and the return signal path based on the output of the calibration pulse generator via the calibration signal path.