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The linearity of response and spatial uniformity of response characteristics of two commercially available single-photon avalanche photodiode (SPAD) detection systems were experimentally investigated using a dedicated characterization facility that measures these parameters. Both SPAD detection systems are shown to exhibit a nonlinear response. Moreover, the responsivity of both SPAD systems is shown to be spatially nonuniform and the degree of nonuniformity depends on the single-photon detection probability setting of these systems. The experimentally observed spatial uniformity of response behavior of these detectors is explained in terms of the spatial nonuniformity in the breakdown voltage V_b across the active area of the device. The experimentally observed linearity of response characteristics of these detectors at high count rates can be explained in terms of “pulse collisions.” However, the origin of the experimentally observed linearity of response characteristics of the same detectors at low count rates is currently unknown.