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One of the main drawbacks of single-photon avalanche diode arrays is optical crosstalk between adjacent detectors. In the past, this phenomenon was basically ascribed to light propagating from one detector to another through a direct optical path. Accordingly, deep trenches coated with metal were introduced as optical isolation barriers between pixels. This solution, however, was unable to completely prevent the crosstalk. In this letter, we demonstrate that a strong contribution to optical crosstalk comes from photons reflected at the bottom of the chip. These photons can bypass trenches making them less effective.