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This paper reports on the development of a SPAD device and its subsequent use in an actively quenched single photon counting imaging system. The device was fabricated in a UMC 0.18 μm CMOS process. The device has a cross-section of circular p+/n-well pn junction SPAD with 10 μm diameter active area. A low-doped p-guard ring (t-well layer) encircles the active area to prevent the premature reverse breakdown on a reverse biased diode breakdown.