The C16531 series is a photon counting module that can detect low-level light. These modules consist of a thermoelectric cooled single photon avalanche diode (SPAD), an amplifier, a comparator, a SPAD bias circuit, and a temperature controller. The photosensitive area is available in two sizes of φ50 μm and φ100 μm, and such small photosensitive areas offer a low dark count. These modules operate by simply connecting to an external power supply (±5 V).