The photon counting detector modules of the PDM series are all solid-state instruments designed and manufactured by Micro-Photon-Devices (MPD) of Bolzano, Italy. They have photon detection efficiency of 48% at 550 nm and generate a TTL output pulse per detected photon. With fast-timing option (additional circuit card installed) they provide better than 50 ps FWHM photon timing resolution (NIM output pulse) for a wide range of detection wavelengths. The excellent photon detection efficiency and superior timing resolution is obtained through the use of epitaxial silicon Single Photon Avalanche Diodes (SPAD) and patented integrated Active Quenching Circuits(iAQC), specifically designed and optimized for time-resolved photon counting applications.