The DPC-BA and the DPC-MA photon counting detector modules from C&L Instruments Inc. include a package with C-mount threads for use with camera ports on standard microscopes, and a photon counting photomultiplier tube with internal power supply and pulse-forming electronics. They also can be used with fiber optic inputs. They interface to the company's PCI bus DPC data cards. The BA version uses a bi-alkali photocathode; the MA, a multialkali for extended red response. A calibrated iris controls the light level, and a holder is provided for optional filters.