An optical blood monitoring system for blocking unwanted light from reachingsensors in a sensor clipassembly fastened to a blood chamber connected in an extracorporeal bloodtreatment system. The bloodchamber has an internal flow cavity for communicating the extracorporeal bloodflow and viewingwindows to enable the sensor clip assembly to illuminate the blood with lightas the blood flows throughthe blood chamber in order to monitor characteristics of the blood. The sensorclip assembly includesopposing heads with LED emitters and photodetectors. In one embodiment, lensesin the heads aresurrounded by shrouds extending from the lenses so that when the sensor clipassembly is fastened to theblood chamber the shrouds block unwanted light from reaching thephotodetectors.
