The commissioning of the CMS electromagnetic calorimeter light monitoring system

The Compact Muon Solenoid (CMS) Experiment, located at the Large Hadron Collider at CERN, is a general purpose particle detector experiment with a high resolution electromagnetic calorimeter (ECAL). I report on the commissioning of the light monitoring system, which tracks changes in transparency of the lead tungstate crystals and is thus crucial for maintaining the energy resolution of the calorimeter.