Performance of the light monitoring system for CMS-ECAL

The electromagnetic calorimeter (ECAL) of CMS is constituted of nearly 80 000 PbWO4 crystals. Small variations in the light transmission of these crystals are expected in the harsh high-luminosity conditions at LHC. This feature requires frequent transmission measurements over the full ECAL in order to obtain short-term corrections of the measured energies of physics events. Therefore, an optical monitoring system was developed consisting of a laser source and an optical fibre distribution system. The validation of the light injection system with its final laser and read out electronics and the feasibility determining the short-term correction were the main goals of a dedicated test beam during summer 2002. Data were collected with a 400 crystal final production module, partially equipped with electronics (100 channels) for these tests. The monitoring system, its performance and the results obtained during this period are presented.