The CMS detector at LHC will be equipped with a high precision lead tungstate electromagnetic calorimeter. To ensure the stability of the calorimetric response at a level of a few per mille, every channel of the detector is monitored with a laser system. This system allows to correct for fluctuations in the detector response with high precision, in particular the expected radiation induced changes of the crystal transparency, which are on the level of several per cent at nominal luminosity. Here we report results from long term tests of the system on fully equipped supermodules of the CMS ECAL which are performed after the final step of the construction but before insertion of the modules in CMS. The in‐situ monitoring strategy for the CMS ECAL is discussed in detail and the performance of the full monitoring procedure as achieved in the test beam is illustrated.

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