The challenges involving the calibration of the CMS Electromagnetic Calorimeter at the LHC

The CMS ECAL is a high resolution electromagnetic crystal calorimeter which relies on precision calibration in order to achieve and maintain its design performance. A set of inter-calibration procedures is carried out to normalize the differences in crystal light yield and photodetector response between channels. Different physics channels such as low mass di-photon resonances, electrons from W and Z decays and the azimuthal symmetry of low energy deposits from minimum bias events are used. A laser monitoring system is used to measure and correct for response changes, which arise mainly from the harsh radiation environment at the LHC. The challenges, techniques and results are discussed and include the combined precision of the inter-calibration and absolute energy calibration for the three year period from 2010-2012. An assessment is made for the performance to be expected from 2015 onwards following the re-start of the LHC.