Measurement of CMS Luminosity

The CMS luminosity system is used to monitor the performance of the LHC in real time, to provide an overall normalization for physics analyses, and to produce bunch-by-bunch luminosities useful for accelerator diagnostics and optimization. Horizontal and vertical separation scans were used to optimize the delivered luminosity and to measure the beam sizes in the interaction regions. Combined with the measured beam currents, the beam sizes are used to determine the luminosity of the LHC to an accuracy of 11\%. This paper describes the methods used to measure the online and offline luminosity in CMS, and presents preliminary analysis on the absolute luminosity calibration.