Luminosity measurements at LEP

Fast luminosity measurements are vital for the optimisation of the machine conditions needed for physics. At LEP this has been achieved since the startup by means of 16 small tungsen-silicon calorimeters measuring the rate of Bhabba scattering events. To increase the counting rate the detectors are placed close to the beams and mounted on collimator jaws. The rate of Bhabba scattering is calculated using the rate of coincidental detections of e- and e+ at both sides of the interaction point. The correction term arising from accidental off-momentum particle coincidence is calculated from the background rates. This technique could be successfully used at beam energies around 45 GeV since the correction term was small.Starting in '95 however, the energy of LEP has been increased up to 91.5 GeV per beam. In these conditions the background event rate almost doubles while the Bhabba cross section adopted and presented in this paper consists of checking the collinearity in the vertical plane of the particle tracks. This is obtained by measuring the vertical centre position of the showers inside the calorimeters using silicon strip detectors.