On-line luminosity measurements at LEP

At each LEP interaction point, the luminosity is measured on-line by small angle Bhabha monitors. These monitors are optimized to observe in all bunches relative luminosity changes in a few seconds. The description of the detectors is given, together with the method used to calculate the luminosity after background correction. Optimization of the LEP performances was done with beam separation scans using the luminosity measurements. Those scans also provide a unique measurement of the vertical beam size at the interaction point.