Luminosity and Beam Measurements used for Performance Optimisation in the LEP Collider

The vertical beam-beam parameter in LEP reached 0.083 in 1999. In order to achieve and maintain this high performance a number of different observables are continuously monitored and optimised. The beam sizes are measured using X-ray detectors and UV telescopes. The luminosity is determined directly with tungsten-silicon calorimeters and indirectly through an accurate measurement of the beam lifetime. The tune shift is measured from the tune spectrum in collision. Beam-beam deflection scans provide information about the beam sizes and separation at the interaction points. The different measurements are shortly reviewed and their resolution and time response is analysed. Their use for the optimisation of LEP is described.