Measuring precisely luminosity at LHC using the ATLAS LUCID: past experience and future plans

The ATLAS luminosity monitor, LUCID, has many innovative aspects implemented. Its photomultipliers tubes are used as detector elements by using the Cherenkov light produced by charged particles above threshold crossing the quartz windows. The analog shaping of the readout chain has been improved, in order to cope with the 25 ns bunch spacing of the LHC machine. The presentation will describe the detector, its calibration system, the performances obtained in RUN 2, and the preliminary ideas for HL-LHC future developments, starting from the lessons learned in RUN 2.