LUCID: the ATLAS Luminosity Detector

A precise measurement of luminosity is a key component of the ATLAS program: its uncertainty is a systematics for all cross-section measurements, from Standard Model processes to new dis- coveries, and for some precise measurements it can be dominant. To be predictive a precision compatible with PDF uncertainty ( 1-2%) is desired. LUCID (LUminosity Cherenkov Integrating Detector) is a luminometer sensitive to charged particles generated by the pp collisions. It is the only ATLAS dedicated detector for this purpose and the referred one during LHC RUN 2. After the RUN 1 shutdown, LUCID had to be redesigned in order to cope with the data taking condi- tions foreseen for RUN 2.The new detector, calibration system and electronics will be described, together with the results of the 2017 luminosity measurement.