A precise measurement of the LHC luminosity is a key component of the ATLAS program: its uncertainty is a systematic error in all cross-section measurements, from the Standard Model processes to new discoveries, and for some precise measurements it can be the dominant error.
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 produced at the interaction point. It is the only ATLAS detector totally dedicated to this purpose and the main one during LHC RUN 2. After the RUN 1 shutdown, LUCID had to be redesigned in order to cope with the data taking conditions foreseen for RUN 2. The new detector, calibration system and electronics will be described, together with a preliminary result of the systematic uncertainty on the 2017 luminosity measurement.