• R. Bartoldus, I. Aracena, P. Grenier, D.W. Miller, E. Strauss, D. Su
  SLAC, Menlo Park, California
• J. Beringer, P. Loscutoff
  LBNL, Berkeley, California
• H. Burkhardt, S.M. White
  CERN, Geneva
• W. Kozanecki
  CEA, Gif-sur-Yvette
• J. Walder
  Lancaster University, Lancaster

We present results from the measurement of the 3-D luminosity distribution with the ATLAS Inner Detector during early running. The spatial distribution of pp interactions is reconstructed by a dedicated algorithm in the High-Level Trigger that fits tracks and primary event vertices in real time, and by an offline algorithm that takes full advantage of the high tracking efficiency and resolution. The number of vertices provides online monitoring of the instantaneous luminosity, while luminous-centroid motion mirrors IP-orbit and RF-phase drifts. The x, y and z luminous widths reflect the evolution of the transverse and longitudinal emittances. The length scales of the IP orbit bumps, which directly impact the accuracy of the transverse convolved beam sizes measured during van der Meer scans, are calibrated offline against the measured displacement of the luminous centroid; this significantly improves the accuracy of the absolute luminosity calibration. The simultaneous determination, during such scans, of the transverse convolved beam sizes (from the luminosity variation) and of the corresponding luminous sizes can be used to disentangle the transverse IP sizes of the two beams.