The Magnetic Distortion Measurement System of the LHCb RICH 2 Detector

The Ring Imaging Cherenkov detector of the LHCb experiment will use Hybrid Photon Detectors (HPDs) to detect the Cherenkov photons. This paper reports on the characterization of image distortions in the HPDs due to the presence of a controlled and variable magnetic field. A video projector shines a well defined pattern onto an HPD which is subject to such a field. We present a detailed description of the experimental set-up and of the analysis algorithms developed to achieve a spatial resolution better than 1 mm2.