  The Hamamatsu R11410 and R8520 photomultiplier tubes (PMTs) are designed for
applications in dark matter detectors using liquid xenon, featuring excellent
sensitivity to VUV light and stable operability at cryogenic temperatures. For
eleven R11410 and seven R8520 PMTs, we measured the relative photo-detection
sensitivity at 470nm as a function of the position of the incident light on the
photocathode. Considering 80% of the photocathode surfaces, the observed
non-uniformity values are in the ranges of (5-10)% and (25-30)% for the R11410
and R8520 models, respectively. We found that the non-uniformity in the inner
region of the photocathode surface is dominated by light reflections on the
internal components of the photosensors and that the border regions contribute
dominantly to the observed non-uniformity.
