In order to characterize optical modules we have set up various testing facilities.
A test bench with dark boxes with and without magnetic shields, in which the PMTs are exposed to illumination coming from red, green or blue LEDs or very fast solid state laser pulsers (to measure time characteristics).
Another dark box equipped with a mechanical system allowing a blue LED to scan the active area of the photo-cathode is used to control the homogeneity of the parameters listed above as a function of the position of the light spot on the photo-cathode.
A water tank is used to study the overall response of the optical module to Cherenkov light emitted by vertical cosmic muons in water. The module can be rotated around a horizontal axis, making it possible to measure the optical module response at different muon angles of incidence. This set-up has also been used in the 200 GeV M2 muon beam at CERN.
Several types of photo-tubes have been tested (see details in ).
8 PMTs are now used extensively in site
measurement tests, so some experience on their behavior in-situ has