  There is growing interest in the use of noble liquid detectors to study
particle properties and search for new phenomena. In particular, they are
extremely suitable for performing direct searches for dark matter. In this kind
of experiments, the light produced after an interaction within the sensitive
volume is usually read-out by photomultipliers. The need to go to masses in the
tonne scale to explore deeper regions of the parameter space, calls for the use
of large area photomultipliers. In this paper we address the need to perform
laboratory calibration measurements of these large photomultipliers, in
particular to characterize its behaviour at cryogenic temperatures where no
reference from the manufacturer is available. We present comparative tests of
phototubes from two companies. The tests are performed in conditions similar to
those of operation in a real experiment. Measurements of the most relevant
phototube parameters (quantum efficiency, gain, linearity, etc.) both at room
and liquid Argon temperatures are reported. The results show that the studied
phototubes comply with the stringent requirements posed by current dark matter
searches performed with noble-liquid detectors.
