  A method is described that allows calibration and assessment of the linearity
of response of an array of photomultiplier tubes. The method does not require
knowledge of the photomultiplier single photoelectron response model and uses
science data directly, thus eliminating the need for dedicated data sets. In
this manner all photomultiplier working conditions (e.g. temperature, external
fields, etc) are exactly matched between calibration and science acquisitions.
This is of particular importance in low background experiments such as
ZEPLIN-III, where methods involving the use of external light sources for
calibration are severely constrained.
