  Absolute calibration of the Pierre Auger Observatory fluorescence detectors
uses a 375 nm light source at the telescope aperture. This end-to-end technique
accounts for the combined effects of all detector components in a single
measurement. The relative response has been measured at wavelengths of 320,
337, 355, 380 and 405 nm, defining a spectral response curve which has been
normalized to the absolute calibration. Before and after each night of data
taking a relative calibration of the phototubes is performed. This relative
calibration is used to track both short and long term changes in the detector's
response. A cross check of the calibration in some phototubes is performed
using an independent laser technique. Overall uncertainties, current results
and future plans are discussed.
