  The fluorescence yield data used for shower reconstruction in the Auger,
HiRes and TA experiments are different, not only in the overall absolute value
but also in the wavelength spectrum and the various atmospheric dependencies.
The effect on the energy reconstruction of using different fluorescence yield
parameterizations is discussed. In addition, the impact of a change in the
fluorescence spectrum depends on the optical efficiency of the telescopes. A
simple analytical procedure allows us to evaluate the combined effect of
fluorescence yield and optical efficiency showing a non-negligible deviation
between the energy scales of TA and Auger. However no relevant effect is found
in the comparison between HiRes and Auger. Finally we show that a similar
procedure could also be applied with real data.
