  Calibrating the absolute energy scale of air showers initiated by ultra-high
energy cosmic rays is an important experimental issue. Currently, the
corresponding systematic uncertainty amounts to 14-21% using the fluorescence
technique. Here we describe a new, independent method which can be applied if
ultra-high energy photons are observed. While such photon-initiated showers
have not yet been identified, the capabilities of present and future cosmic-ray
detectors may allow their discovery. The method makes use of the geomagnetic
conversion of UHE photons (preshower effect), which significantly affects the
subsequent longitudinal shower development. The conversion probability depends
on photon energy and can be calculated accurately by QED. The comparison of the
observed fraction of converted photon events to the expected one allows the
determination of the absolute energy scale of the observed photon air showers
and, thus, an energy calibration of the air shower experiment. We provide
details of the method and estimate the accuracy that can be reached as a
function of the number of observed photon showers. Already a very small number
of UHE photons may help to test and fix the absolute energy scale.
