  Calibration of the absolute energy scale at high-energy photon (gamma-gamma,
gamma-electron) colliders is discussed. The luminosity spectrum at photon
colliders is broad and has a rather sharp high-energy edge, which can be used,
for example, to measure the mass of the Higgs boson in the process gamma-gamma
to H or masses of charged scalars by observing the cross-section threshold. In
addition to the precise knowledge of the edge energy of the luminosity
spectrum, it is even more important to have a way to calibrate the absolute
energy scale of the detector. At first sight, Compton scattering itself
provides a unique way to determine the beam energies and produce particles of
known energies that could be used for detector calibration. The energy scale is
given by the electron mass m_e and laser photon energy \omega_0. However, this
does not work at realistic photon colliders due to large nonlinear effects in
Compton scattering at the conversion region (\xi^2 \sim 0.3). It is argued that
the process gamma-electron to eZ_0 provides the best way to calibrate the
energy scale of the detector, where the energy scale is given by m_Z.
