  A photon collider luminosity and its energy are determined by the parameters
of an electron-electron linear collider (energy, power, beam emittances) and
collision effects. The main collision effect is the coherent e+e- pair
creation. At low energies (2E < 0.5 - 1 TeV) this process is suppressed due to
repulsion of electron beams. In this region gamma-gamma luminosity above
10^{35} cm-2 s-1 is possible (10^{33} is sufficient). At higher energies the
limited average beam power and coherent pair creation restrict the maximum
energy of photon colliders (with sufficient luminosity) at 2E ~ 5 TeV.
Obtaining high luminosities requires the development of new methods of
production beams with low emittances such as a laser cooling.
