  The process gamma gamma -> l+ l- is highly suppressed when the total angular
momentum of the two colliding photons is zero so that it cannot be used for
luminosity determination. This configuration, however is needed for Higgs
production at a photon collider. It will be shown that the process gamma gamma
-> l+ l- gamma can be used in this case to measure the luminosity of a collider
with a precision that is good enough not to limit the error on the partial
decay width Gamma(H -> gamma gamma).
