Physics background as a systematic effect in luminosity measurement at International Linear Collider

In order to achieve required precision of luminosity measurement at the International Linear Collider (ILC), that is, of order of 10-4, systematic effects have to be understood at the level of this precision. Apart from machine background originating from pairs converted from beamstrahlung, physics background from 2- photon processes is one of the main systematic effects. Properties of these processes, as well as their separation from the Bhabha signal have been studied.