This relativistic ignition criterion is derived here for the first time. As an example we take r0t ¼ 1000 g/cm3, r0f ¼ 1 g/cm3 implying a foil thickness of about 100 mm. The laser energy WL and kinetic energy Wkf of the accelerated foil with thickness l and cross section area S are (Eliezer et al., 2013)