For the determination of UV femtosecond laser pulse durations we use the autocorrelation technique based on two photon absorption (TPA) in CaF2 . To picture TPA of 197 nm laser radiation we measure either the energy transmission through a sample or the laser induced fluorescence (LIF) of self trapped excitons (STE) at 278 nm. Both ways yield second order autocorrelations, which are easy to analyze and allow to derive the real pulse lengths being about 370 fs in our case.