With low density polyethylene (LDPE) film as substrate, polyethyleneterephthalate (PET) film as filter, and an high pressure mercury (HPM 15) lamp as UV radiation source, the function of far UV radiation was examined. The results show that when the far UV (200-300 nm) was eliminated, the rate of polymerization of acrylic acid in the interlayer between two LDPE films initiated by benzophenone (BP) dramatically decreased, and the grafting efficiency became close to zero. The decisive effect of the far UV is further confirmed in UV-VIS spectra measuring the hydrogen abstraction reaction of the excited BP. For polymerization systems containing allylic hydrogens, this effect is smaller, while hydroxycyclohexyl phenyl ketone (HHPK) and benzoyldimethylketal (BDK), which are typical photocleaving initiators, show little sensitivity to the far UV. Based on an energy graph of the excited states, the bond energies and the relevant photochemical reactions a tentative interpretation of the results has been made.