A flash photolysis device is described which makes irradiation down to the LiF‐cutoff (∼1100 Å) in long reaction vessels feasible. The number of quanta per flash entering the reaction cell between ∼1100 and 1400 Å and ∼1100 to 1650 Å is measured by chemical actinometry. The light pulse has a 1/e duration of less than 2 &mgr;sec, largely independent of the discharge energy and the length of the light source.