An actinometric method to characterize performance of reflecting UVC reactors used for water treatment

•Problems of actinometry for UV reactors used in water treatment are discussed.•An easy, optimized actinometrical procedure for UV reactors is proposed.•Average fluence rate is estimated directly for reflections and non-parallel light.•The actinometry procedure has been validated by biodosimetry.•Practical implications of the developed actinometry are discussed.