Abstract The experimental determination of the radiant energy field generated by an ultraviolet tubular radiation source (located at the focal axis of a cylindrical reflector with a parabolic cross-section) was performed. In order to carry out “point” measurements and on an absolute basis, microreactors operated in a batch-recycling system were employed. The actinometric reaction of the photodecomposition of uranyl oxalate was conducted in the microprobes and changes in concentration with time were measured in the batch vessel. Polychromatic radiation was accounted for in the theory and used in the experiments. The experimental values were compared with the theoretical predictions; good agreement was obtained. The errors were never higher than 20%. This is important because the theoretical model has no adjustable parameters. The results thus obtained constitute a significant contribution to the validation of the extense source model with volumetric emission and its applications to photoreactor design.