Within the last twelve months, a round robin evaluation focusing the irradiation conditions for photocatalytical testings and the measurement of these conditions was performed. Different radiation sources and measurement devices were tested to illustrate the problem of incomparability of measured irradiances from different laboratories around Europe. Three different types of radiation sources (fluorescence tubes, light emitting diodes and continuum sources) commonly used in photocatalytic setups were utilized to get an overview of measured irradiance values in the ultraviolet range. Irradiance is a fundamental parameter in photocatalysis and has to be determined very precisely. Therefore, a technical specification under the direction of the CEN Technical Committee 386 "Photocatalysis" Working Group 6 "Light Sources", was elaborated which will be published in the end of the year. In this technical specification, spectral areas for photocatalytic tests are closely defined to reach more comparable photocatalytic efficiencies. Furthermore, two approaches to achieve this recommended irradiation conditions will be presented. (C) 2014 Elsevier BY. All rights reserved.