Das Projekt "Traceability for surface spectral solar ultraviolet radiation: Spectral Irradiance Traceability" wird vom Umweltbundesamt gefördert und von Physikalisch-Technische Bundesanstalt durchgeführt. The aim of the research is to shorten the traceability chain of the solar UV measurements to the SI unit and to reduce the associated transfer uncertainties. The goal is to approach uncertainties in the field comparable to those currently achieved only for primary spectral irradiance scale realisations at NMI level, i.e., at the level of 1Prozent. Traceability of the solar UV irradiance measurements in regional and national UV monitoring networks to the SI units has to be ensured with the uncertainties of 1Prozent. This is needed in order to be able to track long-term changes in surface solar UV irradiance due to changes in the total ozone column, aerosols and cloud cover. Presently, the traceability of spectral solar UV irradiance measurements in Europe is assessed using a chain of fragile source-based transfer standards and a transportable reference spectroradiometer. Since every successive step in the source-based traceability chain introduces an additional uncertainty contribution, the state-of-the-art uncertainty of the source-based calibration of the spectroradiometer under field conditions is at the level of 4Prozent, which does not meet present-day requirements.