One of the more difficult tasks in absolute solar spectroradiometry is to reliably transfer the laboratory irradiance scale, based usually on measurements of calibrated 1000 W quartz-halogen lamps, to outdoor measurements of solar radiation. The two main problems in that respect are firstly the large intensity gradient of the solar ultraviolet radiation, which covers more than 6 orders of magnitude between 290 and 400 nm, and secondly the practical aspects of calibrating a spectroradiometer in the field and ensuring its stability over days and sometimes weeks.The EU-founded project QASUME (Quality Assurance of Spectral Ultraviolet Measurements in Europe through the development of a transportable unit) was launched in December 2001. It aims at providing Quality Assurance to solar UV measurements conducted presently by spectroradiometers operating in Europe by establishing a reliable transportable spectroradiometer system which can be transported to any UV monitoring site in Europe and provide co-located measurements with the local site instrument.Since the travelling unit was designed to measure at locations far removed from its laboratory, a portable irradiance scale was devised. It is composed of a travelling dark room (called calibrator from now on), acurrent unit and a set of 100 and 250 W tungsten halogen lamps. Small power lamps were selected as radiation sources instead of FEL lamps firstly because of the size of the calibrator which was designed for a small heat load, and secondly because the construction of the 100 and 250 W lamps should make them less vulnerable to transportation.The travelling lamps are calibrated in the ECUV laboratory within the calibrator using the travelling spectroradiometer. The travelling lamp set is composed of six 100 W lamps and three 250 W lamps. In the field, usually two to three lamps are used to establish a reliable calibration of the spectroradiometer, once at the beginning and once at the end of a campaign. In case of disagreement, additional lamps can be added to the calibration set.The system has been used in more than 15 field campaigns in 2002, and 2003. Its performance as a whole has been very satisfactory; none of the lamps broke or had to be discarded. Lamp output changes as small as 0.5% could be diagnosed and remedied by using additional lamps.To further investigate the performance of the travelling calibrator, a direct intercomparison between the irradiance scale of the portable calibrator and the primary reference held at Physikalisch-Technische Bundesanstalt (PTB), Braunschweig is foreseen for June 2004. The whole travelling system, including the spectroradiometer system and the portable calibrator will measure the calibrated black body operated at PTB. This intercomparison will investigate the uncertainties associated with the travelling irradiance scale due to the several calibration transfers needed from the primary standard (black body) to the field standard (250 W). Spectral solar ultraviolet radiation is usually measured outdoors using double grating spectrophotometers. The calibration of these instruments is normally performed in a dark room facility of the respective laboratory and not at the site of the measurement. Therefore some means of assessing the calibration status of the instrument after it has been transported to the measurement site is necessary. This task is usually performed using small to medium power Tungsten-Halogen lamps in custom-made holders. Therefore the uncertainties of solar UV measurements are in part affected by the rather long chain of transfer standards from the primary standard of a national metrology institution to the solar UV measurement performed in the field. This project aims in reducing this long chain of calibration transfer and to decrease the uncertainties due to transportation of the transfer standards, which are usually used for this purpose. Within the shared-cost project QASUME (Quality Assurance of Solar Ultraviolet Measurements in Europe through the development of a transportable Unit), a transportable spectrometer system used in conjunction with a portable dark room facility has been developed to provide reference solar UV irradiance measurements to European solar UV monitoring stations.One task of this project will be to select the most reliable type of lamp to use for the transfer of the irradiance scale from the primary standard of PTB to the field spectrometer taking into account stability under transportation as well as lamp intensity. A second task will be to study the long term stability of this system during its application under actual field conditions during its usage within QASUME. Finally, the possibility of using the whole spectrometer system with dark room facility for the dissemination of the irradiance scale to UV monitoring laboratories in Europe will be investigated.
Progress report submitted in May 2004.