Intercomparison of lamp and detector-based UV-irradiance scales for solar UV radiometry

The high uncertainty of calibration is the most serious factor limiting acc urate measurements of the solar UV, needed for the assessment of global UV trends. In this study, the calibration of EEL-type standard lamps traceable to three primary standard laboratories were compared with a transfer uncer tainty of +/-1.4% (2 sigma). In comparison with the reference lamp, the spe ctral UV irradiance scales agreed within 1.5%. However, the difference betw een a new lamp and the present reference was 2%, which is close to the limi ts of total uncertainty. It was interesting to observe that one of the scal es, based on a cryogenic absolute radiometer, was in good agreement with th e scales based on blackbody radiation sources. Examination of the long-term stability of the lamp-based UV scale in Finland showed a significant decre ase of 6% in the scale when the standard lamp was changed to a lamp directl y traceable to the primary standard of the National Institute of Standards and Technology (NIST). Previously, the lamp was traceable to the NIST throu gh the secondary standard of Optronic Laboratories Inc. The main obstacle i n improving the global conformity of UV scales is the instability of haloge n standard lamps. On the basis of the present study, it is strongly recomme nded to use detector stabilization of the standard lamps and to investigate the use of portable detector standards in the intercomparisons between pri mary standards laboratories.