The high uncertainty of calibration is the most serious factor limiting accurate measurements of the solar UV, needed for the assessment of global UV trends. In this study, the calibration of FEL-type standard lamps traceable to three primary standard laboratories were compared with a transfer uncertainty of ¿1.4% (2&sgr;). In comparison with the reference lamp, the spectral UV irradiance scales agreed within 1.5%. However, the difference between a new lamp and the present reference was 2%, which is close to the limits of total uncertainty. It was interesting to observe that one of the scales, based on a cryogenic absolute radiometer, was in good agreement with the scales based on blackbody radiation sources. Examination of the long-term stability of the lamp-based UV scale in Finland showed a significant decrease of 6% in the scale when the standard lamp was changed to a lamp directly traceable to the primary standard of the National Institute of Standards and Technology (NIST). Previously, the lamp was traceable to the NIST through the secondary standard of Optronic Laboratories Inc. The main obstacle in improving the global conformity of UV scales is the instability of halogen standard lamps. On the basis of the present study, it is strongly recommended to use detector stabilization of the standard lamps and to investigate the use of portable detector standards in the intercomparisons between primary standards laboratories. ¿ 2000 American Geophysical Union