An overview is given of the results of an intercomparison of spectral emission scales of three national standards laboratories in the air-ultraviolet. The measurement scales agreed within combined uncertainties, but the uncertainties associated with the measurements on the transfer standards used were higher than many in the wider user community appreciate or, indeed, require. Primary measurement scales can usually be achieved with a much lower uncertainty than can be assigned to the calibration of the transfer standards in common use. This is due to a combination of the performance of the transfer standards themselves and the difficulty in transferring the scale between two very different types of source. Where the level of uncertainty which can be achieved under strictly controlled conditions in a national standards laboratory is insufficient to meet the requirements, and sometimes the claims, of the wider measurement community, it is fairly obvious that further work is required. An overview is also presented of the ongoing work at the National Physical Laboratory to improve the uncertainty associated with ultraviolet measurement scares in the UK, the agreement between measurements internationally and the range of services offered.
