Evaluating the performance of filter radiometers as a means of improving the uncertainty of ultraviolet measurements

Improving the accuracy of the measurement of ultraviolet (UV) radiation is one of the most challenging problems facing radiometrists today. The diffic ulties arise in part from the stability and performance of the optical comp onents required to make the measurements, such as sources, detectors, filte rs and diffusers, as well as from the relative intensity of the UV radiatio n compared with the visible background. A particular application of note is the need to measure changes in the level of terrestrial solar UVB radiatio n. This is required to assess the effects of ozone depletion and the potent ially damaging impact on the biological ecosystem. To have a meaningful glo bal measurement programme, uncertainties of around 1% to 2% are required. H owever, this is less than or close to the quoted uncertainties of primary U V spectral irradiance scales maintained by national standards laboratories. This paper describes work being undertaken at the National Physical Labora tory (NPL) to improve the measurement of UV radiation, in terms of establis hing both primary spectral emission scales and improved transfer standards. Most of this work is centred around improving the design and characterizat ion of filter radiometers. The paper presents details on the performance of the components used to construct the filter radiometers, as well as that o f the complete devices. Calibration of the complete filter radiometers has been carried out using radiation from both monochromators and tunable laser s, and a comparison of the results obtained by the two methods is presented .