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 difficulties arise in part from the stability and performance of the optical components required to make the measurements, such as sources, detectors, filters and diffusers, as well as from the relative intensity of the UV radiation compared with the visible background. A particular application of note is the need to measure changes in the level of terrestrial solar UVB radiation. This is required to assess the effects of ozone depletion and the potentially damaging impact on the biological ecosystem. To have a meaningful global measurement programme, uncertainties of around 1% to 2% are required. However, this is less than or close to the quoted uncertainties of primary UV spectral irradiance scales maintained by national standards laboratories. This paper describes work being undertaken at the National Physical Laboratory (NPL) to improve the measurement of UV radiation, in terms of establishing both primary spectral emission scales and improved transfer standards. Most of this work is centred around improving the design and characterization of filter radiometers. The paper presents details on the performance of the components used to construct the filter radiometers, as well as that of the complete devices. Calibration of the complete filter radiometers has been carried out using radiation from both monochromators and tunable lasers, and a comparison of the results obtained by the two methods is presented.