The Optical Radiation Measurement (ORM) group of the National Physical Laboratory is responsible for the development of the UK's primary radiometric quantities. One of its principle aims in recent years has been to improve the accuracy of "user" measurements through improved traceability routes to the SI system of units. The Earth Observation (EO) community is arguably the most demanding, requiring specialist techniques and transfer standards.
To improve accuracy, NPL has developed a range of lamp illuminated integrating spheres, called Transfer Standard Absolute Radiance Source (TSARS). The TSARS is calibrated directly against the NPL primary blackbody source with uncertainties <±0.75% which can be maintained and confirmed following transportation, through detector stabilisation. To demonstrate the performance of NPL's new TSARS, it was taken to TNO TPD in Delft for inclusion in the radiometric characterisation of the Global Ozone Monitoring Experiment (GOME 2) - FM3.
Overall agreement to <± 1% with both TNO TPD and NASA was achieved, demonstrating consistency in international radiometric scales, since the TNO and NASA sources were calibrated traceable to NIST. It also demonstrates the practicality of achieving calibrations with uncertainties <±1%, and since the NPL TSARS is portable, allows similar levels of uncertainty in "field situations" e.g. field spectrometers, aircraft radiometers.
NPL, in conjunction with many NMIs, has been seeking to improve the accuracy of its primary scales of spectral irradiance and radiance. In common with other laboratories this has been done through the use of an ultra-high temperature blackbody characterized using filter radiometers calibrated against a cryogenic raediometer. While such work is of importance to the Earth Observation community, it is also recognized that of at least equal importance is an improvement in the quality of the scales that are provided to the end user. This paper will describe new transfer standard sources of both spectral radiance and iradiance that have been developed not only to improve accuracy to the end user, but also to provide it in a form that is both robust and convenient to use. For example the radiance source has a spatial non-uniformity of <0.05% over a 50 mm diameter aperture and can maintain its accuracy for more than 100 hrs of operation.