New procedure for interpolating NIST FEL lamp irradiances

The Shuttle Solar Backscatter Ultraviolet (SSBUV) programme uses 1000 W, quartz-halogen, tungsten coiled ® lament (FEL) lamps as the primary radiometric calibration source in the 250 nm to 405 nm wavelength region. The National Institute for Standards and Technology (NIST) recommends using the product of a ® fth-order polynomial and a Planck function as a ® tting function for interpolating the NIST-calibrated FEL lamp irradiances to users’ wavelengths. The NIST ® tting procedure can result in more than a 1% overshoot in some data sets. This problem is related to the unconstrained properties of the ® fth-order polynomial. Overshooting during lamp irradiance interpolation gives rise to corresponding errors in the SSBUV calibration and solar spectral irradiance measurements. Errors of this magnitude are unacceptably large for the SSBUV, and we have therefore developed a new interpolation scheme. A new ® tting procedure has been developed to replace the NIST procedure in the SSBUV calibrations. Although it uses fewer ® tting parameters, the new ® tting function provides a better ® t than the NIST ® tting function does. The overshoot problems experienced with the NIST procedure do not exist in the newly interpolated lamp irradiances. We have also investigated the properties of the NIST FEL lamp interpolation scheme in the visible and near-infrared ranges, ® nding that the overshoot problem experienced in the ultraviolet extends into these regions, with potential errors of the order of 0.5% to 1%.