Determination of deuterium lamp irradiance drift in the SSBUV laboratory

Abstract

National Institute of Standards and Technology (NIST)-traceable deuterium arc (D2) lamps are used in the Shuttle Solar Backscatter Ultraviolet (SSBUV) programme as radiometric standards in the 200 nm to 250 nm wavelength region. These lamps are inherently less stable than the quartz-halogen tungsten filament (FEL) lamps used in the programme for calibration at longer wavelengths. In order to monitor changes in the D2 lamp output, the SSBUV D2 lamp irradiance measurements have been compared with FEL lamp irradiance measurements. Techniques have been developed to reduce noise in the SSBUV measurements and to analyse the stability of the FEL lamps. The D2 lamp irradiance drifts have been found to be as large as -13% at 200 nm relative to 250 nm over a period of about seven years. These independently-derived D2 lamp irradiance drifts agree with the NIST measurements of the SSBUV lamps, confirming the long-term stability of the NIST working standards for lamp irradiance in the 200 nm to 250 nm wavelength region. The D2 lamp irradiances used for the SSBUV calibrations are those reported by the NIST, corrected for the lamp drift at the time of the instrument calibration. These derived D2 lamp irradiances not only fill long-term gaps for which no updated NIST calibrations are available, but also allow any short-term changes to be traced in situ from a NIST calibration to the user's laboratory, thus improving the accuracy of the SSBUV instrument's calibration.