Determination of deuterium lamp irradiance drift in the SSBUV laboratory

National Institute of Standards and Technology (NIST)-traceable deuterium a re (D-2) lamps are used in the Shuttle Solar Backscatter Ultraviolet (SSBUV ) programme as radiometric standards in the 200 nm to 250 nm wavelength reg ion. These lamps are inherently less stable than the quartz-halogen tungste n filament (FEL) lamps used in the programme for calibration at longer wave lengths. In order to monitor changes in the D-2 lamp output, the SSBUV D-2 lamp irradiance measurements have been compared with FEL lamp irradiance me asurements. Techniques have been developed to reduce noise in the SSBUV mea surements and to analyse the stability of the FEL lamps. The D-2 lamp irrad iance drifts have been found to be as large as -13% at 200 nm relative to 2 50 nm over a period of about seven years. These independently-derived D-2 l amp irradiance drifts agree with the NIST measurements of the SSBUV lamps, confirming the long-term stability of the NIST working standards for lamp i rradiance in the 200 nm to 250 nm wavelength region. The D-2 lamp irradianc es used for the SSBUV calibrations are those reported by the NIST, correcte d for the lamp drift at the time of the instrument calibration. These deriv ed D-2 lamp irradiances not only fill long-term gaps for which no updated N IST 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 impr oving the accuracy of the SSBUV instrument's calibration.