Characterization and Calibration of a Fourier Transform Spectroradiometer for Solar UV Irradiance Measurements

Transcription

1 EMRP-ENV03 Solar UV WP 3: Improvement of Reference Spectroradiometers Characterization and Calibration of a Fourier Transform Spectroradiometer for Solar UV Irradiance Measurements Peter Meindl, Christian Monte, Martin Wähmer Physikalisch-Technische Bundesanstalt (PTB) Braunschweig & Berlin, Germany The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.

2 Portable reference spectroradiometer system QASUME Quality Assurance of Spectral UV Measurements in Europe (QASUME): Bentham DM-150 double grating monochromator measures the spectrum sequentially several minutes of scanning time (difficult in the case of fast varying atmospheric conditions) on-site wavelength calibration using spectral emission lines (e.g. mercury) and extraterrestrial spectra Photomultiplier tube nonlinear response instability of spectral responsivity (on-site calibrator necessary) Uncertainty for solar UV irradiance measurements: 4,6 % to 8,8 % (k=2) depending on wavelength and solar zenith angle dominated by radiometric calibration and wavelength uncertainty for l< 310 nm Appl. Opt. 44, 2005,

3 Motivation: Why using an FTS as a reference instrument? High wavenumber accuracy and wavenumbers are inherently traced to SI by built-in HeNe laser with low uncertainty No on-site wavenumber calibration necessary! Low uncertainty contribution to solar UV measurements (esp. for l < 310 nm)! High throughput circular aperture has larger area compared to linear slits no diffraction losses to higher-order spectra Semiconductor detectors usable instead of photomultiplier tube? FTS covers broad spectral ranges with high resolution, and all wavenumbers are measured simultaneously Faster than grating spectroradiometer? Instrumental distortions are often accurately calculable and correctable S.P.Davis, M.C.Abrams and J.W.Brault: Fourier Transform Spectrometry (Academic Press, 2001). Herres, Gronholz: Understanding FT-IR Data Processing, Part 1-3. Comp. Appl. Lab. 2 (1984), p.216.

4 Concept of FTS spectroradiometer Commercially available FTS Bruker Vertex80v Global entrance optics CMS Schreder, Austria Semiconductor detectors GaP diode (Bruker) Si diode (Bruker) Si diode (Hamamatsu S8552) Bruker FTS Vertex80v Global entrance optics for irradiance measurements (CMS Schreder, Austria)

5 Wavelength traceability to SI fixed Mirror movable Mirror 10 I I(X) I(X) X 5 10

6 Determination of interferogram sampling positions fixed Mirror l vac = nm HeNe Laser movable Mirror l vac I HeNe (X) I(X) Detector 2 X interferogram sampling positions

7 Wavelength traceability to SI Deviations of the measured mercury peak wavenumbers from the literature data and the deviation of the measured HeNe wavenumber from the SI value. normal air pressure FTS evacuated Refractive index of standard air (15 C, hpa, 450 ppm CO 2, 0 % humidity) Mercury spectrum: Applied Optics Vol. 35 (1), 1996, Physica Scripta, Vol. 63, 2001,

8 Wavelength uncertainty Wavelength uncertainty of the Bruker VERTEX 80v in case of a calibration using an external HeNe laser and FTS usage at normal air pressure (i.e. standard operation) wavelength uncertainty contribution 500 nm 333 nm 250 nm wavenumber calibration 2.5 pm 1.1 pm 0.6 pm dispersion 2.5 pm 2.8 pm 5.0 pm large aperture 3.0 pm 1.3 pm 0.8 pm combined uncertainty 4.6 pm 3.3 pm 5.1 pm calibration using the internal HeNe laser and normal air pressure Wavelength scale of the FTS: combined uncertainty 5.4 pm 3.8 pm 5.4 pm direct traceable to the SI calibration using an external no HeNe on-site laser recalibration and or wavelength check necessary FTS evacuated wavelength uncertainty 5 pm in the range 250 nm to 500 nm combined uncertainty 3.9 pm 1.7 pm 1.0 pm well below 50 pm (demanded in JRP ENV03)

9 SNR for non-calibrated solar UV radiation measurements Source: Sun, 23.July 2013 Resolution: 10 cm -1 Average of 512 interfrerograms in about 300 s Results: FTS usable from 300 nm to 500 nm Steep increase of solar irradiance around 300 nm is not resolvable GaP detector gives a better SNR compared to Si detectors

10 Detector characteristics com DigiTect_Detectors.pdf

11 Calibration of spectral irradiance responsivity Spectral irradiance?

12 Calibration of spectral irradiance responsivity Gold fixed-point Black Body Radiator (ITS90) Radiation Thermometer LP3 High Temperature Black Body Radiator BB3200pg + aperture Cryogenic radiometer Si-trap detector + aperture Filter Radiometer Blackbody BB3200pg + aperture Spectroradiometer Spectral irradiance standard FTS PTB-FB 4.1 Photometry and Applied Radiometry FTS QASUME

13 SNR for FTS calibration at high-temperature black body Source: HTBB at T =2795 K Resolution: 10 cm -1 Results: Calibration possible from 360 nm to 500 nm GaP detector gives a better SNR compared to Si detectors

14 Spectral irradiance responsivity of FTS + GEO Spectral irradiance responsivity of the FTS with global entrance optics for different types of semiconductor detectors. Calibration against HTBB at T = 2795 K.

15 Relative difference of spectral responsivity Comparison of spectral irradiance calibrations Relative difference of two calibrations against: HTBB at T =2795 K and Secondary spectral irradiance standard (tungsten halogen lamp) Detector: GaP, Resolution: 10 cm -1

16 SNR for calibration measurements Calibration against: HTBB at T =2795 K and Secondary spectral irradiance standard (tungsten halogen lamp) Detector: GaP, Resolution: 10 cm -1

17 Relative difference of spectral responsivity Stability of spectral irradiance calibrations Relative difference of two calibrations against the secondary spectral irradiance standard (tungsten halogen lamp) time interval 1 month Detector: GaP, Resolution: 10 cm -1

18 Solar UV irradiance

19 Results Wavelength scale of the FTS is traceable to SI via built-in HeNe laser wavelength uncertainty: 5 pm (250 nm to 500 nm) FTS + GaP/Si + GEO + measurement times equivalent to QASUME Solar UV measurements possible from 300 nm to 500 nm Calibration of spectral irradiance responsivity with HTBB at 2795 K is limited to 360 nm to 500 nm Solar UV irradiance measurements limited to 360 nm to 500 nm Performance of QASUME is not reached with FTS + GaP/Si detectors + GEO Advantage of stable spectral responsivity of GaP/Si detectors can not be applied

20 Outlook Improvements: Calibration against HTBB at higher temperatures (> 3000 K) Application of a channel photomultiplier with high dynamic range and up to 3 orders of magnitude higher detectivity Wavelength scale traceable to SI, measurements probably faster than QASUME with high dynamic range but: on-site calibrator probably needed as with QASUME Optimization of the global entrance optics Evaluation of uncertainty budgets for the spectral irradiance calibration and for solar UV irradiance measurements with FTS Comparison measurements of the FTS and QASUME