Irradiance Calibration Using a Cryogenic Radiometer and a Broadband Light Source

Size: px

Start display at page:

Download "Irradiance Calibration Using a Cryogenic Radiometer and a Broadband Light Source"

Lora King
6 years ago
Views:

1 Irradiance Calibration Using a Cryogenic Radiometer and a Broadband Light Source Jeff Morrill (1), Donald McMullin (2), Linton Floyd (3), Steven Lorentz (4), and Clarence Korendyke (1) (1) Naval Research Laboratory, Washington DC (2) Space Systems Research Corp., Alex. VA (3) Interferometrics Inc., Herndon VA (4) L1 Standards and Technology, New Windsor MD 1

2 Outline The Problem with the TSI Database. NIST/NASA TSI Workshop July, Calibration Methods. Cryogenic Radiometer and a Broadband Source. Laboratory Facility at NRL Conclusions 2

3 The Problem 1375 Total Solar Irradiance Database 1370 TSI [W/m 2 ] Year Butler et al.,

4 TSI Workshop at NIST NASA Sponsored, NIST Hosted TSI Workshop July Goals: Identify/assess potential sources of differences. Recommend measurements and algorithms to address these differences PI s from all major TSI instruments attended. ERBE, ACRIM, VIRGO, DIARAD, PMO, TIM Reviewed potential instrumental causes 4

5 Instrumental Design Differences Cavity Coating Diffuse vs Specular Black - Probably not Order of FOV and Limiting Baffles * Effects scattering and diffraction More work Effects of Baffle Edges Effects scattering and diffraction More work Difference in Data Collection and Analysis Frequency vs Time Domain Analysis Long-Term Degradation Multiple Cavity Techniques 5

6 Potential Sources of Errors Aperture Area Not likely based on recent results Diffraction Needs to be treated Scattering Needs to be treated Thermal Background Due to illuminated shutter Power Application TIM vs all others Power Demodulation TIM vs all others Joule Heating Reason cryogenic radiometers are used in laboratory studies. 6

7 Calibration Methods TURF at LASP (Univ. of CO) Entrance aperture filled with laser beam rastered in spiral pattern. High power cryogenic radiometer used as calibration standard. Uncertainty: Illumination not identical to the Sun. 7

8 Calibration Methods Table Mountain (JPL) Large vacuum chamber houses several instruments Sun as a (broadband) source illuminates instruments simultaneously Uncertainty: Transmission of window varies and bright sky as background. 8

9 One Solution To The Problem Question: Can we help reduce or remove the discrepancy in the database with a different calibration technique? Goal: Produce source and environment for calibration that is closer to the observational conditions. Broad band source in vacuum Overfill aperture with 1AU divergent beam Measure beam with cryogenic radiometer 9

10 Differences Between Collimated White-Light and Monochromatic Laser Illumination Solar Irradiance is neither perfectly collimated nor of a single wavelength Illumination with a Broad Band Source with a spectrum and divergence similar to the Sun may provide an improved calibration of TSI monitors. Lasers are more easily stabilized and significant effort will be applied to this issue in development of a broad band source. 10

11 Differences Between Collimated and Monochromatic Illumination Ray Trace Results ACRIM SORCE/TIM

12 Cryogenic Radiometer with a Broadband Source 11

13 Laboratory Facility at NRL Building A-13: Clean Room and Mechanical Grey Room Baffled Optical Beam line Light Source and Collimating Optics Vacuum Bell Jar and optical table Cryogenic Radiometer 12

14 Optical Beam Line 13

15 Vacuum Bell Jar and Optical Table 14

16 High Power Cryogenic Radiometer Manufactured by L-1 Standards and Technology (similar to LASP unit) Removable cavity: EUV (< 200 nm) and UV / Vis /IR (> 200nm) Power sensitivity 65 mw rather than the ~ 10 mw of earlier models 15

17 Current Light Sources and Collimators White Light Source EUV Source Collimator Aligned with Optical Beam Line Entrance Collimator without Cover 16

18 Alignment Procedures Using a variety of alignment tools the light sources, collimators, and cryogenic radiometer will be aligned to the optical axis of the vacuum beam line and bell jar 17

19 Future Light Source and Collimator 18

20 High Power Light Source 19

21 Properties of High Power Light Source Intensity Distribution Along Arc Spectrum Close-up of Arc Lamp 20

22 Conclusions Using a high power cryogenic radiometer and a broadband source should provide additional information to help resolve the current discrepancy between various TSI data sets. Near Solar illumination in vacuum will better replicate conditions of space based observations. Cryogenic radiometer will ensure highly accurate calibrations. Lamp stability like to be main difficulty. 21

ACRIM3 Characterization by the LASP/TRF and the Total Solar Irradiance Database

ACRIM3 Characterization by the LASP/TRF and the Total Solar Irradiance Database ACRIM3 SCIENCE TEAM Dr. Richard C. Willson ACRIM Principal Investigator ACRIMSAT PROJECT Sandy Kwan JPL Project Manager Dr.