Comparison of the Total Solar Irradiance Radiometer Facility Cryogenic Radiometer against the

1. Comparison of the Total Solar Irradiance Radiometer Facility Cryogenic Radiometer against the NIST Primary Optical Watt Radiometer Joseph P. Rice and Allan W. Smith Optical Technology Division National Institute of Standards and Technology (NIST) Gaithersburg, Maryland 20899 Greg A. Kopp, David M. Harber, and Karl F. Heuerman Laboratory for Atmospheric and Space Physics (LASP) University of Colorado Boulder, Colorado 80303 Steven R. Lorentz L-1 Standards and Technology New Windsor, Maryland 21776 Contact: joe.rice@nist.gov

2. Motivation for this Talk and for the Next Talk • There is a well-publicized calibration issue with absolute Total Solar Irradiance • (TSI) measurements: an unexplained 0.37% difference • Aperture area (as discussed on Monday by Jim Butler) not able to explain it, so it • appears to be something about the way the power measurements are being done • Such a problem does not occur with related cryogenic radiometer measurements • Why not apply cryogenic radiometry to solve the problem?

3. Exo-atmospheric Total Solar Irradiance (TSI) Measurements There is an unexplained 0.37% difference between TIM and VIRGO or ACRIM

4. International Intercomparison of Cryogenic Radiometers • Standards labs can measure responsivity of traps to <1 mW laser power to about 0.02% • This was in the late 1990’s, and NIST numbers are from HACR (predecessor to POWR). BIPM report:

5. Liquid Nitrogen Liquid He at 2K Cryogenic Electrical Substitution Radiometry • Thermalized optical laser power is compared to thermalized electrical power in a black cavity • Generally, active cavity radiometers in vacuum at 2 K to 5 K • Primary standard at NIST and in most other industrialized nations for optical power responsivity of transfer detectors such as Si-diode trap detectors • Intercompared internationally via portable transfer detectors at 0.02% (k=2) uncertainty Primary Optical Watt Radiometer (POWR)

6. Intercomparison of Present-Day Standard NIST Cryogenic Radiometers

7. Introduction to the Intercomparison Reported in This Talk • LASP has now developed a facility for pre-flight calibration of TSI Instruments • Total Solar Irradiance (TSI) Radiometer Facility (TRF) • System-level calibration in irradiance mode at TSI irradiance level (68 mW for TIM) • This is the first ever facility capable of this feat at less than 0.1% uncertainty level • Motivated by the need for improved TSI measurement accuracy • Supported by the NASA Glory Project • Used for Glory Total Irradiance Monitor (TIM) (David Harber’s Talk, next) • The irradiance scale is based upon a new cryogenic radiometer: TRF Radiometer • Cryogenic radiometers are in use worldwide and yield the lowest uncertainty • Typical uncertainty of order 0.01% (k=1) (=100 ppm), but only at 2 mW power level • The TRF Radiometer is optimized for 68 mW power level: first of its kind • What is the radiant power scale uncertainty of the TRF Radiometer? • 1. Can be determined from the components, as for any active cavity radiometer AND/OR • 2. Can be assigned based in large part upon transfer from a NIST cryogenic radiometer, such as the NIST Primary Optical Watt Radiometer (POWR) • This talk describes a scale comparison of the TRF Radiometer with the NIST POWR • Result: NIST Correction of TRF native scale by +306 ppm with an uncertainty of 98 ppm (k=1) is required to calibrate it on the NIST POWR scale

8. Translation Stage • Align translation stage so that laser beam enters POWR. • Adjust ½ wave plate to turn power to 2 mW. • Record POWR shuttered power measurements and both Si trap photodiode signals. Experiment Description Part 1 Beam From 532 nm Laser Beamsplitter 2 Trap Photodiode 2 POWR Brewster Window Shutters POWR Trap Photodiode 1 ½ Wave Plate 2 mW Beamsplitter 1 Intensity Stabilizer Spatial Filter Polarizer TRF Radiometer Brewster Window TRF Radiometer

9. Translation Stage • Move translation stage so that laser beam enters TRF Radiometer. • Adjust ½ wave plate to turn power up to 68 mW. • Record TRF Radiometer shuttered power measurements and both Si trap photodiode signals. Experiment Description Part 2 Beam From 532 nm Laser Beamsplitter 2 POWR Trap Photodiode 2 POWR Brewster Window Shutters Trap Photodiode 1 ½ Wave Plate 68 mW Beamsplitter 1 Intensity Stabilizer Spatial Filter Polarizer TRF Radiometer Brewster Window TRF Radiometer

10. Typical Raw Data Trap Photodiode Signals TRF Radiometer

11. Results Corrections Shuttered Laser Power Entering TRF Aperture Based only on POWR (i.e. what TRF should measure) Trap Photodiode Responsivity (1) Trap Photodiode Response (2)

12. Window Transmittance Scans in Air Relative window transmittance at 0 mm position was corrected. TRF Radiometer Window POWR Window

13. Stress-Induced Birefringence Changes Window Reflectance This common effect, though small with the 6 mm thick POWR window, was significant with the 3 mm thick TRF Radiometer window, and was corrected for both. POWR Window Reflectance: Venting from vacuum to atmosphere TRF Radiometer Window Reflectance: Alternating between vacuum and atmosphere

14. Summary • A scale comparison of the NIST POWR and the TRF Radiometer was performed • 532 nm • Radiant power (underfilled apertures), as opposed to irradiance mode (overfilled apertures) • POWR at 2 mW, TRF Radiometer at 68 mW, two trap photodiodes used as transfer • The TRF Radiometer shuttered power measurement reads low by the following amount: 306 ppm +/- 98 ppm (k=1) • The TRF Radiometer native scale used here had not been explicitly corrected for its nonequivalence, cavity reflectance (about 38 ppm), or electrical power scale calibration • Applying the recommended correction above intrinsically corrects for these effects • A detailed report on this comparison is being written for a published journal article We thank the NASA Glory Project for supporting this work.