NIST IR Spectrophotometry: Current Capabilities and Planned Extensions to Support Climate Science

1. NIST IR Spectrophotometry: Current Capabilities and Planned Extensions to Support Climate Science from 2.5 to 100 mm Simon Kaplan Leonard Hanssen Sergey Mekhontsev Climate Change/CLARREO meeting June 12, 2008 NIST, Gaithersburg, MD

2. Outline Overview/CLARREO Requirements Current Capabilities for Transmittance, Reflectance, and Emittance (Indirect) Direct Emittance System Proposed Methods for l>20 mm Conclusions

3. IR Spectrophotometry for CLARREO Possible areas where IR properties of materials data may be needed for design/modelling/calibration of CLARREO-generation radiometers: Blackbody coatings – diffuse, long-wave (l>20 mm). Dependence on angle and temperature, BRDF. E = 0.999, 3 bounces => U < 0.03? Polarization dependence of scan mirrors, FT spectrometer optics? rel. U < 0.5 %?

4. NIST IR Spectrophotometry Capabilities Overview

5. FT-IR Spectrophotometer Systems for Optical Property Characterization Biorad 60-A • l range: 0.8 - 50 µm • highest resolution: 0.5 cm-1 • purge for FT and all instrumentation • high stability and repeatability • external beam to multiple instruments • step-scan mode for specialized operation Bomem DA • l range: 0.2 - 1000 µm • highest resolution: 0.01 cm-1 • internal vacuum / purge, external purge • improved design for stability • versatile, 5 beam ports, source port

6. FT-IR Setup for Transmittance, Reflectance & Emittance Aperture Plate

7. Integrating Sphere for Specular and Diffuse Samples • Specifications • l range: 1.0 - 18 µm • 6 inch diameter • gold-electroplated plasma-sprayed metal • coating • MCT detector w/ concentrator optics • baffling in sphere • 8° incidence angle • Capabilities • Reflectance, Transmittance & Emittance • Temperatures 15 - 200 °C • absolute & relative, specular & diffuse, R, T & E • uncertainties (2s): • specular: ≤ 0.3% • diffuse: 1.5 - 3.5% • larger for angle dependent structure • can measure R of accessories samples • can sort out scatter from total R & T

8. Temperature Dependent Reflectance/Emittance Examples Black Paint (diffuse) SiC polished

9. Goniometer for Variable Angle Transmittance, Reflectance, & Emittance Goniometer: Input Optics on railSample in center on stagesDetector on rail mounted to ring Brewster Angle High Contrast Broadband Polarizer Sample Cryostat for Reflectance and Transmittance (10 - 600 K) NEW:Si:As Detector/Cryostatfor 2 - 28 µm Measurements

10. Goniometer Measurement Examples • Absolute T and R vs angle, temperature, polarization. • New As:Si BIB detector for l out to 28 mm. • Uncertainty (k=2)  0.5 %.

11. Long Wavelength Measurement Examples • For l > 20 mm, currently limited to specular measurements. • Reflectance relative to Au reference mirror. • No angle dependence in reflectance. • Uncertainty  1 to 3 %.

12. Direct Emittance System

13. Critical Elements of Direct Emittance Measurement • Accurate Reference BB temperature • (use fixed point BB’s and reference thermometers) • Knowledge of Reference BB emissivity • (direct & indirect measurements & modeling) • Accurate sample temperature • (integrating sphere & filter radiometer) • FT Spectrometer performance in spectral radiance comparison • (reduce non-linear effects, inter-reflections, etc.)

14. HDR - NIR Sphere Reflectometer Sample heater Integrating Sphere Sample Water-cooled Flange

15. Temperature Method Comparison:Non-Contact vs. Contact • Last column show agreement level of two methods • Table shows effect of convection loss correction • Agreement is very good; better (too good?) than anticipated from uncertainty budgeting • Errors and uncertainties greater for non-specular samples • Hanssen, L. M., C. P. Cagran, A. V. Prokhorov, S. N. Mekhontsev, and V. B. Khromchenko. 2007. “Use of a High-Temperature Integrating Sphere Reflectometer for Surface-Temperature Measurements” Int J. Thermophys. 28(2):566-580.

16. Temperature Dependent Spectral Emittance: 2 Examples Pt-10%Rh SiC

17. Example UncertaintyBudget • Uncertainty for sample reflectance and temperature measurement & FT spectral emittance data • Main contribution from FT repeatability • Uncertainty for Pt-Rh larger than for SiC

18. Proposed Instrumentation for Variable Angle/Diffuse Reflectance for l > 20 mm • Hohlraum Black Sphere Emissometer - center mount: sphere and sample are source - viewed by FTIR • Enables temperature dependent measurements, high efficiency for LWIR • Enables both reflectance and emittance measurements • Allows low etendue restrictions for grazing angle measurements • Functions as a controlled background system • Set up in purge enclosure or CBS3 system • Laser-Based BRDF system – extend current system (HeNe, CO2, etc.) with several lines out to 100 mm using either CO2-pumped FIR gas laser or QCLs.

21. Involves only relative radiance measurements. • Does not require accurate measurements of sample or enclosure temperature. • No assumptions of sample emittance dependence on temperature. • Requires two enclosures for minimizing overall measurement time. • Potentially best method for temperature and angle dependent emittance and reflectance.

23. BRDF Experimental Setup Mirror/Beamsplitter Detector Unit I Sample Detector Unit II Polarizer-analyzer attenuator Half-Wave Plate Tilt Goniometer Sample Rotation Folding mirror Filter Wheel Detector Rotation Black Enclosure Spatial filter Stabilized CO2 Laser System features: Folding mirror • Out-of-plane capability w/ sample tilt & normal rotation • Retro-reflection setup w/ beamsplitter • Set of interchangeable sample mounts • Mueller Matrix w/linear polarizers & retarders • 1.32, 3.39, 0.78 µm, 1 - 5 µm tunable lasers in process to be added (10.6 µm)

24. Conclusions / Comments • NIST currently has IR spectrophotometry facilities covering 1 to 18 mm for high-accuracy reflectance, transmittance, and emittance. • NIST IR BRDF system, including CHILR instrument, operates at several IR laser wavelengths. • Extension to longer wavelengths to support FIR climate science requires: • Construction of new controlled background temperature vacuum chamber with variable temperature black sphere emissometer/reflectometer – 2 to 3 years effort, and/or • Extension of BRDF system to longer wavelength laser lines, e. g. FIR gas lasers – 2 year effort.