Characterization of the VIIRS Blackbody Emittance Jeremy Kloepfer, Chris Taylor, and Vijay Murgai jeremiah_kloepfer@raytheon.com Conference on Characterization and Radiometric Calibration for Remote Sensing, Utah State University, Space Dynamics Laboratory, August, 2013 Copyright 2013 Raytheon Company. All rights reserved. Customer Success Is Our Mission is a registered trademark of Raytheon Company.
Abstract High emissivity is a key performance parameter for blackbody calibrators, reducing calibration sensitivity to the thermal background and blackbody emittance non-uniformity. The Visible Infrared Imaging Radiometer Suite (VIIRS) blackbody element on the Suomi National Polar-orbiting Partnership (NPP) satellite and the upcoming Joint Polar-orbiting Satellite System (JPSS) have an emittance requirement greater than 0.996. In order to achieve a high value over a large aperture (~12.5 cm diameter), we combine a low reflectance specular coating with an innovative repeating v-groove cavity design. The emittance is then characterized with a custom laser test station that measures the Total Integrated Reflectance (TIR) from the blackbody. Measurements show that the resulting emissivity is greater than 0.997 at 3.39 µm. Details on the experimental setup and data analysis and results are presented herein and discuss the drivers and uncertainties that go into testing a high emittance design. 8/18/2013 2
VIIRS Overview Visible Infrared Imaging Radiometer Suite (VIIRS) Instrument Specifications Visible/Near IR Bands 9 plus day/night pan band Mid-Wave IR Bands 8 Long-Wave IR Bands 4 Imaging Optics 19.1 cm aperture, 114 cm f.l. Orbit Average Power 200 Watts Weight 275 kg Scanned Swath Ground Sample Distance ±56, 3000 km 8/18/2013 <1.6 km @ end of scan 3
VIIRS Blackbody VIIRS Blackbody 8/18/2013 4
Blackbody Optical Requirements Requirement Description Compliance Emittance Emittance Characterization Particulate Contamination Directional Reflectance Characterization The design of the Blackbody source shall have an effective emittance greater than 0.996 for the MWIR and LWIR spectral bands. The emittance shall be characterized to an uncertainty less than ± 0.002. The minimum emittance and emittance knowledge requirement shall be met over a cleanliness level of CL400 and cleaner. The directional illumination to integrated scatter shall be characterized at 5 collecting solid angles corresponding to 1/2 cone angles of 90, 70, 55, 40, and 20 degrees. Pass Pass Pass Pass 8/18/2013 5
Equations: Blackbody Emittance Determined from Reflectance of Gold Standard Parameters: E BB blackbody emittance R BB blackbody reflectance R Au Au mirror reflectance from spectrophotometer tests V BB blackbody signal from TIR test (in volts) V Au Au mirror reference signal from TIR test (in volts) σ one sigma standard deviation (measurement uncertainty) 8/18/2013 6
Test Setup Gold Reference Mirror in Gimbal Mount Laser Variable Aperture Laser Beam Entrance Aperture 2 Inch Collecting Aperture (Front Port) Beam Splitter 0.629 µm / 3.39 µm LASER Z Beam Chopper Variable Aperture Integrating Sphere (Gold) 10 Mounting Plate on Translation Stage Blackbody Assembly (or Witness Sample) Stability Monitor Detector (InSb) Primary Detector (InSb) Geometry not to scale 8/18/2013 7
Test Setup 8/18/2013 8
Measurement Locations 16 15 14 13 12 11 Peak: 10 9 8 7 6 5 4 3 2 1 Au Mirror Reference Scan: Background Scan: Au reference scans At a set Z distance Made during test Background scans Made at start and end of test Blackbody scans: 3 elevations, forward and reverse 5 Z distances Approximate Clear Aperture 8/18/2013 9
Reflectance Measurement Data Results for Several Solid Angles 0.007 11 10 9 8 0.006 Peak 0.005 0.004 171 SA cn 0.003 0.002 0.001 Valley 160 SA cn 151 SA cn 110 SA cn 40 SA cn 0 0 0.5 1 1.5 2 2.5 3 3.5 Position (in) Plot of calculated blackbody total integrated reflectance versus position Each line an average of 6 scans at different solid angle collects Light poorly trapped at tops of peaks so reflectance spikes Expected fall off in signal at smaller solid angles 8/18/2013 10
Reflectance Measurement Data Reflectance Uniformity 0.007 0.006 0.005 High side flat Peak: 16 15 14 13 12 11 10 9 0.004 0.003 0.002 0.001 Cntr Hi Lo 0 0 1 2 3 4 5 6 7 Position (in) Plot of calculated blackbody total integrated reflectance versus position Each line collected at same solid angle but different location Very good signal uniformity across entire blackbody 8/18/2013 11
Emittance Results R BB = 0.0018 E BB = 0.9982 σ BB = 0.0004 Estimated blackbody emittance is 0.9982 at 3.39 µm Based on a fit of the average total integrated reflectance at each SA Uncertainty a roll-up of several test uncertainties including: Blackbody signal variation Au signal variation and reflectance knowledge Test station alignment uncertainties that reduce half cone angle knowledge 8/18/2013 12
Emittance at Other Wavelengths Band Center λ (µm) M12 3.7 I4 3.74 M13 4.05 M14 8.55 M15 10.8 M16 12 I5 11.45 Detailed optical raytrace model was constructed to calculate the emittance at other wavelengths and lower cleanliness levels Model was correlated with specular and total reflectance and scatter measurements at other wavelengths to ground and optimize predictions Results show emittance >0.997 across all bands of interest At 8.55 µm, the wavelength with lowest emittance, CL400 contamination drops the emittance to 0.9968 8/18/2013 13
Summary Summary VIIRS on-board blackbody calibrator requires emittance >0.996 A custom laser test station was constructed to measure the total integrated reflectance at 3.39 µm Measurements show uniform emittance exceeding 0.998 across the blackbody Simulated emittance at other wavelengths >0.997 8/18/2013 14