Optical Property Characterization of Far IR Materials Critical for CLARREO Mission Support

The Climate Absolute Radiance and Refractivity Observatory (CLARREO) and other climate science and remote sensing applications require accurate thermal and far infrared (IR) spectral radiance measurements, which are dependent on reference blackbody sources that have high and well-characterized spectral emissivity. The design and performance of blackbody sources are strongly affected by the spectral emittance of available coatings. Hence the emittance or reflectance of such coatings need to be accurately characterized. The measurement metrology for the reflectance of near-ambient targets in the thermal IR spectral range (2.5 to 15 microns) is well established and validated. The equivalent metrology for the far IR spectral range (15 to 50 microns and longer) is currently being established at National Measurement Institutes (NMIs), such as the National Institute of Standards and Technology (NIST) and at the Physikalisch-Technische Bundesanstalt (PTB), but will take several years to become operational and validated. Other existing facilities, such as at Surface Optics Corporation (SOC), do not have well characterized uncertainty budgets and are not traceable to the SI, but may be useful to provide support of the R&D for CLARREO. The direct measurement of the spectral emittance can also be used to obtain or validate reflectance data, but until cryo-vacuum facilities become fully functional at an NMI, such measurements are possible only for targets at elevated temperatures (typically above 200 °C). This limitation prevents the characterization of several critically important coatings. This paper describes ongoing efforts to establish the uncertainty of reflectance measurements at the SOC calibration facility over the far IR spectral range. For this purpose, NIST and PTB emittance facilities are used to measure the spectral emittance of transfer standard targets at 200 °C at wavelengths from 10 to 100 microns. Subsequently, these samples are measured at the SOC facility at the same temperature as well as at ambient. As a result of this effort, a variable angle HDR capability in the required spectral and temperature range with known uncertainty will become available for immediate support of the engineering (development) of CLARREO subsystems and transfer standards.