The Optical Properties measurements laboratory at The USA National Institute of Standards & Technology (NIST), a part of the Optical Technology Division of the PHYSICS Laboratory has been developing a full spectral emissivity (emittance) measurement capability.

The laboratory has established high accuracy infrared reflectance and transmittance capabilities for wavelengths between 1 µm and 18 µm. Near normal absolute spectral reflectance and transmittance of both specular and diffuse samples can be measured from near ambient to 200 °C using a custom integrating sphere and Fourier transform (FT) spectrometer. Additional capabilities for specular samples include transmittance down to 10 K using an optical cryostat, as well as variable angle transmittance and reflectance using a custom goniometer and polarizers.

_{Layout of the NIST Setup for Direct and Indirect Infrared Spectral Emittance Measurements}

Spectral directional emittance can be determined indirectly form reflectance and transmittance measurements described on the NIST Page: Infrared Spectrophotometry. These capabilities have limits of temperature, measurement geometry and sample type.

To expand the spectral emittance capabilities, a separate facility has been developed for its measurement at NIST using the direct method of radiance comparison of the sample with a blackbody(BB) reference source.

The facility consists of a set of reference blackbody sources mounted on a motorized stage for selection; interchangeable sample heater/mounts on motorized translation and rotation stages; a removable visible/near-infrared integrating sphere for measuring the sample temperature above 500 K; and low scatter interface optics to image the 3 mm to 5 mm central region of the sample or BB source onto a water cooled field stop.

Each BB contains calibrated platinum resistance thermometer (PRT) or thermocouple (TC) temperature sensors.

The spectral emissivities of the BBs have been calculated using a Monte Carlo ray tracing algorithm with input of the measured spectral reflectance of the cavity wall materials or coatings.

_{Integrating sphere for non-contact temperature measurement with sample heater in place}

_{Heaters for Transparent (Left) and Opaque (Center) Samples; Heater (up to 600 °C) and Set of Samples for Emittance Measurements (Right)}

The sample emittance is determined through a series of measurement steps.

The first step is a measurement of the sample’s hemispherical-directional reflectance at the measurement temperature and at a single wavelength matched to the filter radiometer.

The second step is a relative radiance measurement of the sample to a BB at the same wavelength.

The third step is to compare the sample spectral radiance to that of the reference blackbody source as a ratio with the FTIR .

_{Three Steps of Spectral Directional Emittance Scale Realization}

Finally, here’s a few results:

_{Spectral emittance of SiC and Pt10Rh samples}.

References

• Infrared spectral emissivity characterization facility at NIST,
  L.M. Hanssen, S.N. Mekhontsev, and V.B. Khromchenko,Proc. SPIE 5405, 112 (2004).
• Temperature- and angle-resolved infrared spectral directional emissivity of SiC, Alumina, and Pt for temperatures up to 1000 °C, C.P. Cagran, L.M. Hanssen, M. Noorma, and S.N. Mekhontsev,Intl. J. Thermophysical Prop. (submitted 2006).
• Use of a high temperature reflectometer for surface temperature measurements,
  L.M. Hanssen, M. Noorma, A.V. Prokhorov, S.N. Mekhontsev, and C.P. Cagran, Intl. J. Thermophysical Prop. (submitted 2006).

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