Infrared Blackbody Spectral Characterization

In order to satisfy the needs with blackbodies infrared calibration and support of the secondary-level blackbody calibration laboratories, a preliminary spectral radiance scale realization has been rundertaken at NIST in the temperature range of 232 °C to 962 °C and spectral range of 2.5 µm to 20 µm. Spectral radiance scale realization process includes the use of Sn, Zn, Al and Ag fixed-point blackbodies (BB), as well as the transfer of the spectral radiance scale to transfer standard BBs based on water, Cs and Na heat pipes. Sources are compared by means of a Fourier Transform Infrared (FTIR) spectrometer, as well as a set of filter radiometers and a radiation pyrometer. Scales of spectral emissivity and radiance temperature of BB can be derived from the spectral radiance scale. The approach to scale realization isillustrated in figure below.

The spectral radiance scale is derived from a set of fixed-point BBs, the design and characterization of which was the first task. Their temperatures are derived from ITS-90. Secondly, the transfer standard pyrometers to maintain and interpolate the temperature scale at particular wavelengths have been developed. The third task was the construction of a spectral comparator consisting of a spectrometer and fore-optics for spectral scale transfer. Finally, a set of variable temperature blackbodies to maintain the scale and interpolate over the temperature range has been built.

A set of fixed-point BBs.

For highest accuracy, variable temperature blackbodies are calibrated in spectral radiance against a pair of fixed-point blackbodies with interchangeable crucibles of In, Sn, and Zn, and Al, Ag, and Cu, respectively. Fixed-point blackbodies spectral emissivity also needs to be accurately characterized. We employ a multi-prong approach: (1) Monte Carlo ray-trace modeling and calculations, (2) hemispherical reflectance measurements of the crucible cavity material flat sample, as well as the cavity itself, (3) direct spectral emittance measurements of the same samples using the facility, and (4) comparison of the fixed point blackbodies with each other as well as with variable temperature heat pipe blackbodies, using filter radiometers and the facility’s Fourier transform spectrometer.

A set of variable-temperature BBs

The Monte Carlo code is used to predict the cavity emissivity with input of the cavity shape and the emissivity and specularity of the cavity material. The reflectance measurements provide emissivity data of both the material and the cavity at room temperature. The results are used to compare with and validate the code results. The direct emittance measurements of the material provide the temperature dependence of the material emittance as code input. The code predicted results for the cavities at their operating temperature (freeze points) is then compared with the relative spectral radiance measurements. Use of this complete set of evaluation tools enables us to obtain the spectral emissivity of the blackbodies with reliably determined uncertainties.

The spectral comparator contains fore-optics and a spectrometer. The fore-optics contains two off-axis aspherical mirrors and has a low level of out-of-field scatter in the visible and IR. Medium resolution Fourier Transform spectrometer BOMEM DA3 is used for relative measurements of spectral radiance.

Two transfer standard pyrometers, RT900 and RT1550, with spectral responsivity centered at 900 nm and 1550 nm, respectively, are used to interpolate the temperature scale between fixed point temperatures, as well as to measure radiance temperature of the customer blackbody.

RT900 Si Transfer Standard Pyrometer

RT1550 InGaAs Transfer Standard Pyrometer

IR Spectral Radiance and Blackbody Spectral Emissivity Scale Realization

Customer BB source calibration procedure

Horizontal/vertical uniformity scan using Transfer Standard Pyrometer
Short term temporal stability measurement with the Transfer Standard Pyrometer.
Radiance temperature measurements at the BB center with the Transfer Standard Pyrometer for each temperature set points of interest.
Spectral radiance measurements at each temperature set point of interest with FTIR comparator
Effective emissivity calculation from measured spectral radiance, using either set point or Pyrometer radiation temperature as a reference temperature

Spectral Radiance Scale Transfer Example:
Al Fixed Point to Na heat-pipe Variable Temperature Blackbody

In the spectral band of 8 µm to 14 µm, the standard deviation of the mean for spectral radiance was typically at the level of 0.1%. In the spectral band from 3 µm to 5 µm, scatter of the results for most temperatures is substantially larger, possibly relating to use of a wide band pyroelectric detector, and needs further attention. The results show no systematic spectral features for the transfer standard blackbodies but reveal some systematic features for one of the customer BBs. The results will be used to optimize the design of the dedicated Advanced Infrared Radiometry and Imaging (AIRI) Facility at NIST.

Publications:
IR spectral characterization of customer blackbody sources: first calibration results,
S. Mekhontsev, M. Noorma, A. Prokhorov, and L. Hanssen,
Thermosense XXVIII, ed. by Jonathan J. Miles, G. Raymond Peacock, and Kathryn M. Knettel, Proc. of SPIE 6205, 620503 (2006).

Emissivity evaluation of fixed-point blackbodies,
S. Mekhontsev, V. Khromchenko, A. Prokhorov, and L. Hanssen,
9th International Symposium on Temperature and Thermal Measurements in Industry and Science (TEMPMEKO 2004), June 22-25, 2004, Dubrovnik, Croatia, Proceedings, Vol. 1, ed. by D. Zvizdic (2004), pp. 581-586.