Here - PMOD/WRC

filters with 4⋅ 10 -6 nm/K thermal drift, and a light weight
carbon fiber frame with a very low coefficient of axial
expansion. The RT1550 is also equipped with a
water-cooled lens mount for additional temperature
stabilization. The total length of each pyrometer is 550 mm,
and they have an integrated electronic module with a
single USB connection to a laptop computer that contains
the control software.
Calibration of pyrometers
Fixed-point BBs at the freezing temperatures of Sn, Zn, Al,
and Ag have been used as reference standard sources of
spectral radiance. In addition, a Au fixed point has been
used to verify the calibration of an RT900. As an example,
the freezing plateau of the Zn is shown in Fig. 4.
The defining equation of ITS-90, which is based on
Planck’s radiation law, has been used with a RT900
pyrometer to compare the calibrations with Al, Ag, and Au
fixed points. The deviations, shown in Table 1 for an
RT900, are calculated relative to the calibration at the Ag
fixed point.
Temperature, ºC
419.5275
419.5270
419.5265
419.5260
419.5255
dedicated to the spectral radiance characterization of IR
radiation sources as well as to the maintenance and
dissemination of the radiance temperature scale below the
silver point.
This new facility will employ two different fixed-point
BB designs with different crucible geometry and furnace
designs: a traditional NIST design 6 as well as more a
recent one 7 . We have performed a first round of
comparisons of these two different BBs, which indicates
the existence of a small but statistically significant
difference in radiance temperatures. A consistent offset is
found even in the case of using the same furnace with
different crucibles.
One possible cause of such a systematic offset in
measurement with two different furnaces is the
out-of-field-scatter (SSE) that is very dependent on the
furnace design. However, the SSE correction cannot be
easily applied because of the partially obscured
observation conditions of many features of the radiation
field. Because of this, we have decided to take an approach
originally suggested by Jones and Tapping 8 and
demonstrated in Fig. 5 below. A fixed-point blackbody is
used in a regular configuration. The only difference is that
in place of the metal filled crucible we use an empty one
that has had the rear half of the cavity removed. In this
case the ideal field of view of the pyrometer is totally filled
with a highly absorbing cone attached to BB rear flange.
419.5250
0:00:00 1:00:00 2:00:00 3:00:00 4:00:00
Time
Figure 4. Freezing plateau of Zn measured with an RT1550.
An RT1550 has been calibrated with Sn, Zn, Al, and Ag
fixed points. Next, the Sakuma-Hattori interpolation
equation 3 has been fitted with the measurement results
using a nonlinear fitting software 4 . The deviations, shown
in Table 1 for the RT1550, demonstrate the difference
between the defined fixed-point temperatures and the
temperatures achieved with the Sakuma-Hattori
interpolation equation.
Table 1. Evaluation Results of Pyrometers
Fixed ITS-90 freezing Deviation from ITS-90, mK
point temperature, C RT900 RT1550
Sn 231.928 n/a 3
Zn 419.527 n/a 23
Al 660.323 -18 -13
Ag 961.780 0 44
Au 1064.18 25 n/a
We do not have sufficient history to fully account for long
term stability, but after round-trip transportation to a
customer in California and one year of use, the RT1500
and RT900 agreed to within 0.1 ºC.
Discussion
Pyrometer development was performed in the framework
of a project to build a customer facility for IR spectral
emissivity calibration 5 . Currently both pyrometer models
are being incorporated into a new NIST facility that will be
Figure 5. Out-of-field scatter experiment schematic
The results of the crucible comparison and out-of-field
scatter contribution for both furnaces should be available
by the time of the conference.
Acknowledgments The authors would like to thank Peter
Saunders (MSL, New Zeeland) for providing the nonlinear fitting
software used for the calculation of Sakuma-Hattori parameters,
and Howard Yoon (NIST) for his advice on lens selection.
References
1. Preston-Thomas, H., The ITS-90,” Metrologia 27, 3-10, 1990.
2. Ohtsuka M., Bedford R.E., Measurement of size-of-source
effects in an optical pyrometer, Measurement 7, 2-6, 1988.
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scales of radiation thermometers, in Proc. TEMPMEKO 1997,
305-310, 1997.
4. Saunders, P., White, D. R., Physical basis of interpolation
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2003.
5. Hanssen, L.M., Mekhontsev, S.N., and Khromchenko, V.B.,
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Proc. SPIE 5405, 1-12, 2004.
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