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Transfer standard pyrometers for radiance temperature measurements below the freezing
temperature of silver at NIST
M. Noorma 1,2 , S. Mekhontsev 1 , V. Khromchenko 1 , A. Gura 1 , M. Litorja 1 , B. Tsai 1 , and L. Hanssen 1
1 National Institute of Standards and Technology (NIST), Gaithersburg MD, USA
2 Metrology Research Institute, Helsinki University of Technology (TKK), Espoo, Finland
Abstract. New transfer standard pyrometers operating at
900 nm and 1.55 µm have been designed, characterized,
and calibrated with defined fixed points of ITS-90. The
pyrometers are optimized for radiance temperature
measurements in the range between the freezing
temperatures of Sn (231.928 ºC) and Ag (961.78 ºC). The
calibrations at different fixed points demonstrate good
agreement. The size of source correction for a source with
40 mm diameter has been measured to be as low as 0.01 %.
These instruments feature a compact and ruggedized
design. The pyrometers may be used to interpolate,
maintain and disseminate radiance temperature scales as
well as for inter-laboratory comparisons.
Introduction
According to the ITS-90, the temperature scale between
the triple point of equilibrium hydrogen and the freezing
point of silver is defined by means of a platinum resistance
thermometer (PRT) calibrated at a specified set of fixed
points, and use of specified functions for interpolation at
intervening temperatures 1 . Thus blackbodies (BB) with
known emissivity equipped with a calibrated PRT can be
used as reference standards for radiometric temperature
measurements.
Another approach for radiometric temperature
measurements, which is used by several national
metrology laboratories including NIST, is based on
fixed-point BB sources with their temperature assigned
according to ITS-90, but relies on accurate pyrometers for
interpolation to other temperatures. This eliminates errors
due potential temperature differences between the PRT and
radiating cavity.
In both cases, high-accuracy transfer standard
radiometers are needed for interpolation of the radiance
temperatures scale and its transfer from the reference
sources to customer BBs.
Figure 1. Transfer standard pyrometer RT1550
Relative responsivity
1.E+01
1.E+00
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
1.E-06
1.E-07
400 600 800 1000 1200 1400 1600 1800
Wavelength, nm
Figure 2. Relative spectral responsivities of an RT900 and an
RT1550
Pyrometers
Two first generation pyrometers, “RT900,” were built at
NIST in 2003. They are based on a highly linear silicon
photodiode and a bandpass filter and can be used above
500 ºC. After substantial modification of both the optical
unit and control software, in 2005 we built two second
generation pyrometers, “RT1550,” (shown in Fig. 1) with
an InGaAs detector and a bandpass filter at 1.55 µm, and
which can be used down to 200 ºC. The relative spectral
responsivities of the pyrometers are shown in Fig. 2.
Both radiometers have a 3 mm spot size at 500 mm
from the front plane, and an AR-coated GRIN lens with a
47 mm optical diameter. The size of source effect (SSE)
has been studied with a special source with a central
obscuration 2 with results shown in Fig. 3.
Other features include a built-in laser diode for
backward tracing of the axial beam, a field stop made of
SSE
1.6E-04
1.4E-04
1.2E-04
1.0E-04
8.0E-05
6.0E-05
4.0E-05
2.0E-05
0.0E+00
RT1550
RT900
0 10 20 30 40 50
Source diameter, mm
Figure 3. SSE of an RT900 and an RT1550 as a function of
source diameter.
polished Ni-plated Invar, non-hygroscopic refractory oxide
Proceedings NEWRAD, 17-19 October 2005, Davos, Switzerland 313