Comparison of Calibration Methods for Radiation Thermomemeter ...

Comparison of Calibration Methods for Radiation Thermomemeter below the
Freezing Point of Copper at NIMT
P. Wongnut 1,2 , T. Keawprasert 1 , U. Norranim 1
1 National Institute of Metrology Thailand, 3/4-5, Klong 5, Klong Luang, Pathumthani 12120, Thailand
2 To whom correspondence should be addressed. E-mail: phichet@nimt.or.th
NIMT has established the calibration facility for
radiation thermometers in the temperature range from
o
o
157 C to 1085 C by comparison with fixed-points. To
determine the radiance temperature by integration
scale, a double monochromator-based facility has
been set up for the relative spectral responsivity
calibration over the region from 250 to 1200 nm. Both
calibration methods use Sakuma-Hattori equation to
fit the measurement results. The spectral integration
temperature scale is compared with the radiance scale
obtained by using the fixed-point blackbody sources
INTRODUCTION
At National Institute of Metrology Thailand (NIMT),
the calibration of standard radiation thermometers was
normally based on a series of fixed-point blackbody
sources with the temperature range from the freezing
point of indium (156.5985 o C) to copper (1084.62 o C),
without their spectral responsivity measurement [1].
Recently a monochromator facility has been set
up to calibrate the spectral responsivity of standard
radiation thermometers of NIMT. Such a 0.65 μm
standard radiation thermometer was calibrated using a
copper point blackbody source in combination with a
spectral responsivity measurement and a non-linearity
measurement, and then it can be used to realize the
ITS-90 scale up to 2,500 o C.
Likely, in case of a 0.9 μm standard radiation
thermometer and a 1.6 μm standard radiation
thermometer, it is able to determine the independent
scale with the spectral responsivity calibration to
verify the scale from the multi fixed-points method
[2]. In this article, both of calibration methods are
performed for the 0.9 μm standard radiation
thermometer to compare the integration scale with the
multi fixed-point temperature scale at the fixed-point
temperatures in the range from Zn point to Cu point.
CALIBRATION METHODS
Fixed-Point Method
A 0.9 μm standard radiation thermometer is
calibrated with the fixed-points from zinc (419.527◦C)
to copper (1084.62◦C). Each fixed-point blackbody
has an aperture of 6 mm diameter and an effective
emissivity of 0.9995±0.0005.
All calibration data points of each artefact are
fitted with the Plank version of Sakuma-Hattori
equation. The residual errors at any fixed-points for
all radiation thermometers are less than ±5 mK.
Spectral Responsivity Method
The double-grating monochromator with 250 focal
length covers the spectral range from 250 nm to
1200 nm by using a diffraction grating with 1200
lines/mm with a 750 nm blaze wavelength. A
mercury lamp is used to calibrate and initialize the
wavelength scale of the monochromator. Therefore,
its accuracy in the wavelength scale is better than
0.1 nm. A standard Si photo diode, which absolute
spectral irradiance is traceable to NMIJ, is directly
compared to the radiation thermometer.
For measurements, the spectral bandwidth of 1.0
nm is performed in the spectral bandpass of the
radiation thermometer, and 10 nm in the blocking
region. Calibration results of all standard radiation
thermometers are shown in Fig. 1. The fine structure
is investigated around the peak of spectral
responsivity with the spectral bandwidth of 0.1 nm.
Then an independent radiance temperature scale
is realized by integrating the spectral responsivity
R(λ) multiplied by the Planck function L(λ, T ),
∫
V ( T ) = a L(
λ,
T ) R(
λ)
dλ
where the coefficient a was determined from signal
at the silver or copper point. And the Sakuma-
Hattori equation is used as the calibration relation.
Relative Spectral 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
400 500 600 700 800 900 1000 1100 1200
Wavelength / nm
650 nm radiation thermometer
900 nm radiation thermometer
Figure 1. Relative spectral responsivity of the standard
radiation thermometers of NIMT.

DISCUSSION
The combined uncertainty (k = 1) due to the fixedpoint
blackbodies excluding drift is now 80 mK at Zn
point to 100 mK at Cu point. Other sources of error
due to the radiation thermometer such as size of
source effect, nonlinearity and gain ratio are not
described in this article.
In the measurements of spectral responsivity,
relative integral errors in the bandpass are less than
10 -4 for both 0.65 μm and 0.9 μm thermometers.
Additionally the out-of-band responsivities of all
thermometers are approximately 10 -5 . From the
investigations in the fine structure, any interference
effects are not found with the spectral bandwidth of
0.1 nm. That shows that all standard radiation
thermometers have excellent optics systems which
can eliminate the interference error. The relative
uncertainties are 0.4 % and 0.5% for 0.65 μm and 0.9
μm radiation thermometer respectively. The main
component of uncertainty is due to the standard photo
diode. However, with this relative uncertainty level,
the uncertainty in temperature of the integration scale
down to the Zn point is in the same level of the fixedpoint
scale. More details for the uncertainty
calculation is described in [3].
For 0.9 μm radiation thermometer, the differences
of radiance temperature with combined uncertainty
between the fixed-point scale and the integral scale at
the fixed-point temperatures are shown in Fig.2.
While the same difference at the Ag point is 66 mK
for 0.65 μm radiation thermometer.
spectral responsivity. A 0.9 μm standard radiation
thermometer is calibrated with fixed-points of Zn,
Al, Ag and Cu to perform its temperature scale. To
verify the scale, an independent scale obtained from
the integration of relative spectral responsivity and
Planck’s law is realized. The maximum difference
of both temperature scales is 0.71 ± 0.26 o C at the
fixed-point temperature of Zinc.
REFERENCES
[1] F. Sakuma. and M. Kobayashi, in Proceeding of.
TEMPMEKO 1997, 305-310, 1997.
[2] H.J.Jung, in Proceedings of TEMPMEKO 1996, 235-
244, 1997.
[3] J. Hartmann and L.Werner, Int J Thermophys,
DOI 10.1007/s10765-008-0383-3.
0.8
Difference from fixed-point scale
0.4
0.0
-0.4
-0.8
-1.2
400 600 800 1000 1200
Fixed-point temperature / o C
Figure 2. Differences of temperature at the 4 fixed-points
CONCLUSION
The double monochromator is set up to perform the
spectral responsivity calibration from 250 nm to 1200
nm for standard radiation thermometers. A 0.65 μm
standard radiation thermometer is calibrated with Ag
or Cu fixed-point blackbody to extend its calibration
range up to 2500 o C, combination with its relative