Instrumentation & Measurement Magazine 25-7 - 24

Table 1 - Stability and Uncertainty archived for two radiometers using two calibration methods
UV RADIODMETER
BLUE RADIODMETER
CALIBRATION
METHODS
Diaphragm
ND filters
Stability STDEV
48.3%
7%
Expanded
Uncertainty (%)
12.8%
9.4%
Stability
STDEV
35.5%
4%
Expanded
Uncertainty (%)
5.9%
2.7%
reliable. In addition, Fig. 4 shows the R chart, and it appeared
that in the case of the diaphragm calibration method (Fig. 4a),
there is more than one indication of an out-of-control condition,
while in the case of the ND-filters calibration method,
there is not. Therefore, the variability of the ND-filters calibration
process is still stable and somehow controlled, and hence,
the precision and accuracy are satisfied.
On the other hand, Fig. 5 demonstrates the STDEV in the
stability of calibration methods, diaphragm and ND-filters
for nine randomly selected experiment days. As noticed from
the measurements, there is a significant difference in the reproducibility
STDEV between the diaphragm and ND-filters
methods. The figure illustrates that the ND-filter calibration
method over the experimental days is more stable and has
fewer disparities than the diaphragm calibration method. In
addition, and as a result, the uncertainty due to repeatability
and hence reproducibility is expected to be improved in the
ND-filters method. Table 1 shows that the diaphragm method's
uncertainty value is higher than the ND-filters method for
the two tested radiometers, approximately between double
and triple in value. It is definitely because the most valuable
and affected source of uncertainty was the disparities in reproducibility
[11].
Concerning the UV radiometers, the uncertainty in measurements
is considerably large compared to any other bands
due to the characteristic low response at the UV band. Briefly,
some of these errors include out-of-band contributions to the
signal, non-ideal geometric properties (non-ideal cosine response
in the detectors), and poor matching to a defined action
spectrum. In addition, UV radiation itself induces aging of the
optical elements of UV detectors over time [12].
The noticed advantage of the ND-filters calibration method
is that personal errors in repeatability and reproducibility are
reduced compared to the diaphragm method. The reason is
simply that it is difficult practically in the diaphragm method
to adjust the same nominal value of irradiance measurements
each day, which is reversely affected on accuracy, and precision
of repeated irradiance values. Consequently, the ND-filters
calibration method exhibits more stability in varying irradiance
levels, because the only controller of the irradiance values
neutrality is the ND-filters transmittance, which changes relatively
after a long time. Therefore, this method can be helpful
for the operators in sites to calibrate any test detectors. As a result,
the tools of the demanded calibration will be decreased
and easy to carry, and it will save time and energy of the operator
in the site
24
Conclusion
Two common radiometer calibration methods are compared in
this study: the ND-filters and diaphragm methods. As crucial
as the periodic calibration of devices is, the need to reconsider
which of the utilized methods is more stable and reliable in the
long term appears. Consequently, this study aims to compare
and verify which of these methods is more stable. The results
indicate that the ND-filters calibration method exhibits more
stability, i.e., achieving stabilized accuracy and precision of
repeated irradiance values. The stability STDEV for the NDfilter
method is around one-eighth of the other method. In
addition, the estimated uncertainty value is less than the diaphragm
method, approximately between half and one-third
the value. Consequently, as an advantage, this method can be
more helpful for the operators to use to calibrate any test detectors
due to the ability to control and reduce personal errors in
repeatability and reproducibility as much as possible. Hence,
it will ease the on-site calibration process, saving the operator
time and energy.
References
[1] C. Wyatt, Radiometric Calibration: Theory and Methods, 1st ed.
Amsterdam, The Netherlands: Elsevier Inc., 1978.
[2] International vocabulary of metrology-Basic and general concepts and
associated terms (VIM), 3rd ed. JCGM 200:2012, Joint Committee
For Guides In Metrology, 2012.
[3] ISO 3534-2:2006 Statistics-Vocabulary and symbols-Part 2: Applied
statistics, p. 125, International Organization for Standardization
(ISO), 2006.
[4] F. Pai, T. Yeh, and Y. Hung, " Analysis on accuracy of bias, linearity
and stability of measurement system in ball screw processes by
simulation, " Sustainability, vol. 7, pp. 15464-15486, 2015.
[5] C. Wu, W.Pearn, and S. Kotz, " An overview of theory and practice
on process capability indices for quality assurance, " Int. J. Prod.
Econ., vol. 117, no. 2, pp. 338-359, 2009.
[6] E. Jarošová, " Control charts for processes with an inherent
between-sample variation, " STATISTIKA, vol. 98, no. 2, pp.
150-160, 2018.
[7] A. Thompson and H. M. Chen, " Beamcon III, a linearity
measurement instrument for optical detectors, " J. Res. Natl. Inst.
Stand. Technol., vol. 99, no. 6, p. 751, 1994.
[8] C. Johnson, H. Yoon, J. Rice, and A. Parr, " Principles of optical
radiometry and measurement uncertainty, " Experimental Methods
in the Physical Sciences vol. 47, pp. 13-37, 2014.
[9] D. Montgomery, Statistical Quality Control: A Modern Introduction,
7th ed. Hoboken, New Jersey: Wiley, 2012.
IEEE Instrumentation & Measurement Magazine
October 2022

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