Citation for published version:
Chatterjee, K, Tuli, S, Pickering, SG & Almond, DP 2012, 'An objective comparison of pulsed, lock-in, and
frequency modulated thermal wave imaging', AIP Conference Proceedings, vol. 1430, pp. 1812-1815.
https://doi.org/10.1063/1.4716431
DOI:
10.1063/1.4716431
Publication date:
2012
Document Version
Publisher's PDF, also known as Version of record
Link to publication
Copyright (2012) American Institute of Physics. This article may be downloaded for personal use only. Any other
use requires prior permission of the author and the American Institute of Physics.
The following article appeared in Chatterjee, K, Tuli, S, Pickering, SG & Almond, DP 2012, 'An objective
comparison of pulsed, lock-in, and frequency modulated thermal wave imaging' AIP Conference Proceedings,
vol 1430, pp. 1812-1815, and may be found at http://link.aip.org/link/doi/10.1063/1.4716431
University of Bath
Alternative formats
If you require this document in an alternative format, please contact:
openaccess@bath.ac.uk
General rights
Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners
and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.
Take down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately
and investigate your claim.
Download date: 09. Aug. 2022
AN OBJECTIVE COMPARISON OF PULSED, LOCK-IN,
AND FREQUENCY MODULATED THERMAL WAVE IMAGING
K. Chatterjee
a
, S. Tuli
a
, S. G. Pickering
b
, and D. P. Almond
b
a
Centre for Applied Research in Electronics,
Indian Institute of Technology, Delhi 110016, India
b
UK Research Centre in NDE, Department of Mechanical Engineering,
University of Bath, Bath BA2 7AY, UK
ABSTRACT. An objective comparison of three different thermal non-destructive evaluation (NDE)
techniques – pulsed thermography (PT), lock-in (LI) thermography, and frequency modulated
thermal wave imaging (FMTWI), has been carried out on a CFRP sample. The matched energy
comparison shows that on the basis of computed SNR, the shallow defects are better detected by
PT, while deeper defects are detected equally by all techniques.
Keywords: Pulsed Thermography, Lock-in Thermography, FMTWI, Matched Energy Comparison
PACS: 81.70.-q
INTRODUCTION
Pulsed [1] and lock-in (LI) [2] thermography are the most commonly used thermo-
graphic nondestructive evaluation techniques. In pulsed thermography, the thermal images
are recorded under pulsed heating of the test sample, often by an optical flash. By contrast,
in LI, the thermal images are captured under periodic heating. The excitation frequency is
chosen based on the diffusion length of the thermal signal. Since LI results suffer from blind
frequency effect [3, 4], it is advised to repeat the experiments at multiple frequencies. The
effort can be shortened by the application of frequency modulated thermal wave imaging
(FMTWI), which is a superposition of multiple LI experiments [5, 6]. Fourier transformation
is extensively used to generate phase and amplitude images from LI and FMTWI videos. Ob-
jective comparison of pulsed, LI, and FMTWI is not easy. A comparison based on computed
signal-to-noise ratio (SNR) under matched energy condition has just been reported by the
authors [4], and is summarized here.
EXPERIMENT
Figure 1a is a drawing of the test piece employed in all tests described in this paper.
It is an approximately 7 mm thick carbon fiber composite board containing artificial defects.
The test piece was painted with acetone soluble black acrylic paint to provide a greater surface
absorptivity and emissivity. The 6 mm diameter defects only are considered for the compari-
son reported in this work.
Figure 1b shows the experimental setup for LI and FMTWI tests. For LI thermogra-
phy, 16.7 mHz, 33.3 mHz, 50.0 mHz, 66.7 mHz, and 83.3 mHz frequencies were used, while
Review of Progress in Quantitative Nondestructive Evaluation
AIP Conf. Proc. 1430, 1812-1815 (2012); doi: 10.1063/1.4716431
© 2012 American Institute of Physics 978-0-7354-1013-8/$30.00
1812
Downloaded 10 Jul 2013 to 138.38.54.59. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://proceedings.aip.org/about/rights_permissions
SECTION A-A
1
1.25
0.25
0.50
0.75
7
Front view
6
6
2
11
11
12.75
4
12.50
R
6
A A
(a) CFRP sample (dimensions in mm).
FUNCTION
GENERATOR
WATER
#1
BATH
WATER
#2
BATH
HEAT
#1
SOURCE
HEAT
#2
SOURCE
OPTICAL
OPTICAL + IR
OPTICAL
OPTICAL + IR
IR CAMERA
AMPLIFIER
POWER
SAMPLE UNDER TEST
(b) Experimental setup
FIGURE 1. Test piece and experimental setup.
a 20 mHz to 80 mHz up chirp was chosen for FMTWI. The heat source outputs were filtered
for infra-red radiation with the help of water bath.
The comparison work was carried out under matched effective excitation energy con-
dition. Since the energy level of the flash lamp used in pulsed experiment could not be varied,
it was used as the reference. In LI and FMTWI, the flood lamps oscillation amplitude was so
controlled that the effective absorbed AC energy was identical to that of the pulsed experi-
ment. It is worth mentioning that the DC part of the heating in LI and FMTWI does not take
part in the calculation of phase and amplitude images. Thus only the effective AC part was
matched to the absorbed pulsed energy.
To achieve this, the pulsed experiment was performed first. The temperature vs. time
data, which exhibits 1/
√
t behaviour on a sound region, was established. The absorbed en-
ergy was estimated from the frame after 1 second of the flash, as describe in the pulsed
section of Table 1, and mathematically equated to the effective energy in LI thermography to
calculate the amplitude of surface temperature oscillation under matched excitation energy
1813
Downloaded 10 Jul 2013 to 138.38.54.59. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://proceedings.aip.org/about/rights_permissions
TABLE 1. Equations governing matched energy comparison for the three thermal NDE techniques.
Pulsed
T
P
= E
P
2e
√
πt E
P
= T
P
t=1
2e
√
π
T
P
: Surface temperature, E
P
: Absorbed energy, t: Time
LI
T
AC
= W
0
/e
√
ω E
LI
= 2W
0
D/π T
AC
= π T
P
t=1
.
D
√
2f
f: Excitation frequency, ω = 2πf , T
AC
: Surface temperature oscillation amplitude,
E
LI
: Total effective absorbed energy in LI, W
0
: Peak incident energy, D: Test duration
FMTWI
E
F M
= 37.9W
0
E
LI
= 38.2W
0
E
F M
: Total effective absorbed energy in FMTWI
condition. The power of the heat sources were then set to achieve this oscillation amplitude.
The LI section of Table 1 summarizes the method.
In FMTWI, the effective energy is almost identical to that of LI, as shown in the
third section of Table 1. Hence the same power settings of the heat sources, as used in LI
experiment, were repeated during FMTWI.
RESULTS
The comparison of pulsed, LI and FMTWI images is summarized in Fig. 2. It shows
that for the shallowest defect, pulsed thermography produces the best signal-to-noise ratio,
and LI and FMTWI magnitude images follow. It is interesting to note that the phase images
produce the worst signal-to-noise ratio. For the deeper 1.25 mm defect, all techniques turn
out to be equally good/bad.
0
20
40
60
80
100
120
140
160
0.25 0.5 0.75 1 1.25
SNR
Defect depth (mm)
Pulsed thermography
Lock-in/FMTWI phase
Lock-in/FMTWI amplitude
FIGURE 2. Plot of SNR as a function of defect depth.
1814
Downloaded 10 Jul 2013 to 138.38.54.59. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://proceedings.aip.org/about/rights_permissions
CONCLUSION
In this paper, three thermographic techniques (viz. pulsed, LI and FMTWI) are com-
pared based on effective excitation energy matching. It is shown that while for shallow de-
fects the pulsed technique provides the best signal-to-noise ratio, its performance decreases
for deeper defects. In the latter case, LI and FMTWI become comparable, if not better.
ACKNOWLEDGMENTS
This work was supported by British Council under its UKIERI programme. It also
formed part of a Naval Research Board, DRDO India supported project and the core research
programme of the UK Research Centre in NDE funded by the Engineering and Physical
Science Research Council.
REFERENCES
1. S. K. Lau, D. P. Almond, and J. M. Milne, NDT&E Int., 24, pp. 195–202, (1991).
2. G. Busse, D. Wu, and W. Karpen, J. Appl. Phys., 71, p. 3962, (1992).
3. S. G. Pickering and D. P. Almond, NDT&E Int., 41, pp. 501–509, (2008).
4. K. Chatterjee, S. Tuli, S. G. Pickering, and D. P. Almond, NDT&E Int., 44, pp. 655–667,
(2011).
5. S. Tuli and R. Mulaveesala, QIRT, 2(1), p. 41, (2005).
6. S. Tuli and R. Mulaveesala, Appl. Phys. Lett., 89, p. 191913, (2006).
1815
Downloaded 10 Jul 2013 to 138.38.54.59. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://proceedings.aip.org/about/rights_permissions
