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Vol. 60, No. 10 / 1 April 2021 / Applied Optics

Hybrid optical imaging with near-infrared,
mid-infrared, and terahertz wavelengths for
nondestructive inspection [Invited]
Yu Tokizane,1 Hikaru Ejiri,2 Takeo Minamikawa,1
1,
AND Takeshi Yasui *Safumi Suzuki,3 Masahiro Asada,41Institute of Post-LED Photonics (pLED), Tokushima University, 2-1 Minami-Josanjima, Tokushima, Tokushima 770-8506, Japan
Faculty of Science and Technology, Tokushima University, 2-1 Minami-Josanjima, Tokushima, Tokushima 770-8506, Japan
3
Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8552, Japan
4
Institute of Innovative Research, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8552, Japan*Corresponding author: yasui.takeshi@tokushima-u.ac.jp
2

Received 16 November 2020; revised 2 February 2021; accepted 2 February 2021; posted 17 February 2021 (Doc. ID 415131);published 8 March 2021

Optical imaging is a powerful tool for nondestructive inspection, with high spatial resolution and low invasiveness.
As light–material interactions vary a great deal depending on the wavelength, it is difficult to select the best imaging wavelength without prior knowledge of the optical properties of the material. To overcome this difficulty, weconstructed a hybrid optical imaging system using three different wavelengths: near-infrared (NIR), mid-infrared(MIR), and terahertz (THz) regions. The same imaging optics were integrated with different light sources anddetectors. Depending on the light–material interaction and detection sensitivity, NIR and THz imaging indicatedsome potential for nondestructive inspection, but MIR imaging showed difficulty. A combination of NIR and THz
imaging will be a powerful tool for optical nondestructive inspection. © 2021 Optical Society of America
https://doi.org/10.1364/AO.415131

1. INTRODUCTION
Nondestructive inspection [1] is a powerful tool for examining
defects in industrial products, ensuring their safety and reliability. Conventionally, x rays [2,3] and ultrasound [4,5] have been
widely used for nondestructive inspection. X rays have excellent
material penetration but high invasiveness to the human body,leading to health problems, because its high photon energy(typically, 0.1 ∼ 100 keV) causes photoionization of cells. Also,
the excessively high transmission often hampers obtaining
good image contrast in nonmetal materials such as plastics,
rubber, and ceramics. On the other hand, ultrasound has good
penetration and low invasiveness; however, ultrasound testing
cannot be used for remote sensing due to the large attenuation ofultrasound in air. Also, the limited spatial resolution is anotherproblem with ultrasound imaging.
Optical imaging is another modality for nondestructive
inspection with low invasiveness and high spatial resolution,
covering a wide range of wavelengths from visible to infrared(IR), and terahertz (THz) waves. Invasiveness of these wavelengths is significantly lower than that of x-ray because of
low photon energy: typically, >1000 meV for visible light,
10–1000 meV for infrared light, and 1–10 meV for THz waves.
Since materials have a wide variety of interactions with opticalwaves depending on the wavelength, the probing capability and1559-128X/21/10B100-06 Journal © 2021 Optical Society of America

penetration power differ depending on the wavelength. Also,
the spatial resolution depends on the wavelength due to thediffraction limit. Unfortunately, as visible light has low penetration power due to large absorption and/or scattering regardlessof the submicrometer spatial resolution, its use for nondestructive inspection is limited to the surfaces of industrial products.
Infrared light has sensitivity to molecular vibrations [6] as well as
better penetration power than the visible light. More specifically,near-infrared light (NIR light, wavelength = 0.8–2.5 µm) is
sensitive to the overtones of molecular vibrations, and benefits
from the availability of mature lights sources and detectors. A
combination of NIR imaging and spectroscopic analysis is used
for nondestructive analysis of biological materials [7] and foods[8]. Mid-infrared light (MIR light, wavelength = 2.5–30 µm)
is more sensitive to molecular vibrations due to its fundamental
tones and interacts with the rich MIR spectral fingerprints ofmaterials. Therefore, MIR imaging has been used for examining
drug–polymer blends [9] and biological tissues [10]. Morerecently, THz waves have appeared as a new imaging modality
for nondestructive inspection due to the moderate penetrationin nonmetal, nonpolar materials, low invasiveness, and unique
THz spectral fingerprints [11,12]. THz imaging has beeneffectively applied for nondestructive inspection of reinforced

Research Article
plastics [13], ceramics [14], wood [15], illegal drugs [16], andcultural heritage [17].
Conventionally, imaging systems in NIR, MIR, and THz
regions have been well established as an independent systemand have been individually employed for investigation of
objects with known optical properties. However, if a material
has entirely unknown optical properties, it is not necessarily
so easy to select the suitable imaging wavelength. To guess thesuitable imaging wavelength prior to the practical imaging, ahybrid optical imaging system capable of covering wavelengths
from NIR to MIR and THz regions would be a powerful tool.
Based on the guess, one can select the practical imaging systemin the NIR, MIR, or THz region. However, there have been noattempts to develop such a hybrid optical imaging system. In
this article, we have integrated NIR imaging, MIR imaging, andTHz imaging systems to construct a hybrid NIR/MIR/THz
imaging system by using the same imaging optics with different light sources and detectors. We then demonstrated its
effectiveness in nondestructive inspection of nonmetal objects.
2. EXPERIMENTAL SETUP
Figure 1 shows the experimental setup of the hybrid
NIR/MIR/THz imaging system. We prepared three kindsof light sources with three largely different wavelengths: an
NIR external cavity laser diode (NIR-ECLD; RIO, Santa
Clara, CA, U.S.; Colorado, RIO-03235-5-04-0, wavelength= 1527.6–1565.6 nm, power = 20 mW), an MIR quantum
cascade laser (MIR-QCL; DRS Daylight Solutions, San Diego,
CA, U.S.; MIRCAT-QT-2100, wavelength = 6–11 µm, power<300 mW), and a THz resonant tunneling diode [18,19](THz-RTD; wavelength = 698 µm, frequency = 0.430 THz,
power = 2.3 µW). The laser beam used for imaging was selectedby flip mirrors (FMs). The selected laser beam was focused
onto a sample with an off-axis parabolic mirror (OAPM1;
focal length = 75 mm), and was then collimated by anotheroff-axis parabolic mirror (OAPM2; focal length = 75 mm)
after passing through the sample. We prepared three kinds ofdetectors with sensitivities to different wavelengths: an NIR
InGaAs-photodetector (NIR-PD; Electro-Optics Technology,
Inc., Traverse, MI, U.S.; ET-3020, wavelength sensitivity =950–1650 nm, frequency response <2.5 MHz), an MIR thermal power meter (MIR-PM; Thorlabs Inc., Newton, NJ, U.S.;
S425C, wavelength sensitivity = 190 nm–20 µm, response time

Fig. 1. Experimental setup. NIR-ECLD, NIR external cavity laser
diode; MIR-QCL, MIR quantum cascade laser; THz-RTD, THz
resonant tunneling diode; FMs, flip mirrors; OAPM1 and OAPM2,
off-axis parabolic mirrors; NIR-PD, NIR InGaAs photodetector;
MIR-PM, MIR thermal power meter; THz-PE, THz pyroelectricdetector; LIA, lock-in amplifier.

Vol. 60, No. 10 / 1 April 2021 / Applied Optics

B101= 0.6 s), and a THz pyroelectric detector (THz-PE; PHLUXi,
Inc., Harumi, Tokyo, Japan; PYD-1, wavelength sensitivity= NIR–300 µm, frequency response <300 Hz). A lock-in
amplifier (LIA; NF Corporation, Yokohama, Japan; LI5640,
frequency range <100 kHz) was adopted to achieve good asignal-to-noise ratio (SNR) in THz imaging and NIR imaging.
Unfortunately, the slow response of the MIR-PM prevented us
from adopting lock-in detection in MIR imaging. For imaging
measurement, the lateral position of the sample was linearlyraster-scanned by using a translational stage (SURUGA SEIKI
Co., Ltd., Shizuoka, Shizuoka, Japan; K161-85, travel length =80 mm, travel step = 2 µm).
3. RESULTS
A. Imaging Performance

To evaluate the spatial resolution of the hybrid NIR/MIR/THz
imaging system, we perform knife-edge measurements usinga razor blade at three different wavelengths: λNIR = 1.6 µm
for NIR imaging, λMIR = 6.3 µm for MIR imaging, andλTHz = 698 µm for THz imaging. Figure 2 shows the resultof the knife-edge measurements in (a) NIR imaging, (b) MIR
imaging, and (c) THz imaging. The dependence of the knifeedge profile on the wavelength can be clearly confirmed. From
curve fitting analysis with an error function, we determined
the focal spot size: 43 µm for NIR imaging, 94 µm for MIR
imaging, and 1104 µm for THz imaging, as shown in Fig. 2(d).
Table 1 shows the comparison of focal spot size between theexperimental and theoretical values in NIR, MIR, and THz
imaging. The experimental value of the spatial resolution was
in moderate agreement with the theoretical value, indicating
imaging performance close to the diffraction limit. There isroom for improvement of spatial resolution by using an opticalbeam with a larger diameter and/or an OAPM1 with a shorter
focal length.
We next investigated a dynamic range (DR) of optical intensity measurement in NIR, MIR, and THz imaging, which
is defined as a ratio of a mean of signal to that of noise. The
resulting DR was 5177 in NIR imaging (LIA time constant =100 ms), 55 in MIR imaging (number of signal accumulation= 100), and 68 in THz imaging (LIA time constant = 100 ms).
We also investigated an SNR of optical intensity in the same
system. When SNR is defined as a ratio of a mean to a standard
deviation of signal, the resulting SNR was 3976 in NIR imaging(LIA time constant = 100 ms), 341 in MIR imaging (numberof signal accumulations = 100), and 81 in THz imaging (LIA
time constant = 100 ms). Although the averaging effect of theacquired signal was different among those imaging systems,
moderate DR and SNR were achieved. The difference of imaging performance in the hybrid NIR/MIR/THz imaging system
is also summarized in Table 1.
To highlight the difference in imaging performance among
different wavelengths, we visualized a region of a metal bladein the optical chopper [see red square region in Fig. 3(a)].
Figures 3(b)–3(d) show transmission images of the metal bladeat λNIR = 1.6 µm, λMIR = 6.3 µm, and λTHz = 698 µm,
respectively. The image region was 5.2 mm by 5.2 mm, corresponding to 100 pixels by 100 pixels. Therefore, the pixel
interval (= 52 µm) is almost the same as the focus spot size inB102

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Fig. 2. Knife-edge measurement. Knife-edge profile in (a) NIR imaging, (b) MIR imaging, and (c) THz imaging; (d) comparison of spatial resolution among NIR, MIR, and THz imaging.

Table 1. Imaging Performance of the Hybrid
NIR/MIR/THz Imaging System

Wavelength (µm)
Spatial resolution Theoretical(µm)
Experimental
DR
SNR

NIR
Imaging

MIR
Imaging

THz
Imaging

1.6
22
44
5177
3976

6.3
29
94
55
341

698
1554
1104
68
81

the NIR imaging, whereas the pixel interval is smaller than thefocus spot size in MIR and THz imaging. The NIR image andMIR image show the clear shape of the metal blade with high
SNR and similar spatial resolution; the similar spatial resolution
between the images well reflects the results in Figs. 2(a) and 2(b).
On the other hand, the shape of the metal blade was blurred in
THz imaging due to the larger beam waist; however, a moderate
SNR was obtained.
B. Internal Structure of Prepaid Transportation Card

Fig. 3. Hybrid optical imaging of a metal blade in an optical chopper. (a) Optical photograph, (b) NIR image, (c) MIR image, and(d) THz image.

The first sample was a prepaid transportation card (size =54 mm by 85 mm, thickness = 0.75 mm) because nondestructive inspection of plastic products with internal structure is apotential application of our hybrid NIR/MIR/THz imaging
system for nondestructive inspection. We performed hybrid
imaging of the card sample. Figure 4(a) shows an NIR image(image size = 126 pixels by 402 pixels, image acquisition time
= 11,614 s). Internal metal structures were visualized withhigh spatial resolution despite the considerably low transmission (typically, transmittance of −20 to −30 dB), benefittingfrom the high sensitivity of the NIR-PD. We confirmed that asquare-shaped antenna extended along the card edges and wasconnected with an IC chip. However, the shadows of the surface
printed areas significantly overlapped the image of the internal
structures, hampering clear visualization of the internal structure. We acquired no images in MIR imaging due to insufficienttransmission (not shown). Figure 4(b) shows a THz image(image size = 112 pixels by 154 pixels, image acquisition time =5,959 s). In this case, the internal structure of the antenna andintegrated circuit chip was well visualized without the influenceof surface printed areas. Although the spatial resolution wasResearch Article

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Thus, THz imaging is the only option for nondestructive
inspection of bulky resin materials.
D. Internal Break in Power Cable

Detection of void defects is often required in nondestructive
inspection of bulky resin material. To investigate the suitabilityof our hybrid NIR/MIR/THz imaging system for this application, we prepared a black acrylic cube (thickness = 4 mm)with an internal cylindrical hole (hole length = 3 mm, holediameter = 8 mm). Figure 5(a) shows an NIR image (image size
= 20 mm by 20 mm, corresponding to 125 pixels by 125 pixels,
image acquisition time = 1,513 s). The NIR image showed noimage contrast of air voids because the signal level was below thedetection limit in the imaging system, implying strong absorption and/or scattering in the black bulky sample. We acquired
no images in MIR imaging (not shown). Figure 5(b) shows a
THz image (image size = 20 mm by 20 mm, corresponding
to 40 pixels by 40 pixels, image acquisition time = 492 s). The
high transmittance of the THz beam in resin material enabled
us to clearly visualize the details of a single hole in the sample.

Power cables are an important electric infrastructure in daily
life, and internal breaks directly lead to stoppage of the power
supply. To find and repair internal breaks, visual inspection has
been widely used; in this case, an inspector finds a part where
an internal break or corrosion is suspected, opens the cablecoating mechanically, checks the internal metal wire, and covers
the wire with a new cable coating after repairing it. However,in this method, the suspected power cable has to be opened
whether or not it needs to be repaired. Opening the cable, as
well as incomplete covering with the cable coating, may lead tosecondary corrosion of the repaired wire due to residual moisture in the cable. Also, such repair work always involves the riskof an electric shock accident during maintenance. Therefore,
there is a considerable need for nondestructive inspection ofinternal breaks in power cables. If polyvinyl chloride (PVC)
and polyethylene (PE) used for cable coatings have moderate transmission to NIR, MIR, or a THz beam, our hybrid
NIR/MIR/THz imaging system may be a promising candidate
for such nondestructive inspection.
To evaluate this, for the samples, we used two usual insulated cables, one covered with a black coating and one covered
with a white coating (coating diameter = 3.1 mm, diameterof internal metal wire = 1.6 mm), as shown in Fig. 6(a). To
simulate breaks, after opening the cable coatings with a knife,
the internal metal wires were cut with a nipper and then werecovered with the original coatings again. Figure 6(b) shows an
NIR image of the cut cables [see blue square region in Fig. 6(a),image size = 16 mm by 5.8 mm, corresponding to 100 pixels
by 36 pixels, image acquisition time = 368 s]. While the cut inthe cable with the black coating was not visualized [see upper
region of Fig. 6(b)], the cut in the cable with the white coating
was visualized with high spatial resolution [see lower regionof Fig. 6(b)]. This is mainly due to the difference of the coating material absorptions and indicated that NIR imaging was
influenced by the material coloring, although it has potential
for nondestructive inspection of breaks in power cables. MIR
imaging could not visualize the cuts in the cables (not shown).
Figure 6(c) shows a THz image of the cut cables [see green square

Fig. 5. Hybrid optical imaging of air void of bulky resin material.
(a) NIR image and (b) THz image.

Fig. 6. Hybrid imaging of internal disconnection of power cable.
(a) Optical photograph, (b) NIR image, and (c) THz image.

Fig. 4. Hybrid optical imaging of a prepaid transportation card.
(a) NIR image and (b) THz image. Sample size is 54 mm width by
85 mm height.

lower than that in NIR imaging, details of internal structures
could still be confirmed. In this way, NIR imaging is powerfulfor nondestructive inspection of card samples having little surface printing with high spatial resolution, whereas THz imaging
is useful for nondestructive inspection of card samples having a
lot of surface printing with moderate spatial resolution.
C. Air Void in Bulky Resin Material

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region in Fig. 6(a), image size = 22 mm by 9.5 mm, pixel size =44 pixels by 19 pixels, image acquisition time = 320 s]. The cutsin both cables were well visualized in the THz image without
any influence of coating material coloring, again highlighting
the good material transmission of the THz waves. Although thespatial resolution of the THz image was lower than that of the
NIR image, it was still possible to visualize the cut.
4. DISCUSSION
Here we discuss the possibility of NIR, MIR, and THz imaging
for nondestructive inspection. While the spatial resolution
clearly indicated wavelength dependence based on the diffraction limit (see Figs. 2 and 3), there were large differences inmaterial transmittance among these wavelengths (see Figs. 4–6).
First, NIR imaging suffered from relatively low transmittance;
however, the combination of a mature high-sensitivity IR detector and the lock-in detection enabled us to visualize the internal
structure of the thin sample. Also, we confirmed an interesting
feature of NIR imaging: namely, it is sensitive to absorption by
printed areas or coloring in the sample. Although the shadows ofthese areas hindered clear visualization of the internal structure,
the feasibility of obtaining an NIR image of both surface andinternal structures in specific applications was shown. Second,
MIR imaging is an interesting candidate for nondestructive
inspection featuring both internal visualization and spectroscopic chemical analysis of the sample. However, MIR imaging
could not be applied at all for nondestructive inspection ofeither thin or thick samples. This was due to the too strong
absorption of excessive IR spectral fingerprints. Although useof lock-in detection with a fast MIR detector may improve thissituation, one has to consider the risk that the too strong absorption may cause thermal damage to the sample. Third, THz
imaging visualized the internal structure of all samples regardlessof the material thickness without the influence of absorption
by printed areas and coloring in the sample. The only weaknessof THz imaging was the slightly blurred image due to the lower
spatial resolution. Although the use of higher-frequency THz
waves will improve the spatial resolution, it limits the hightransmittance through many materials due to lower skin depth.
Thus, the compatibility of these imaging modalities differs
depending on the sample. The proposed hybrid optical imaging
system is a powerful tool for investigating the compatibility andselecting the best imaging modality.
As discussed above, NIR imaging and THz imaging haveboth merits and demerits: high spatial resolution and low transmittance for the former, and high transmittance and low spatial
resolution for the latter. If the merits of both imaging modalities
are combined while eliminating their demerits, a hybrid opticalimaging technique will become more promising for nondestructive inspection. One potential method of combining thecharacteristics of different optical imaging methods is image
fusion [20]. Image fusion is a process of combining two or more
images into a single image, and the resulting single image ismore informative and accurate than each original image. For
example, IR and visible image fusion has been applied for facerecognition [21] and other applications [20]. Also, THz andvisible image fusion has been used for hidden object detection[22]. THz and NIR image fusion will combine the merits of

THz and NIR imaging and eliminate their demerits, achieving
high transmittance with high spatial resolution. A combinationof hybrid NIR/THz imaging with an image fusion algorithm is a
topic of our future work.
One may consider performing spectroscopy before imaging
and then use a suitable wavelength imaging system for opticalnondestructive inspection. Unfortunately, spectrometers in
NIR, MIR, and THz regions are based on different principles
and completely different systems (for example, dispersion-type
NIR spectrometers, Fourier-transform MIR spectrometers,
and THz time-domain spectrometers). It is not cost-effective
to have all of them unless spectroscopic chemical analysis is themain purpose. If just a monochromatic image in the NIR, MIR,
or THz region is required for nondestructive inspection, theproposed system is one practical solution.
Also, one may consider that the imaging performance (forexample, image acquisition time or portability) of the hybrid
imaging system is inferior to that of the existing imaging systems
in the NIR, MIR, or THz region. Although that is a fact, weconsider that an important role of the hybrid NIR/MIR/THz
imaging system is to guess the suitable imaging wavelength prior
to the practical imaging. To this end, simultaneous acquisition
of NIR, MIR, and THz images by a single system is important.
Based on the guess, one can select or construct the practicalimaging system in the NIR, MIR, or THz region for practicalapplications.
5. CONCLUSION
We constructed a hybrid NIR/MIR/THz imaging system for
optical nondestructive inspection by using the same imaging
optics with different light sources and detectors. NIR imaging
indicated the potential to visualize both surface and internal
structures of a thin sample with high spatial resolution due tothe moderate material transmission and high-sensitivity IR
detector. MIR imaging showed difficulty for nondestructive
inspection because of the too strong absorption of excessive
MIR spectral fingerprints; its use should be limited, not tonondestructive inspection, but to chemical analysis of ultrathin
sliced samples. THz imaging showed high promise for nondestructive inspection of both thin and bulky samples, benefittingfrom the high material transmission. We discussed the possibility of improving the lower spatial resolution of THz imaging by
employing image fusion with an NIR image. Although NIR,
MIR, and THz images were obtained sequentially by switchinglight sources and detectors with FMs in this work, use of broadband dichroic mirrors would allow us to obtain the three images
simultaneously. As a result, one could easily determine the best
imaging wavelength without the need for prior knowledge of thesample properties. Also, a combination of such simultaneously
acquired multiple images with image fusion would leads to more
informative, accurate, nondestructive inspection by making
good use of the light–material interaction characteristics in eachwavelength region with image fusion algorithms.
Funding. Collaborative Research Based on Industrial Demand,
Japan Science and Technology Agency (Terahertz-wave); Cabinet Office,
Government of Japan (Subsidy for Reg. Univ. and Reg. Ind. Creation).
Disclosures. The authors declare no conflicts of interest.

Research Article
REFERENCES
1. D. E. Bray and D. McBride, Nondestructive Testing Techniques(Wiley, 1992).
2. P. J. Withers and M. Preuss, “Fatigue and damage in structural
materials studied by x-ray tomography,” Annu. Rev. Mater. Res. 42,
81–103 (2012).
3. J. R. Rygg, O. S. Jones, J. E. Field, M. A. Barrios, L. R. Benedetti,
G. W. Collins, D. C. Eder, M. J. Edwards, J. L. Kline, J. J. Kroll, O. L.
Landen, T. Ma, A. Pak, J. L. Peterson, K. Raman, R. P. J. Town, and
D. K. Bradley, “2D x-ray radiography of imploding capsules at thenational ignition facility,” Phys. Rev. Lett. 112, 195001 (2014).
4. M. T. M. Khairi, M. A. Md Yunus, M. Faramarzi, G. P. Sean, J.
Pusppanathan, and A. Abid, “Ultrasound computed tomography for material inspection: principles, design and applications,”
Measurement 146, 490–523 (2019).
5. M. H. Skjelvareid, Y. Birkelund, and Y. Larsen, “Internal pipeline
inspection using virtual source synthetic aperture ultrasoundimaging,” NDT&E Int. 54, 151–158 (2013).
6. J. M. Chalmers and P. R. Griffiths, Handbook of Vibrational
Spectroscopy (Wiley, 2002).
7. M. Manley, “Near-infrared spectroscopy and hyperspectral imaging:
non-destructive analysis of biological materials,” Chem. Soc. Rev.
43, 8200–8214 (2014).
8. D. F. Barbin, G. ElMasry, D.-W. Sun, and P. Allen, “Non-destructive
determination of chemical composition in intact and minced pork
using near-infrared hyperspectral imaging,” Food Chem. 138,
1162–1171 (2013).
9. B. Van Eerdenbrugh, M. Lo, K. Kjoller, C. Marcott, and L. S.
Taylor, “Nanoscale mid-infrared imaging of phase separation in adrug–polymer blend,” J. Pharm. Sci. 101, 2066–2073 (2012).
10. K. Haase, N. Kröger-Lui, A. Pucci, A. Schönhals, and W. Petrich,“Real-time mid-infrared imaging of living microorganisms,” J.
Biophoton. 9, 61–66 (2016).
11. D. M. Mittleman, “Twenty years of terahertz imaging,” Opt. Express
26, 9417–9431 (2018).

Vol. 60, No. 10 / 1 April 2021 / Applied Optics

B105

12. S. Zhong, “Progress in terahertz nondestructive testing: a review,”
Front. Mech. Eng. 14, 273–281 (2019).
13. C. Jördens, M. Scheller, S. Wietzke, D. Romeike, C. Jansen, T.
Zentgraf, K. Wiesauer, V. Reisecker, and M. Koch, “Terahertz
spectroscopy to study the orientation of glass fibres in reinforced
plastics,” Compos. Sci. Technol. 70, 472–477 (2010).
14. D.-D. Zhang, J.-J. Ren, J. Gu, L.-J. Li, J.-Y. Zhang, W.-H. Xiong, Y.-F.
Zhong, and T.-Y. Zhou, “Nondestructive testing of bonding defects
in multilayered ceramic matrix composites using THz time domain
spectroscopy and imaging,” Compos. Struct. 251, 112624 (2020).
15. K. Krügener, E.-M. Stübling, R. Jachim, B. Kietz, M. Koch, and W.
Viöl, “THz tomography for detecting damages on wood caused by
insects,” Appl. Opt. 58, 6063–6066 (2019).
16. M. Kato, S. R. Tripathi, K. Murate, K. Imayama, and K. Kawase, “Nondestructive drug inspection in covering materials using a terahertz
spectral imaging system with injection-seeded terahertz parametric
generation and detection,” Opt. Express 24, 6425–6432 (2016).
17. K. Fukunaga and I. Hosako, “Innovative non-invasive analysis
techniques for cultural heritage using terahertz technology,” C. R.
Physique 11, 519–526 (2010).
18. M. Asada, S. Suzuki, and N. Kishimoto, “Resonant tunneling diodes
for sub-terahertz and terahertz oscillators,” Jpn. J. Appl. Phys. 47,
4375–4384 (2008).
19. S. Suzuki, M. Asada, A. Teranishi, H. Sugiyama, and H. Yokoyama,“Fundamental oscillation of resonant tunneling diodes above 1 THz
at room temperature,” Appl. Phys. Lett. 97, 242102 (2010).
20. J. Ma, Y. Ma, and C. Li, “Infrared and visible image fusion methods
and applications: a survey,” Inf. Fusion 45, 153–178 (2019).
21. R. Singh, M. Vatsa, and A. Noore, “Integrated multilevel image fusion
and match score fusion of visible and infrared face images for robust
face recognition,” Pattern Recogn. 41, 880–893 (2008).
22. M. Kowalski, N. Palka, M. Piszczek, and M. Szustakowski, “Hidden
object detection system based on fusion of THz and VIS images,”
Acta Phys. Pol. A 124, 490–493 (2013).