Optics Communications 267 (2006) 128–136
www.elsevier.com/locate/optcom

Real-time two-dimensional terahertz tomography of moving objects
Takashi Yasuda a, Takeshi Yasui a,*, Tsutomu Araki a, Emmanuel Abraham bbaGraduate School of Engineering Science, Osaka University, 1–3 Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan
Centre de Physique Moléculaire, Optique et Hertzienne, UMR5798, Université Bordeaux I, 351, Cours de la Libération, 33405 Talence, France

Received 3 December 2005; received in revised form 19 May 2006; accepted 26 May 2006

Abstract
We propose real-time two-dimensional (2D) terahertz (THz) tomography for 2D cross-sectional imaging of moving objects. The proposed method does not require mechanical scanning elements for time delay and sample position, which are essential for conventional
THz tomography. For this purpose, a 2D spatiotemporal THz image is acquired at a frame rate of 10 frame/s by a combination of timeto-space conversion using non-collinear 2D free-space electro-optic sampling and line focusing of a THz beam on a sample. We
demonstrate the use of real-time 2D THz tomography for real-time monitoring of the paint thickness in a moving paint sample andthe drying progress in a wet paint ﬁlm. The proposed method provides a powerful tool for non-ionizing and non-contact probing inthe ﬁelds of nondestructive testing and biomedical diagnostics.
 2006 Elsevier B.V. All rights reserved.

1. Introduction
In ﬁelds of nondestructive testing and evaluation, it isimportant to visualize internal structures of an object asa cross-sectional image. Although X-rays and ultrasonicwaves have been utilized for this purpose in industrial
and biomedical applications, ionizing eﬀect harmful tohuman health in the former and contact measurement in
the latter limit certain applications. Terahertz (THz)tomography [1,2] has recently attracted attention as a possible substitute for these conventional methods because it
can serve as a non-ionizing and non-contact probe. THz
tomography is realized by the time-of-ﬂight measurementof THz pulse echoes when a THz electromagnetic pulse isincident upon a sample in a reﬂection geometry. This technique can be used in various applications to reveal the
cross-section of internal structures from the distributionof the group refractive index in samples. For example,
the THz tomography clearly visualizes the 2D cross-sectionof tablet coating thickness, which is important to control
the release of active pharmaceutical ingredients [3]. In the*

Corresponding author. Tel.: +81 668506217; fax: +81 668506213.
E-mail address: t-yasui@me.es.osaka-u.ac.jp (T. Yasui).

0030-4018/$ - see front matter  2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.optcom.2006.05.063

skin diagnosis, a skin cancer area was recognized as a temporal distortion of the THz pulse echo signal [4]. Anotherapplication of THz tomography is in thickness measurement of the paint ﬁlm of industrial products such as car
bodies. Since conventional thickness meters, including
ultrasonic testing, eddy-current testing, and electromagnetic testing, are based on contact measurement, it is diﬃcult to apply for moving objects or wet paint ﬁlms. One
possible method for non-contact and remote measurement
is optical coherence tomography [5]. However, coloredpaint ﬁlm is usually opaque and causes strong scatteringof visible and infrared light, resulting in decreased reliability of the measurement and low penetration depth [6].
Although a photothermal method [7] is suitable forthickness determination of the opaque painting ﬁlm, thismethod is based on a point-to-point measurement, resulting in high time consuming to construct cross-section
image of the object. We have previously proposed a THz
paintmeter as a tool for testing paint ﬁlms, and have successfully applied it for determination of painting thickness
and its distribution in various kinds of paint ﬁlms that canbe hardly measured by conventional methods (for example,
wet, metallic, paint-oﬀ, or multilayer paint ﬁlm) [6]. Furthermore, the monitoring of dryness in a wet paint ﬁlm

T. Yasuda et al. / Optics Communications 267 (2006) 128–136

has been demonstrated by taking advantage of a distinct
spectroscopic diﬀerence in the THz region between the
wet and dry condition of a paint ﬁlm. However, since usualTHz tomography is also based on a point-to-point measurement, it is necessary to perform 2D mechanical scanning of the time delay and sample position to construct a
2D tomographic image of the sample. The resultant hightime consumption of this method has limited its application
to stationary objects. If THz tomography can be extended
to moving objects, such as industrial products on a moving
stage or the human body, the ﬁelds that THz tomography
can be applied to will be greatly increased. For this, realtime image acquisition is essential.
Rapid image acquisition can be achieved if instead of amechanical stage we use an alternative technique for measuring the time delay and sample position. One eﬀective
method to realize a stage-free conﬁguration is a combination of a single-shot measurement of the temporal waveform and its one-dimensional (1D) transverse imaging,enabling real-time 2D spatiotemporal imaging. Such realtime 2D spatiotemporal imaging technique has been
proposed in optical region and eﬀectively applied to highspeed optical echography of biological tissues [8]. On theother hand, in THz region, there are a few reports on single-shot temporal measurement of THz pulses requiring no
mechanical stage for time delay. These techniques are
based on two-dimensional (2D) free-space electro-optic
sampling (2D-FSEOS) [9]. By combining a linearly chirped
optical probe pulse and a multi-channel spectrometer, thetemporal waveform of a THz pulse can be converted into
the wavelength spectrum of an optical probe pulse [10].
Such a time-to-wavelength conversion method can beapplied to 2D spatiotemporal THz imaging [11]. However,
the time resolution in this method is limited by a tradeoﬀ
inherent in combining frequency and time domain methods[12], though this limitation can be overcome by the noncollinear cross-correlation of electro-optic (EO) modulated
optical pulses, requiring a more complicated optical setup[13]. One possible method for single-shot measurement of129

THz waveforms is the use of a long probe pulse and a
streak camera [14]. In this method, the temporal proﬁleof the EO modulated probe pulse is transformed into thespatial distribution of the probe light intensity throughtime-to-space conversion in a streak tube and is thendetected with a 2D charge-coupled-device (CCD) camera.
Although this method may be also extended to 2D spatiotemporal THz imaging, the time resolution of this method
is limited by the response of the streak camera, typicallyfrom subpicosecond to picosecond. Furthermore, this single-shot measurement method needs expensive and complicated streak camera. Another possible method is the use of
2D-FSEOS with the THz beam and optical probe pulse ina non-collinear geometry (non-collinear 2D-FSEOS) [15].
In this geometry, direct time-to-space conversion occurs
through the spatial overlap of the two non-collinear beams
in an EO crystal and the resulting spatial distribution of theprobe beam is detected with a 1D photodiode array. Thismethod is easily achieved by a slight modiﬁcation of a regular 2D-FSEOS system without any expensive apparatus
or complicated setup, and most advantageously, it has nolimitations on time resolution caused by the intrinsic coupling between frequency and time domain methods [12]or the instrumental response [14]. Despite the simple conﬁguration, suitable for 2D spatiotemporal THz imaging,
there are no reports of its demonstration and hence noreported applications of real-time 2D THz tomography.
In this paper, we propose real-time 2D THz tomography
by the combined use of non-collinear 2D-FSEOS and line
focusing of the THz beam onto a sample coupled with a
CCD camera. After describing the principle and experimental setup of the proposed method, we present THz
tomographic images of a moving paint ﬁlm and the drying
progress of a wet paint ﬁlm.
2. Principle and experimental setup
Fig. 1(a) shows the principle of non-collinear 2DFSEOS [15]. Here, we consider a pulsed THz electric ﬁeld

Fig. 1. (a) Principle of non-collinear 2D-FSEOS. P: polarizer; EO crystal: electro-optic crystal; A: analyzer; L: lens; 1D-PDA: one-dimensional
photodiode array. Pulsed THz electric ﬁeld is composed of a negative and a positive peak with a time delay of Ds. Dh is the spatial displacementtranscribed from the time delay Ds. (b) Relationship between crossed angle and time window with respect to diﬀerent probe beam diameters in noncollinear 2D-FSEOS. The right vertical axis is scaled for theortical maximum probing depth with group refractive index of 2.14.

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to be composed of a negative and a positive peak with atime delay of Ds. The THz beam and the probe beam are
non-collinearly incident on an EO crystal for FSEOS (forexample, ZnTe crystal) at a crossed angle of h in air. The
two wavefronts of the negative and positive peaks of theTHz beam (the solid and broken lines in Fig. 1(a)) overlap
with the wavefront of the probe beam at diﬀerent
transverse positions of the probe beam. As a result of the
non-collinear overlapping of the THz and probe beam
wavefronts, the THz electric ﬁeld at diﬀerent times is translated into diﬀerent positions across the transverse proﬁle ofthe probe beam. To obtain the 1D spatial distribution, theEO modulated probe beam is detected with a 1D photodiode array (1D-PDA) using two perpendicular polarizers ina cross-Nicol conﬁguration (P and A) and an imaging lens
(L). The resultant time-to-space conversion factor isdetermined by
Ds ¼ ðDh  tan hÞ=c

ð1Þwhere Dh is the spatial displacement transcribed from thetime delay Ds and c is the velocity of light in vacuum.
The temporal proﬁle of the THz pulse can then be successively developed along the horizontal 1D-PDA. The measurable time window DT is deﬁned as
DT ¼ ð/  tan hÞ=cð2Þwhere / is the diameter of the probe beam. Fig. 1(b) shows
the measurable time window DT with respect to diﬀerent
probe beam diameters / in the case of ZnTe crystal, where
the lower and upper horizontal coordinates are scaled as acrossed angle in air (h) and that in the ZnTe (hi). From a

relationship of refractive indices between the ZnTe(=2.85 at 800 nm) and air (=1), the total reﬂection of theprobe beam occurs at a back surface of the ZnTe whenhi = 20.5 or h = 90 (not shown in Fig. 1(b)). The maximum / value /max is limited by the size of commerciallyavailable EO crystals. For example, /max = 75 mm for
the ZnTe crystal used in our experiment. The theoreticalmaximum probing depth (MPD) in THz tomography isdetermined bytheoretical MPD ¼ ðcDT Þ=ð2ng Þð3Þwhere ng is the group refractive index of the sample in THz
region. The right vertical axis in Fig. 1(b) is scaled for theoretical MPD with ng = 2.14 although the ng value is afunction of THz frequency. One can set theoretical MPD
by the selection of h and /. However, actual MPD is limited by THz absorption in the sample and/or imaging performances in a THz optical system as discussed later.
To extend this non-collinear 2D-FSEOS to real-time 2D
THz tomography, we prepared line focusing and imaging
optics for the THz beam in a reﬂection conﬁguration anda CCD camera, as shown in Fig. 2. A femtosecond Ti:sapphire regenerative ampliﬁer (Spectra-Physics, Hurricane,pulse energy = 1 mJ, pulse duration = 100 fs, central wavelength = 800 nm, repetition rate = 1 kHz) was employed togenerate and detect the THz pulse. An intense THz pulse isgenerated via optical rectiﬁcation of ampliﬁed femtosecond
pulse light (300-lJ pulse energy) in a 4-mm-thick h1 1 0i
ZnTe crystal (ZnTe1). In the line focusing and imaging systems for the THz beam, we used three kinds of THz lenses
manufactured by PAX, Ltd., Japan. The radiated THz

Fig. 2. Experimental setup of real-time 2D THz tomography. BS: beam splitter; ZnTe1 and ZnTe2: zinc telluride crystals; L1: plano-convex THz lens;
IRF: infrared-cut ﬁlter; CL: cylindrical THz lens; L2: biconvex THz lens; P: polarizer; A: analyzer; L3: plano-convex lens; CCD: charge-coupled-device
camera.

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beam is collimated by a plano-convex THz lens (L1,f = 50 mm). A white polyethylene plate (IRF, thickness = 3 mm) was used as a ﬁlter to block the excess laser
light passing through the ZnTe1. When the CCD camera
is used as a detector for non-collinear 2 D-FSEOS, the spatial distribution Dh develops along the horizontal coordinate of the CCD camera, corresponding speciﬁcally to thetime delay Ds and hence the depth proﬁle along the Z-axisof the sample (see Fig. 2). On the other hand, the verticalcoordinate of the CCD camera can be used for one-dimensional (1D) transverse imaging of the sample along theX-axis. In Fig. 2, the top view and side view of the THz beam
are illustrated to help in understanding the line focusing andimaging optics for the THz beam. To obtain a 1D transverse
image along the X-axis of the sample, the THz beam is ﬁrst
line-focused on the sample using a cylindrical THz lens (CL,f = 50 mm) with the focal line parallel to the X-axis of thesample (see top and side views in Fig. 2), resulting in a lineof 14-mm height being illuminated. Next, the line of theTHz beam reﬂected from the sample is imaged by a biconvex
THz lens (L2, f = 100 mm) onto a 1-mm-thick h1 1 0i ZnTe
crystal for FSEOS (ZnTe2) (see side view in Fig. 2). A probe
beam of 5-mm diameter (1-lJ pulse energy) is incident on
the ZnTe2 non-collinearly with the THz beam at a crossed
angle of h. This results in the formation of a 2D spatiotemporal THz image on the probe beam at the exit window ofthe ZnTe2, in which the temporal proﬁle of the THz pulse
and its 1D transverse distribution along the X-axis develop
in the horizontal and vertical directions in the ZnTe2. EO
sampling is then performed at the near zero optical transmission point using two crossed polarizers (P and A, extinction ratio = 5 · 106) [16]. Finally, the 2D spatiotemporal
THz image is imaged via a plano-convex lens (L3,f = 150 mm) onto a 12-bit thermoelectric-cooled CCD camera (PCO-imaging, SensiCam 360KL, 640 · 480 pixels,
cooled to 15 C, 10 frame/s at an exposure time of 70 ms
and a readout time of 30 ms) as the 2D spatial distributionof the probe beam in real time. The resulting 2D spatiotemporal THz image provides an X–Z cross-sectional image ofthe sample, namely real-time 2D THz tomography.
Our proposed method has three attractive features.
First, it is easily realized by a slight modiﬁcation of conventional 2D-FSEOS, speciﬁcally a change from a collinear toa non-collinear conﬁguration and the use of line focusing
optics for the THz beam. Second, high-speed single-shot
2D THz tomography measurement is possible in principle.
For example, if ampliﬁed femtosecond pulse light with arepetition rate of 1 kHz is used for THz generation anddetection, a frame rate up to 1 kHz is possible [17]. Finally,
and most importantly, this method can be applied to moving objects such as moving products on a belt conveyor or
the human body.
3. Results
Fig. 3(a) shows a 2D spatiotemporal THz image of asingle frame for a plane Al mirror surface at h = 28 (image

131

size = 9 ps · 5 mm). The image was obtained by subtracting the CCD image measured in the absence of the THz
electric ﬁeld (background image) from the EO modulated
image. The white and black areas in the image indicate
positive and negative THz electric ﬁeld, respectively. The
time scale was calibrated from a known time delay produced by a conventional optical delay stage. The unit increment in the time axis was 14.1 fs/pixel, resulting from thetime window of 9 ps and the horizontal width of 640 pixels.
Fig. 3(b) shows a three-dimensional representation of theabove 2D spatiotemporal THz image. The THz pulse echo
reﬂected from the sample appears as a signal line around
4.1 ps (the vertical white line in Fig. 3(a)). This straight-line
proﬁle results from the uniform distribution of the mirror
surface without internal structures. Fig. 3 (c) shows the
temporal waveform of the THz electric ﬁeld extracted froma line proﬁle of the 2D spatiotemporal THz image (line (A)in Fig. 3(a)). As shown, a pulsed THz electric ﬁeld with 0.5ps pulse duration was obtained with a signal-to-noise ratioof 80. The time-resolution limitation of this method ismainly due to mismatching of the group velocity of the
optical pulse with the phase velocity of the THz pulse along
the THz propagation direction in the ZnTe2, which is discussed in detail elsewhere [15]. Fig. 3(d) shows an amplitude spectrum of the THz radiation obtained by Fouriertransform of the temporal waveform in Fig. 3(c). The peakfrequency and bandwidth of the spectrum are 0.55 THz
and 1.04 THz, respectively. The dynamic range of thedetection system is SD = 20log(212  508) = 71 dB by taking into account 12-bit acquisition of the CCD cameraand signal level of the background image (=(mean) ±(standard deviation) = 508 ± 166), whereas a frameto-frame variation of THz signal intensity was 0.3%.
Here, let us consider imaging performances of the present system. Performances of the line focusing and imaging
systems of THz beam are summarized in Table 1, which arecalculated using a peak frequency of THz amplitude spectrum (=0.55 THz; corresponding wavelength = 550 lm)
and parameters of CL or L2. The width of THz focal line
on a sample is 1.7 mm whereas the THz transverse resolution along the X direction of a sample (vertical coordinate
in Fig. 3(a)) is 2.7 mm. From a viewpoint of applicability toan object with curved or stepped surface, we have to consider the depth of focus in THz line illumination and thedepth of ﬁeld in THz imaging. The depth of line focus
and depth of ﬁeld are estimated to be 14.3 mm and43.7 mm in the present system, respectively. These values
indicate that the proposed system is suﬃciently allowable
to a curved or stepped surface with an extreme roughness.
Such allowance is attractive to apply this technique forindustrial products on a moving stage even when an automatic focusing system of the THz beam is not employed.
These values also indicate that the THz line focusing andimaging system well covers the theoretical MPD for /= 5 mm. We further have to consider eﬀects of the ZnTe2
crystal thickness with respect to the depth of focus in theTHz imaging system and the depth of ﬁeld in the probe132

T. Yasuda et al. / Optics Communications 267 (2006) 128–136

Fig. 3. 2D spatiotemporal THz image of a plane Al mirror surface (h = 28, image size = 9 ps · 5 mm). (a) CCD image for an exposure time of 70 ms. The
white and black areas in the image indicate positive and negative THz electric ﬁeld, respectively. (b) 3D representation of the 2D spatiotemporal THz
image. (c) Temporal waveform of the THz electric ﬁeld extracted from a line proﬁle of the 2D spatiotemporal THz image and (d) corresponding spectrumof THz amplitude.

Table 1
Imaging performances of THz and probe optical systems

THz beamLine focusing
Imaging
Probe beam
Imaging

Transverse
resolution

Depthof ﬁeld

Depth offocus

1.7 mm
2.7 mm43.7 mm

14.3 mm±21.6 mm

5.9 lm

150 mm±71 lmimaging system. Imaging performances of the probe beampassing through the ZnTe2 are also indicated in Table 1,
which are calculated using a probe wavelength of 800 nm
and parameters of L3. At the present setup, the focal depthof ±21.6 mm in THz imaging and the ﬁeld depth of
150 mm in the probe imaging well cover the ZnTe2 thickness (=1 mm), resulting in no signiﬁcant eﬀect on the imaging characteristics. Another factor to degrade the spatial
resolution is the spherical aberration induced by the refractive index mismatching between air and ZnTe crystal for
THz beam and optical probe beam. Furthermore, the tilted
surface of the ZnTe crystal to the probe beam may deteriorate the imaging characteristics in the probe imaging system. We consider that these eﬀects limit the actual
transverse resolution. To conﬁrm it, we performed aknife-edge test in the present setup. The resulting trans-verse resolution was 3.1 mm, which is dependent on frequency of THz amplitude spectrum (see Fig. 3(d)). A
little diﬀerence between the theoretical and practical resolutions indicates that the above eﬀects do not deteriorate thespatial resolution signiﬁcantly.
Before performing real-time 2D THz tomography ofpaint ﬁlms, we measured spectral characteristics of an oil,alkyd paint (white color; quick-drying type) with a THz
time-domain spectroscopy [18], which is used as a sample
in the following demonstrations. For this experiment, we
prepared a dry paint ﬁlm (thickness = 4.6 mm) and a wet
one (thickness = 4.5 mm). The resulting spectra of absorption coeﬃcient and phase refractive index are shown in
Fig. 4(a) and (b). One can conﬁrm that there are distinct
diﬀerences in absorption coeﬃcient and refractive index
between the wet paint ﬁlm and dry one. In general, thepaint is composed of a resin (e.g., alkyd resin), a pigment
and an organic solvent (e.g., paint thinner). The drying
process of the paint is temporally advanced by the volatilization of the organic solvent. The volatilization helps thetransformation to the dry condition. Therefore, such spectroscopic diﬀerence between them is mainly due to volatilization of the solvent and/or accompanying chemicalchange of the resin. For example, since the wet paint has
the refractive index dominated by the solvent, the refractive

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133

Fig. 4. Spectra of (a) absorption coeﬃcient and (b) phase refractive index of oil, alkyd paint.

index of the wet paint ﬁlm is smaller than that of the dry
one and thus increases depending on drying degree. The
spectral change accompanying the wet-to-dry transformation results in attenuation, delay, and/or distortion of theTHz pulse echoes. With respect to the dry paint ﬁlm, theTHz pulse can reach to 2.9 mm in depth, which is calculated from absorption coeﬃcient of 7.5 cm1 at 0.55 THz(=spectral peak of THz radiation, see Fig. 3(d)) and a signal-to-noise ratio of 80 (see Fig. 3(c)). Also, we determineda typical ng value of the dry paint ﬁlm from a relationshipbetween geometrical thickness measured with a contacttype thickness meter (=d) and optical thickness with a
THz paintmeter (=ngd) according to an analysis procedure
in a previous paper [6] because the ng value is a function of
THz frequency. The resulting ng value of the dry paint ﬁlm
was 2.14. Theoretical MPD in the present system is0.42 mm for this paint from ng = 2.14 and DT = 6 ps
(h = 20 and / = 5 mm are used in the following demonstrations). From comparison of the penetration depth of
2.9 mm and theoretical MPD of 0.42 mm, the actual
MPD in the present system is 0.42 mm. Further magniﬁcation of DT value and improvement of signal-to-noise ratio
will lead to actual MPD more than several millimeters.

To conﬁrm the performance of the proposed method,
we have demonstrated real-time 2 D THz tomography ofa moving paint ﬁlm. The sample used in the experiment
was a half-paint ﬁlm, shown in Fig. 5, in which the dry
white alkyd painting is layered on half the area of an Al
substrate. The paint thickness around the center of thepainting area was 175 lm, determined beforehand withthe contact, eddy-current type thickness meter. Because
the THz beam is line-focused on the sample (see top andside views in Fig. 2), the sample was continuously moved
along the direction of the focus line (X direction in Figs.
2 and 5) by a translation stage (moving speed = 5 mm/s).
A 2 D spatiotemporal THz movie, or 2D THz tomographic
movie, of the moving half-paint ﬁlm was measured at aframe rate of 10 frame/s when h = 20 (see Movie 1).
Fig. 5 illustrates snapshots of the movie at three diﬀerent
illuminating positions (image size = 6 ps · 5 mm), which
provide 2D cross-sectional image of a sample. In theunpainted area, one THz echo line from the surface of
the Al substrate appears at around 2.8 ps (Fig. 5(a)). In
the painted area, two THz echo lines from the paint surface(2.3 ps) and the paint-substrate interface (4.5 ps) are clearly
separated (Fig. 5(c)). Since the time separation between theFig. 5. 2D THz tomography at three diﬀerent illuminating positions of the moving half-paint ﬁlm (h = 20, image size = 6 ps · 5 mm). (a) Unpainted area,(b) boundary between unpainted and painted areas, and (c) painted area.

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T. Yasuda et al. / Optics Communications 267 (2006) 128–136

two echoes corresponds to optical thickness of the paintﬁlm, the paint thickness can be determined using the grouprefractive index of the paint (2.14) and the time delay, as
shown in the upper horizontal coordinate. Here, the thickness resolution (dmin) is determined from the duration of
the THz pulse (Dt) and a group refractive index of a sample(ng) as follows:d min ¼ ðcDtÞ=ð2ng Þ¼ ð3  108  0:5  1012 Þ=ð2  2:14Þ ¼ 35 lm

ð4Þ

where c is velocity of light in vacuum [6]. Work is in progress to improve this resolution by the application of a separation algorithm of superposed pulse echoes based on alinear optimization technique. On the other hand, thethickness precision is determined by variation of opticalpass length of probe pulse and/or THz pulse caused byair disturbance or mechanical vibration. For example,
when the path length ﬂuctuates within ±10 lm due to those
disturbances, a peak position of THz echo pulse deviates
within ±33 fs; this leads to thickness precision of 2.3 lm.
A 1D distribution of the paint thickness in position range
of 1–4 mm along X axis of the sample in Fig. 5(c) was
determined to be 162 ± 21 lm ((mean) ± (standard deviation)) by the time separation between the two THz echoes,
which is consistent with that measured by the contact-type
thickness meter (175 lm). At the boundary between the unpainted and painted areas, a mixture of one and two echoes
is observed (Fig. 5(b)). We consider that a blurring of edge
at the boundary between the two areas is mainly due toinsuﬃcient transverse resolution rather than aberration or
diﬀraction. In this way, the temporal evolution of the 2D
THz tomography is clearly visualized as the illuminating
position of the THz beam moves across the sample.
Although the proposed system is applied to a single-layer
paint ﬁlm in this demonstration, it can be also extended
to multilayer paint ﬁlms. In this case, an ability to distinguish a paint boundary depends on a relationship between
Fresnel reﬂection in paint ﬁlm and signal-to-noise ratio in
addition to the dmin value. Here, we consider a THz pulse
echo from a paint boundary in two-layer paint ﬁlm, inwhich the second-layer paint ﬁlm with a group refractive
index (ng2) is layered on the ﬁrst-layer paint ﬁlm (white
alkyd, ng1 = 2.14) as shown in an inset of Fig. 6. The ng2
value is selected within a range of 1.5–3.6 because grouprefractive indices of car body paints vary widely within
1.5–3.6 in THz region [6]. Fig. 6 shows a relationshipbetween the ng2 value and ratio of the pulse echo to theincident pulse, which is calculated from Fresnel formula.
Here, negative sign of the ratio indicates inverse of polarity
in the pulse echo. From this relationship and a signalto-noise ratio of 80, the present system can detect the paintboundary with diﬀerence of group refractive index up to
0.08 when THz absorption in paint ﬁlms is negligible.
The remote sensing characteristics of the measurement
mean that the THz paintmeter can monitor paint thickness
even for wet paint for which conventional contact-type

Fig. 6. Relationship between group refractive index of the second-layer
paint ﬁlm and ratio of pulse echo at paint boundary to incident pulse.
Sample of two-layer paint ﬁlm used in this calculation is illustrated as an
inset.

thickness meters cannot be applied [6]. We here applied
real-time 2D THz tomography to the monitoring of thedrying progress of a wet paint ﬁlm via the temporal changeof the paint thickness during the wet-to-dry transformation. For this examination, an alkyd painting was madeon an Al substrate just before the start of the measurement.
A THz tomographic movie of the drying process in the wet
paint ﬁlm was measured over 20 min just after painting (seeMovie 2). Fig. 7 shows snapshots of the THz tomographic
movie at 1 min steps (h = 20, image size = 6 ps · 5 mm),indicating that two THz echo signals temporally evolve
over the period of the measurement. The left echo line
(time = tL) comes from the paint surface and hence relates
to the geometric thickness of the paint ﬁlm. Since the rightecho line (time = tR) is a reﬂection from the paint-substrateinterface, time separation between the left and the rightecho lines (time = tR  tL = Dt) relates to the optical thickness of the paint ﬁlm. We determined the tR and tL valuesby averaging time values of two echo peaks along the vertical coordinate in Fig. 7, respectively, and then calculated
the Dt values from them. Fig. 8 shows the evolution of thetL and Dt values with respect to elapsed time, which indicating geometrical and optical shrinkage of the paint ﬁlm
during the wet-to-dry transformation. The solvent volatilization, triggering the wet-to-dry transformation, causes
increase of the ng value (see Fig. 4(b)) and the geometricalshrinkage (tL in Fig. 8) in the wet paint ﬁlm with drying
progress. From comparison of temporal change between
the tL and Dt values, we can conclude that the opticalshrinkage is dominated by the geometrical shrinkage. At
10 min, the temporal change was almost ﬁnished becauseof the quick-drying paint, implying completion of the drying process. It is also seen from Movie 2 and Fig. 7 that theshrinkage progresses uniformly in-plane. The data shownin Fig. 8 looks very similar to that shown in the previous

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135

Fig. 7. 2D THz tomography of the drying process in the wet paint ﬁlm at 1 min step (h = 20, image size = 6 ps · 5 mm).

sive than the previous oscillator-based one. However, theproposed system enables high-speed data acquisition
attractive for practical applications. For example, a frame
rate of 10 frame/s and an image size of 640 · 480 pixels
in the present system provide nearly 3 Mpixels/s, which isalready 480 times faster than the oscillator-based system
in combination with a 10-Hz optical delay line. Suchhigh-speed data acquisition opens the door to real-time
2D THz tomography of moving objects in real world
and, more speciﬁcally, an in-process THz paintmeter.
4. Conclusions

Fig. 8. Temporal change of tL (left THz echo) and Dt values (timeseparation between the left and right echoes) with respect to elapsed time.

paper done with an oscillator-based THz paintmeter [6].
Such the similar results obtained from the two diﬀerent
methods indicate that the proposed method can monitor
the drying progress correctly, because the two methods
measure the same phenomena with respect to drying process. A little diﬀerence of convergence time between themis due to diﬀerence of drying speed between acryl paintand alkyd one. With respect to system cost, the proposed
ampliﬁer-based THz paintmeter is a few times more expen-

We have proposed real-time 2D THz tomography basedon the combined use of non-collinear 2D-FSEOS and line
focusing of the THz beam on a sample. 2D spatiotemporal
THz images are acquired at a frame rate of 10 frame/s andthen used to visualize a 2D cross-section of the sample. We
have demonstrated real-time monitoring of paint thickness
in a moving paint sample and the drying dynamics in a wet
paint ﬁlm as examples of its application in a THz paintmeter. To the best of our knowledge, this is the ﬁrst time that
2D THz tomography of a moving object has been achievedin real time. This method will be useful in monitoring and/or controlling industrial products on paint coating line andsimilar applications.

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T. Yasuda et al. / Optics Communications 267 (2006) 128–136

Acknowledgements
This work was supported by the Industrial Technology
Research Grant Program 2002 from the New Energy
and Industrial Technology Development Organization(NEDO) of Japan, the Strategic Information and Communications R&D Promotion Programme (SCOPE) from the
Ministry of Internal Aﬀairs and Communications (MIC) of
Japan, Grant-in-Aid for Scientiﬁc Research Nos. 17656088
and 18686008, from the Ministry of Education, Culture,
Sports, Science and Technology of Japan, and the JFE
21st Century Foundation. We are grateful to Drs. Mamoru
Hashimoto and Masahiko Tani in Osaka University for
fruitful discussions.
Appendix A. Supplementary data
Supplementary data associated with this article can befound in the online version, at doi:10.1016/j.optcom.
2006.05.063.
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