Speckle interferometry in the long-wave infrared for combining
holography and thermography in a single sensor. Applications to
nondestructive testing: the FANTOM project
Marc P. Georges*
Centre Spatial de Liège-Université de Liège, Liege Science Park, 4031 Angleur, Belgium
ABSTRACT
Speckle interferometry in the thermal wavelengths range (long-wave infrared, 8-14 µm), combining a CO
2
laser and
recording with an uncooled microbolometer camera is presented. In this wavelength range specklegrams are affected by
the thermal radiation emitted by objects at room temperature. This allows simultaneously capturing temperature and
surface shape information about objects. The FANTOM project is based on this concept and an instrument was
developed to take advantage of this natural data fusion. It has been used in a variety of nondestructive testing
applications where both information are useful, specifically in aeronautical composite structures.
Keywords: Speckle interferometry, CO2 lasers, long-wave infrared, thermography, microbolometer arrays,
nondestructive testing, composite materials, aerospace applications
1. INTRODUCTION
Electronic speckle pattern interferometry (ESPI) and Holographic Interferometry
[1]
are well-known nondestructive
techniques used for the full-field contactless measurement of object surface displacements, which can be determined
through fringes patterns (interferograms) superimposed to the object image. Thermography is an imaging technique
which is used in a wide variety of applications, among which nondestructive testing
[2]
. Following Planck’s law, the
spectral radiance of a blackbody is function of the temperature of the latter
[2]
. In thermography a thermal imager
integrates this spectral radiance over some infrared spectral range and in a given solid angle. The resulting flux is related
to the temperature of the object or scene observed. HI/ESPI and thermography are often used in combination because
they give complementary information, mainly in the field of defect detection
[3][4][5][6]
. Nevertheless in such cases,
separate devices with their own imaging systems are used. Deformation map (from interferograms) and thermal images
require to be resampled for correlating both.
The concept presented in this paper is based on the fact that HI or ESPI in the infrared range offers the possibility of a
natural fusion of information. Indeed in the infrared these interferometric techniques use cameras which are also used in
thermography. Therefore it makes sense to think capturing both information simultaneously on every pixel, which allows
a perfect correlation between thermal and deformation fields.
We consider here the long-wave infrared range (LWIR) with wavelengths between 8 and 14 micrometers and the CO
2
laser as coherent source, with various possible lines around 10 micrometers. Photochemical hologram recording at this
wavelength was studied already in the 1960s years. In a previous paper
[7]
we performed a literature survey of the
different materials. We showed that there was no convincing material for recording holograms to be used in HI, mainly
because of low resolution and some other practical considerations. Therefore electronic hologram recording is preferred
due to the recent development of various thermal focal plane array technologies, which is in constant evolution in terms
of performances and resolution increase, while cost decreases.
The very first evidence of such LWIR electronic recording was provided by Løkberg and Kwon in 1984
[8]
which showed
ESPI with CO
2
lasers associated with pyroelectric vidicon cameras. The vidicon image capture requires variable image
intensity. Therefore only variating speckle patterns could be recorded which limited the application of ESPI on vibrating
objects. In 2003 Allaria et al.
[9]
proposed LWIR digital holography (DH) with a modified pyroelectric sensor array which
allows imaging of static objects. Since then that group showed numerous applications and advantages of LWIR DH. One
advantage of LWIR DH is the possibility of reconstructing numerically objects larger than in visible DH (for a given
distance between the object and the sensor) because the ratio of the wavelength and pixel size is generally 5 to 10 times
larger in LWIR
[7][10]
. Recently it was demonstrated that lensless LWIR DH can capture laser light reflected by humans
Invited Paper
Optical Measurement Systems for Industrial Inspection IX, edited by Peter Lehmann,
Wolfgang Osten, Armando Albertazzi G. Jr., Proc. of SPIE Vol. 9525, 95251L
© 2015 SPIE · CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2191133
Proc. of SPIE Vol. 9525 95251L-1
illuminated through smokes and flames and reconstruct numerically images of sufficient quality to recognize the
presence of people located beyond
[11]
. An up-to-date review of applications based on specific features of LWIR DH can
be found in [12].
In the metrology and nondestructive testing domain, we have shown various developments and applications. Our
preliminary LWIR ESPI results concerned in-plane ESPI
[13]
, out-of-plane ESPI
[14]
and out-of-plane lensless digital
holographic interferometry (DHI)
[14]
. In these methods one observes the optical phase differences that are related to the
object displacement or deformation between two object states. The advantages of LWIR are the displacement
measurement range which is 20 times larger in the LWIR than in the visible and the relaxed stability constraints
[13][14]
.
We pursued these studies by the development of LWIR DHI for the deformation metrology of large aspheric space
reflectors under space simulated environment
[7]
, with various types of illuminations for dealing with the specularity of
surfaces
[15]
and we have demonstrated its application to exotic shapes like off-axis elliptic reflectors
[16]
.
The work presented here is related to developments in LWIR digital holography in view of nondestructive testing of
aeronautical composites in the frame of the European Union funded project FANTOM (Full-field advanced non-
destructive testing technique for online thermo-mechanical measurement on aeronautical structures). We already showed
achievements of LWIR DHI
[17]
and comparison of different LWIR interferometric techniques and optimization of an
ESPI setup
[18]
. These works resulted in the development of a mobile ESPI head which included the CO
2
laser and a
thermographic camera based on a microbolometers array. We demonstrated that the use of such long infrared
wavelengths allows working in nondestructive testing applications in industrial environment. In particular large
composite structures were examined in hangar conditions
[19]
. The nondestructive testing of large structures in industrial
perturbated environments is a hot topic and various works propose alternative solutions to LWIR ESPI [19].
We will recall first the basic principle of digital holography and describe the combination of temperature and
deformation measurement. Then we describe the set-up and present a series of applications, from the thermo-mechanical
measurements on composites to defect detection performed on-site.
2. BASIC CONCEPT
Digital hologram or specklegram recording consists in illuminating an object with a laser and realize interference
between the object and reference beams on a camera array. Their interference produces, at instant t and in every
pixel
(, )
x
y , an intensity pattern
(, )
H
I
xy
given by
()()() ()() ()()
,, ,, ,, 2 ,, . ,, cos ,,
HRO RO
I xyt I xyt I xyt I xyt I x yt xyt
φ
=++
(1)
where
()
,,
R
I
xyt
and
(
)
,,
O
I
xyt
are respectively the intensities of the reference and object beams at the sensor level,
the phase
()()()
,, ,, ,,
RO
x
yt xyt xyt
φϕ ϕ
= −
is the difference of
(
)
,,
R
x
yt
ϕ
and
()
,,
O
x
yt
ϕ
which are
respectively the phases of the reference and object beams. We have omitted in (1) the contributions of incoherent noise
sources and which would have added an intensity term on the right-hand side.
When holographic recording is applied at thermal wavelengths, the total intensity pattern of the hologram can be
expressed as the superposition of the interference pattern
(
)
,,
H
I
xyt
given by (1), and an incoherent thermal
background term
()
,,
Th
I
xyt
:
(, ,) (, ,) (, ,)
HTh
I
xyt I xyt I xyt=+
(2)
Our aim is to obtain a simultaneous image of the object thermal background (so-called thermogram) and a recording of
the phase, moreover in every pixel
(, )
x
y . In order to produce such a thermal image, we cannot use a lensless DH
recording configuration. Indeed the thermal background being incoherent, it cannot be reconstructed and propagated at
an arbitrary distance through the usual DH reconstruction principle. Conversely, by using an image-plane holographic
configuration, the image is focused optically by a lens onto the camera array. Such digital image-plane holography has
Proc. of SPIE Vol. 9525 95251L-2
(a)
Thermal hologram /specklegram
1(x, y)
= ITh(X,Y) + Iy(X,Y)
(b)
(c)
I Th (x Y)
Thermal image
Interference pattern
X
(d)
II I I,II II II'll I
I
ILI
X
been compared to lensless classical DH
[21]
. In these imaging conditions the intensity
Th
I
represents the emitted radiation
related to the temperature of the surface of the object which is imaged on the camera sensor array.
The combination principle is depicted in Figure 1: Figure 1(a) shows a typical simulated holographic pattern recorded at
thermal wavelength, Figure 1(b) shows the thermogram that would be recorded if the thermographic camera was used
without the laser. When the holographic part of the setup is used (i.e. the laser beam is present), the interference pattern
is a high spatial frequency pattern represented in Figure 1(c). Figure 1(d) shows the profiles of both thermal and
interference components on a line y=Y, respectively the red and blue plots.
Figure 1. Principle of combination: (a) hologram or specklegram recorded at thermal wavelengths, (b) thermal part of the
former, (c) interference part of the former, (d) line profile along line y=Y
3. DEVELOPMENT OF THE INSTRUMENT
In addition to develop a technique which is able of simultaneous holographic and thermographic acquisition, the
FANTOM project aimed at developing a mobile interferometer for nondestructive testing to be used in industrial
premices, out of laboratories. At first the consortium focused on the interferometric part of the setup and on the
development of a high resolution high end cooled thermographic camera to be used in the setup. The development of the
interferometer has been presented already in several papers. Various techniques were considered and compared but only
one was kept for building the mobile instrument. These techniques are the off-axis DHI with a frontal lens, the ESPI with
phase-shifting and the shearography, which were investigated and compared in [18]. The best results were obtained by
ESPI, despite some advantages of DHI which offers the possibility of single shot measurements and direct phase
extraction, whereas ESPI requires 4 phase-shifted specklegram acquisitions. Although much simpler than the former,
shearography was discarded. Indeed since it provides derivative of displacement and not displacements directly, we need
spatially integrating interferograms, what could lead to errors. Observing the derivative of displacements is extremely
interesting for flaw or damage detection, but for pure metrology of thermo-mechanical phenomena, ESPI is better suited.
Also the LWIR shearography set-up is by far more cumbersome than its visible equivalent. Finally, LWIR shearography
provides a double image of the object scene with transversal shear, both in holographic part and thermal part of the
signal. Extracting the thermogram from the measurement would be a more difficult task due to this specificity of
shearography, whereas in ESPI we have a single image directly. Figure 2, from (a) to (c), illustrates the big steps of this
study.
Proc. of SPIE Vol. 9525 95251L-3
(a)
(c)
To upper
bench
s -pol: 1
RB .® P Pol
OB
Upper bench
PBS
(b)
M3
To object
PBS: Polarizing beamsplitter
SH: Shutter
L : Lens
IL: Illuminating lens
OL: Objective lens
HWP : Half -wave plate
WP : Water pipes
BC : Beam combiner
CM : Concave mirror
RB : Reference beam
OB : Object beam
MPZT: Mirror + Piezo- element
Mi: Mirrors
(a; 4aboratm set-up
(b) Labor-atop/ compact prototype
(c) mobile instrument
Figure 2. Various steps of the development of the set-up: (a) laboratory set-up, (b) compact prototype, (c) mobile instrument
The interferometric studies and developments were performed on the basis of an uncooled thermographic camera
(Jenoptik Variocam hr, with 640x480 pixels). In parallel Infratec GmbH (FANTOM partner) developed a new cooled
LWIR camera with a 640x512 MCT sensor (HAWK sensor from Selex Galileo). Such cooled IR sensor technology
allow signal-to-noise ratio and frame rate higher than uncooled ones. Usually cooled cameras are used for thermography
NDT and it was initially planned to incorporate such camera into the FANTOM instrument. However the cooling system
is based on an active Stirling cooler which provokes small vibrations on the sensor. Although this is not a problem for
pure thermography acquisition, we found that the small residual vibrations were still a problem for using it in our
prototype, such as the one displayed in Figure 2(b). It was then decided to keep the uncooled camera for the remaining of
the project. Nevertheless the new cooled camera (ImageIR 8300) has been used for active thermography in view of flaw
detection in composites
[22]
.
The compact prototype shown in Figure 2(b) was the necessary step for studying the main functionalities of the future
instrument. Initially the laser was placed outside of the optical bench and its light was sent directly through mirrors.
However this is not practical if one wishes a transportable set-up. Therefore we studied the possibility of using IR fibers.
The hollow silica fibers technology proved to be efficient in transmission but the curvature radius smaller than 20 cm
induced strong losses and damages the fiber cladding
[19]
. Consequently we discarded this possibility and moved to a
design which incorporates the CO
2
laser onboard the instrument. The final mobile instrument is shown in Figure 2(c).
Figure 3 shows the scheme of principle of the instrument which is composed of a lower bench (Figure 3(a)) containing
mainly the laser (Access Laser Co, model Merit S, 8 Watts) and reference/object beams separation assembly (Figure
3(b)), made by polarizing beamsplitter. The upper bench (Figure 3(c)) contains the beam combiner, the uncooled
thermographic camera (Jenoptik Variocam hr, 640x480 pixels). More details on the different parts can be found in [19].
Figure 3. Scheme of principle of the mobile LWIR speckle interferometer: (a) lower bench, (b) separation of beams in the
lower bench, and (c) upper bench.
Proc. of SPIE Vol. 9525 95251L-4
100
95
90
85
80
75
70
7.0
Polarization
P
12
11
10
9
g
8
8
0 6
Cl
5
a 4
3
2
1
0
70 7.5 80 85 90 95
10.0
10.5 11.0 11.5 12.0 12.5 13.0 13.5 140
7.5 8.0 8.5 9.0 9.5
10.0
10.5 11.0 11.5 12.0 12.5 13.0 13.5 14.0
Wavelength (µm)
S
P
NM
-.
1
-=11IM_
_
-,
-
Wavelength (µm)
(a)
(b)
A particular attention has been paid on the design of the beam combiner (BC). One important requirement for the
instrument was that the thermal background must not be disturbed, or the less possible, by the holographic part of the
setup. With respect to this, the most problematic element is the beamsplitter used as BC. Indeed if we choose for
example a beamsplitter with a reflectance of 50% and transmittance 50% (R50/T50) over the entire LWIR spectrum, this
means that only half of the object thermal radiation will reach the camera. This in turn would strongly increase the
uncertainty on the surface temperature measurement. For overcoming this, a specific BS had to be developed for the
application. LWIR beamsplitters are made of coatings on ZnSe substrate. We specified the coating to be most
transmitting over the LWIR spectrum and the less reflecting on the same extent, except for the laser wavelength, say
around 10 µm. An ideal BC should have a 100% reflectance at the laser wavelength and 0 % elsewhere. The BC was
built by II-VI Company and the most suitable coating that was obtained for us has the spectral properties shown in
Figure 4 (these graphs are the computed values provided by the manufacturer, not actual measured ones).
Figure 4. Spectral properties of the beam combiner for s- and p-polarizations: (a) transmittance, (b) reflectance
Since the thermal background is not polarized, the global transmittance is a combination of both s and p ones and the
loss will remain limited to a few percents. This impacts the infrared irradiance on the camera array, i.e.
(
)
,,
Th
I
xyt
,
hence the temperature measurement. Temperature measurement assumes a calibration process, e.g. by using a blackbody
at a precise temperature. The camera has been calibrated in factory prior delivery. A proper calibration could have been
performed with the BC. However we have not considered such recalibration here. Indeed in our nondestructive testing
applications, only temperature variations are needed and not the actual temperature. Therefore we will not pay attention
to the small transmittance loss which will be considered as almost constant between two close temperatures.
Proc. of SPIE Vol. 9525 95251L-5
