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Reliability of 3D Imaging by Digital Holography at
Long IR Wavelength
Anna Pelagotti, Massimiliano Locatelli, Andrea Giovanni Geltrude, Pasquale Poggi, Riccardo Meucci,
Melania Paturzo, Member, IEEE, Lisa Miccio, Student Member, IEEE, and Pietro Ferraro, Senior Member, IEEE
Abstract—Digital Holography (DH) in the infrared (IR) range
presents some peculiar aspects compared with the more common
DH in the visible range. The current major drawback is due to
the size of the pixel pitch of presently available thermal cameras,
which is rather large compared to what would be optimal, and
what is possible with analog films. However, since the CO2 laser
wavelength is 15 times longer than average visible wavelength, a
much higher stability, a wider view angle, and shorter acquisition
distances are achievable, allowing easier acquisition of large object
holograms.
We present test results of DH in the IR range, in several
configurations, in transmission and reflection mode and their
performance when used on several different materials. Moreover,
we show the feasibility of large object holography using a CO2
laser and a digital thermal camera.
Index Terms—CO2 laser, digital holography, infrared radiation,
thermal camera.
I. INTRODUCTION
S
INCE the early stage of holography, many efforts have
been devoted to find reliable materials to record holograms
using infrared (IR) wavelengths [1]–[8], as this spectral range
could open new and interesting perspectives. It was envisaged
that many materials, opaque to the visible radiation but transparent to the IR one, could be tested this way. For example,
using the 10.6- m wavelength, holography could allow the inspection of infrared glasses, such as Ge, InSb, and CdHgTe,
which are impossible to control in the visible and near-IR ranges
[9].
Moreover, it was known that such long IR long wavelength is
especially appropriate to investigate plasma, since the interferometric sensitivity of interferometric measurement of electron
Manuscript received October 18, 2009; revised January 07, 2010; accepted
January 11, 2010. Date of publication June 07, 2010; date of current version:
August 27, 2010. This work was supported by the European Community’s Seventh Framework Programme FP7/2007-2013 under Grant Agreement 216105
(“Real 3D” Project).
A. Pelagotti, M. Locatelli, A. Geltrude, P. Poggi, and R. Meucci are with the
CNR INOA (National Institute of Applied Optics) Florence, 50125 Florence,
Italy (e-mail: [email protected]).
M. Paturzo, L. Miccio, and P. Ferraro are with the CNR INOA (National
Institute of Applied Optics) Pozzuoli (NA), 80078 Pozzuoli, NA, Italy (e-mail:
[email protected]; [email protected]; [email protected]).
This paper has supplementary multimedia material available at http://www.
ieeexpore.ieee.org, provided by the author. This pack contains one supplementary avi file (Augusto vivo.avi) which contains a movie clip generated by 30
amplitude reconstruction holograms acquired by rotating a small metal object
by 6 degrees each time. This material is 577KB in size. Player Information:
Windows Media Player, or any player software supporting XVID.
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JDT.2010.2041186
concentrations increases with the wavelength of probing radiation [9]. Furthermore, the use of long IR wavelengths reduces
the sensitivity of interferometric measurements, and therefore,
IR holography is well suited for optical testing of aspheric optics, or to measure the optical path variation, dispensing from
the use of multiple wavelengths in the visible region [10], [11].
In the last decades, the development of solid-state image sensors, like charge-coupled device (CCD) or CMOS, which are
capable of recording signals in the visible range and up to about
1100 nm, enabled the development of Digital Holography (DH),
where traditional film is replaced by an imaging camera. In
order to numerically simulate the reconstruction process achievable with traditional films, a number of suitable algorithms have
been developed, and to them several post processing possibilities have been added, like plane of focus adjustment [12], [13],
amplitude and phase images computing from a single hologram
[14], recording aberration compensation [15], [16], and more.
However, only lately the technology of pyrocameras, and
focal plane array (FPA) microbolometers offered digital sensors that do not require cryogenic cooling and have enough
pixels, of appropriate size, to make the extension of DH to
the long IR range feasible. In particular, the last generation
microbolometers have a number of useful pixels over 300 K
where the pixel size has been reduced up to 25 m. The above
technological progress stimulated further exploration of DH at
IR long wavelengths [17], [18].
This renewed interest could have further implications also in
homeland security, night vision and biological science as it is extending 3D imaging capabilities from visible to far IR towards
the terahertz (THz) region [19]. In this paper, we report investigation on IR at 10.6 m and demonstrate, by means of experiments, how to exploit the advantages and possibilities offered
by this wavelength. In order to test reliability of DH at long IR,
we experimented four different setups, and imaged objects of
increasing sizes, made of several different materials.
II. CRITICAL ASPECTS IN DH RECONSTRUCTION
In cases like ours, where the distance object camera is significantly larger than the digital recording device aperture, the
numerical reconstruction of object wavefront amplitude can be
performed according to the discretization of the well-known
Fresnel approximation of Rayleigh–Sommerfeld equation [20],
using rather simple algorithms.
In order to reconstruct large objects holograms, a number
of critical aspects occurring in DH have however to be taken
into account. The digital recording and reconstruction process
of hologram has in fact an important drawback compared with
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JOURNAL OF DISPLAY TECHNOLOGY, VOL. 6, NO. 10, OCTOBER 2010
and we further suppose to record the
hologram at the minimum distance allowed by the sampling
theorem, the resolution pixel approaches its minimum value
and becomes independent from the wavelength in use
(6)
Fig. 1. Typical off-axis configuration.
its analog counterpart, which is the general lack of resolution of
digital recording devices compared to films.
This is crucial, since in order to satisfy the Whittaker-Shannon sampling theorem, the fringe period of the
interference pattern has to satisfy the following inequality:
(1)
which establishes that the fringe period has to be larger than
two times the detector pixel pitch , according to (1) where
is the angle between the reference and the object beam.
This limitation fixes a maximum value for , which considering small values of it, reduces to lambda over two times the
pixel pitch (2). It is thus remarkable that presents an inverse
proportionality to
and a direct proportionality to the wavelength
(2)
From this maximum value of , it follows that a minimum
distance value between the object and the detector can be assessed. In the off-axis configuration shown in Fig. 1, the maximum angle and the minimum distance can be easily computed
by geometrical considerations
(3)
(4)
In particular, for objects larger than the sensor, where is the
object lateral dimension and
the CCD lateral dimension, the expression for the minimum distance further simplifies
as indicated in (4), from which we can see that this value is in
inverse proportion to the wavelength used, and in direct proportion to the pixel pitch .
Considering that we are working in Fresnel approximation,
we can assume our computation essentially reduces to a discrete Fourier transform. Then the resolution of the reconstructed
image, along the two axes, shows a proportionality to the reconstruction distance and to the wavelength in use
Besides, for large objects, the expression further simplifies and
becomes dependent only on the object dimension and the CCD
elements (6).
We can therefore infer that despite the important drawback
connected with the current dimensions of the thermal camera
pixels, the use of longer wavelengths in digital holography has
several advantages, especially in the hologram recording of
large objects: first of all the stability of the system becomes
a less critical factor at longer wavelengths, secondly, as one
can see from the previous expressions, the angle between the
reference beam and the object beam, increases with the wavelength and consequently the distance between the camera and
the object can be reduced to more acceptable laboratory values
even for large objects. Moreover, with CO lasers is possible
to reach very high power outputs and this makes it easier to
expand the beam and irradiate large objects. Besides, as we
previously described, the resolution of the reconstruction image
at the minimum distance, is not influenced by the wavelength in
use. In fact, with the same configuration, in terms of distance
and detector pitch , using a laser beam in the visible range it
is possible to reconstruct, at maximum, images of objects about
15 times smaller, as the lateral dimension of the image area
is directly proportional to the wavelength of the source used.
In order to further increase the size of achievable objects we
tested also a different configuration, using a concave lens, as
suggested in [21]. With this setup, the wave-field reflected from
the object’s surface is drastically reduced because it emerges
from the small virtual image of the object created by the lens and
not from the large object itself. While the angle between rays
from edge points of the large object and the normally impinging
plane reference wave is too large to fulfill the sampling theorem,
the angle between rays coming from the virtual object and the
reference wave is much smaller so that it meets the requirements
of (2).
Using a lens with negative focal length we can calculate the
distance the lens must have from the camera and the distance
of the object from the lens. The calculation is based on the
lens formula and on the magnification formula
(7)
denotes the transversal magnification,
Here
eral extension of the virtual image. With:
(5)
is the distance of the object from the camera, and
and
are, respectively, the horizontal and the vertical dimension of the camera pixel. If, for simplicity, we
suppose to have a square detector with square pixels, where
where
is the lat-
(8)
we obtain
(9)
PELAGOTTI et al.: RELIABILITY OF 3D IMAGING BY DIGITAL HOLOGRAPHY AT LONG IR WAVELENGTH
467
Fig. 2. Recording setup with a negative lens.
Fig. 4. Amplitude reconstruction of 1–2 slits (2.24 line pair/mm) of USAF test
target.
Fig. 3. Transmission setup used for acquiring holograms of Figs. 4 and 5.
The object size is reduced by the magnification factor
but
the distances between the virtual images and the lens are not
reduced by the same factor. For them the longitudinal magniapplies which is a consequence of the lens
fication factor
formula. We have
. The minus sign accounts for
the reversal of the position order.
III. EXPERIMENTAL EQUIPMENT
We used the coherent light source produced by two different
CW CO laser both emitting on the
line at 10.59 m on
the fundamental mode
: a low power device, emitting
a 500-mW radiation characterized by a waist of 6 mm and a
divergence of 2 mrad and a high power device emitting a 110-W
radiation characterized by a waist of 10 mm and a divergence of
2 mrad. We used ZnSe beam splitter s and lenses and gold coated
mirrors.
The interferograms were recorded by means of two different imaging cameras: a LiTaO3 thermal camera, Pyrocam3
(SPIRICON), with 124 124 pixels and 85 m pixel pitch
and a ASi thermal camera, Miricle 110 k (Thermoteknix), with
384 288 pixels and 35 m pixel pitch, both used without
camera’s objectives.
Thermal cameras are generally available with a reduced
number of pixel and a larger pixel pitch, compared with CCD
Fig. 5. Amplitude reconstruction of 2–2 square (4.49 line pair/mm) and 0–2
square (1.12 line pair/mm) of USAF test target.
cameras. This is clearly not an advantage since it severely
limits the achievable resolution. However, the wavelength we
used is about 15 times larger than the average visible ones, thus
allowing larger object hologram acquisition, as described in
Section II.
IV. EXPERIMENTAL SETUPS
Images of some slits of the USAF R74 target were acquired
with the interferometric transmission mode set up shown in
Fig. 3.
The low power laser and the Pyrocam thermal camera were
used. By means of a ZnSe lens, with 1 inch focal length, inserted in the object arm between the camera and the object, we
managed to acquire and reconstruct the amplitude of a few millimeters area of the test target (see Figs. 4 and 5).
However, the camera used for these acquisitions had major
drawbacks, i.e., a rather large pixel pitch, which would seriously
limit the achievable spatial resolution, together with very limited
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Fig. 9. Amplitude reconstruction of a lens mount hologram with the setup
described in Fig. 6.
Fig. 6. First reflection setup used.
Fig. 7. Amplitude reconstruction of a 1 euro coin hologram with the setup described in Fig. 6.
Fig. 8. Amplitude reconstruction of a 2 euro coin hologram with the set-up
described in Fig. 6.
number of pixels on the detector. This allows to reconstruct only
a small portion of the object with reasonable resolution.
A simpler interferometric reflection configuration, as shown
in Fig. 6, a higher resolution camera and the high power laser,
attenuated by means of a beam splitter, were used in order to
obtain the amplitude reconstruction of a larger object (1 and
2 euro coins, Figs. 7–8, and a lens mount, Fig. 9).
The object was placed near the beam splitter so that the central part of the laser beam was reflected by the beam splitter
towards the camera, thus working as reference beam, while the
outer part was diffused by the object, thus forming the object
beam. Since the beam splitter reflected only 50% of radiation
impinging on it, the reference beam intensity was comparable
to that of the object beam and a good fringe contrast was obtained. In this configuration it was possible to keep the camera
quite near the object and obtain rather high resolution images.
A more complex configuration, as shown in Fig. 10, was
tested in order to obtain the amplitude reconstruction of a larger
object; in this interferometric set up, the reference beam came
from the back of the object so that its direction could be easily
adjusted to obtain adequate fringe spacing.
In order to further explore the possibilities offered by this IR
DH, we tested the acquisition of objects made of different materials. After a few trials on glass, pottery and marble object,
we concentrated our attention on metal items, with different finishing. One of a pair of similar metal objects representing emperors Augustus and Traianus (see Fig. 11, and in Fig. 12 the acquired hologram), was sanded in order to remove the finishing.
The reconstructed holograms of two objects are noticeably different, as shown in Figs. 13 and 14.
The two small bronzes were placed at different distances from
the detector, with Augustus further away with respect to Traianus by about 7 cm, and the shortest distance from the detector
is of about 22 cm. Using the appropriate distance in the reconstruction formula, we thus could focus either of the two, as illustrated in Figs. 13 and 14.
We have included a supplementary AVI file (Augusto vivo.
avi) which contains a movie clip generated by 30 amplitude
reconstruction holograms taken rotating Augusto by 6 degrees
each time.
Another configuration (see Fig. 15), as described in
Section II, was then used in order to further increase the image
area. In fact, with the setup described in Fig. 10 when objects
with larger dimensions have to be recorded, the recording
distance increases up to several meters and this is not feasible
in practice. To reduce the object angle to values with resolvable
spatial frequencies spectrum we prepared an interferometer
with a negative lens in the object arm, between the object and
the camera, in order to generate a reduced virtual image of the
object in a distance given by the usual lens equation. The wave
field emerging from this virtual image is superimposed with the
reference wave and the resulting hologram is recorded. As an
overall effect we enlarged the field of view of the camera and,
by this means, larger object wave fields could be reconstructed.
With this configuration we managed to obtain a larger (about
2 ) field-of-view respect to the previous configuration, as
shown in Figs. 16 and 17.
V. CONCLUSION
Recent technological progress concerning IR coherent
sources, and, especially, digital FPA capable of recording IR
radiation, stimulated exploration of DH at IR long wavelengths.
PELAGOTTI et al.: RELIABILITY OF 3D IMAGING BY DIGITAL HOLOGRAPHY AT LONG IR WAVELENGTH
Fig. 10. Second reflection mode setup used.
Fig. 11. Two test metal objects, one of them (Augustus) was sanded to obtain a different surface.
Fig. 12. Speckle hologram achieved with the Fig. 10 setup of Fig. 11 objects.
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Fig. 13. Amplitude reconstruction of the two metal objects hologram with
Augustus on focus.
Fig. 14. Amplitude reconstruction of the two metal objects hologram with
Traiano on focus.
Fig. 16. Amplitude reconstruction of Augustus hologram with reflection setup
without negative lens.
Fig. 15. Third reflection setup with negative lens. With this setup the hologram
Fig. 17 was acquired.
This attention is justified since IR radiation offers interesting
possibilities in crucial applications such as homeland security, night vision and biological science, as it is extending
3D imaging capabilities from visible to far IR. We presented
the work done testing digital holography at 10.6 m, both in
transmission and reflection configurations. Thanks to the long
wavelength involved, we managed to reconstruct various size
objects: from less than 1 mm to about 25 cm. We also tested the
possibilities offered by this technique with various materials.
Our future goal is to obtain larger objects reconstructions
(up to about 1 m) by means of negative lenses in the setup,
and further experiment with IR response to specific materials,
known for having an interesting behavior in this region.
Fig. 17. Amplitude reconstruction of Augustus hologram with reflection setup
with negative lens.
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Massimiliano Locatelli received the degree in physics from University of Florence, Florence, Italy, with a thesis on “Digital Holography Applications,” carried out at INOA (National Institute of Applied Optics) of Florence in 2009. He
is currently working toward the Ph.D. degree at LENS (European Laboratory
for Non-linear Spettroscopy), University of Florence, Italy.
Andrea Giovanni Geltrude received the degree in electronic engineering from
University of Catania, Catania, Italy, in April 2008, and is currently working
toward the Ph.D. degree in nonlinear dynamics and complex systems from University of Florence.
He currently carries out its research activities in the laboratories of CNRINOA of Florence where, besides the study of chaotic dynamics in lasers, he
is engaged in activities relating to interferometry and digital holography in the
visible and IR range.photograph and biography not available at time of publication.
Pasquale Poggi is an electronic technician. He joined the National Institute of
Applied Optics in 1976. Since then he has been working on several projects for
the design and development of optoelectronic devices for research and industrial
applications.
Riccardo Meucci Riccardo Meucci received the degree in physics in 1982, and
the Ph.D. degree in optics in 1987, both from the University of Florence, Florence, Italy.
From 1984 to 1978 he served as Researcher in Optics at Istituto di Cibernetica
of the Italian National Research Council (CNR) Arco Felice (Naples), thereafter
at Istituto Nazionale di Ottica (INO), later transformed into Istituto Nazionale
di Ottica Applicata (INOA). Currently, he is Research Director at CNR-INOA.
His scientific activity mainly includes: chaotic instabilities in single mode lasers,
laser transients, dynamical models for the CO2 laser, control of chaos, spatiotemporal instabilities and delayed systems, polarization instabilities and digital
holography in the mid-infrared region.
Melania Paturzo received the degree (cum laude) in physics from the University of Naples "Federico II", Italy, in 2002. Her thesis concerned the study of the
photo-isomerization process in a Langmuir mono-layer by means of second-harmonic generation, and the Ph.D. degree from LENS (European Laboratory for
Non-linear Spectroscopy), University of Florence, Italy, on the topic "Optical
devices based on micro-engineered lithium niobate crystals: from material characterization to experimental demonstrations".
Currently, she is a researcher at CNR-INOA, Naples, Italy. Her research interest is in the field of the optics. Her research activities are related to the investigation of material properties by means of interferometric techniques, fabrication
of periodically poled lithium niobate samples by electric field poling process,
and development of optical techniques to get super-resolution in digital holographic microscopy. She is author and\or co-author of more than 25 papers in
International peer reviewed Journals and of more than 40 conference papers.
Lisa Miccio received the degree in physics (with full marks and cum laude) from
the University of Naples "Federico II," Italy, in 2006., and is currently aworking
toward the Ph.D. degree from Florence University, Florence, Italy.
Her research interest is in the field of the optics. Her research activities are
related to the investigation of material properties by means of optical techniques.
Anna Pelagotti received the degree in electronic engineering from the University of Florence in 1995.
From 1996 to 2002 she worked for Philips Research, Eindhoven, the Netherlands, on Video and Image Processing. In 2003 she joined National Institute of
Applied Optics (INOA) of the National Council of Research of Italy, in the Art
Diagnostics group. In 2006, she co-founded of Art-Test. She is currently back
with INOA where she works on Digital Holography, and continues her activities
on multimodal/multispectral acquisition and processing.
In August 2009, Ms. Pelagotti chaired a special session at the European Signal
Processing Conference. Her work resulted in several patents, one of which received the Best Patent Award of year 2007, by Tuscany Region, and several
scientific papers on Journals and International Conference Proceedings.
Pietro Ferraro, is currently Chief Research Scientist at INOA-CNR. Previously
he worked as Principal Investigator with Alenia Aeronautics. He has published
3 book chapters, 90 papers in journals, 150 papers at International Conferences.
He owns 10 patents. Among his current scientific interests are: holography, interferometry, microscopy, fabrication of nanostructures, ferroelectric crystals,
optical fiber sensors.
Dr. Ferraro has , chaired two International Conferences and served as member
of scientific committee in many SPIE Conferences. He was Guest Editor of eight
special issues on international journals and is in the Editorial Board of Optics
and Lasers in Engineering (Elsevier).