Chapter 3.4 Structural Diagnosis – Optical and Digital Interferometry Applied in Art Conservation Vivi Tornari Institute of Electronic Structure and Laser,
Foundation of Research and Technology
Contents 3.4.2
Principles of Laser Holography
3.4.3.1 Optical Holographic Interferometry 3.4.3.2 Digital Speckle Holographic Interferometry Alterations on structural and mechanical
properties of artworks are an important factor of deterioration causing
slow but steady disintegration of physical characteristics. The effects
of thermal and moisture processes, transportation and handling, various
conservation and restoration actions, as well as the display
arrangement may influence systematically or rapidly the condition of
the concerned artwork, monument or antiquity. A substantial tool to
help conservation researchers and practitioners to visualize the
invisible but constant disintegration process has been introduced by
the use of lasers as applied by the principles of holography and
holographic interferometry. The visualization of small or inborn
discontinuities in the bulk and their consequences on the mechanical
instability of the artwork construction can thus be optically and
digitally obtained. The holographic technology does not use the
irradiance penetration it is instead based on surface reflection of
safely diffused laser beams. The involved in structural diagnosis
holography and related techniques do not require any sample removal or
surface preparation and are completely safe for varnishes and pigments.
In the context of the above reasons the techniques are titled non
destructive, non contacting and non invasive. The methodology to visualize the influences of
interest and the matching defects is based on differential displacement
provoked in time by two slightly different positions of the reflecting
surface of interest. The provoked displacement signifies a relative
optical path change in the reflecting beams that is optically or
digitally converted to a signal of bright and dark patterned outline.
The obtained pattern is the “encoded” response of
the examined artwork indicating through the bright and dark uniform or
not distribution its conservation state. The general range of applications of laser
interference is remarkable with foremost important the invention of
holographic interferometry which serves as a scientific and engineering
tool at many fields for which it is uniquely suited and recognized
[1-3]. The development of holographic interferometry has also
influenced the development of several closely related measurement
techniques based also on the use of laser light including speckle
photography and interferometry, holographic phototelasticity, projected
fringe techniques, holographic contour generation, holographic
techniques incorporating television systems, phase shifting and
wavefront shearing [4-7]. The theory, practice and application of the
techniques are very close and often complementary to holographic
interferometry, which may serve as reference to the field of structural
diagnosis in art conservation in which applications are still
developing. It was holographic interferometry that was
first applied to detect subsurface damage in Donatello statue in 3.4.2 Principles of Laser Holography The ability of holographic techniques to
visualize subsurface anomalies and their effects on structural
condition from surface information without using any penetrating
irradiation is found on the high information content of the method
based on the unique property of holography to generate a record of
phase distribution from surface reflected beams. Phase is a fundamental
light property as is the amplitude, polarization, wavelength; but is a
property which due to the high light frequencies (~1014
Hz) can be captured only by using two identical highly coherent in time
and space laser beams which are capable of producing by minor phase
changes of the order of fractions of wavelength the desirable
interference effects. This interference principle forms a fundamental
difference between photography and holography. Photography records only
the average in time amplitude distribution of reflecting light whereas
holography records all light information amplitude and phase
(explaining its name holo = all and graphy = record). The result is a
high density record of object information in three-dimensional spatial
coordinates with distinct optical properties with most influential in
this application the paraxial viewing, thus observing a 3D scene by
changing the viewing angle, at full vertical and horizontal parallax
and one to one scale for image-to-object reproduction. If the record is
repeated after sometime while the object is slightly altered e.g. due
to a temporal increase on its temperature, its phase characteristics
are also slightly altered affecting the reflecting phase carrying beam.
When the two records are reconstructed and spatially superimposed all
minor phase alterations are visualized in the form of intensity
distributions as the known bright and dark fringes of interference
phenomenon. This is the technique of holographic interferometry which
is used to visualize the structural condition. It is achievable when an
existent subsurface anomaly or mechanically stressed area respond
differently to the thermal alteration than the rest of the object. This
differential response is getting visualized as an anomalous localized
distribution of interference fringes producing thus own interference
fringe pattern. Therefore the number of localized fringe patterns found
in one holographic interferogram is a measure of object’s
defects. A uniform fringe distribution represents a satisfactory
conservation state whereas an interrupted represents a state of
disintegration relevant to the amount of revealed object defectiveness. The mathematical expression of the above can be
found in wave optics for linearly polarized monochromatic light
equations. Holography is a technique to reproduce light waves and the
signal of interest is found in the object wave since it carries the
information of the object. Additionally a second beam is recorded
simultaneously to realize the interference effect which is called
reference beam since it is the original beam from the laser source
without any induced modulation. The result is an intensity variation of
bright and dark fringes, thus varies between maximum and minimum values
of brightness, known as fringe pattern. Dark fringes are contours of
constant phase difference of odd-integer multiples of p and bright fringes of even-integer multiples
of p. In application to interferometry slight
deformations primarily affect the phase and thus the quantity of prime
interest the phase shift is expressed as intensity fluctuations with
phase difference between two records of the object measured by Df=2pN=Nl/2, where N
integer measured by the peaks of fringe pattern along an axis revealing
the relation of the shift to the physical quantity inducing the optical
path alteration and thus the object displacement, and l the laser wavelength. The working procedure after the setup alignment
is completed for the case of double exposure recording starts with the
first capture of the initial object state as shown in Tab. 3.4.1. Then
an excitation is applied which for artwork inspection thermal is well
suited provoking variety of material displacement which allows better
defect visualization. An indicative table of excitation duration is
shown in Tab. 3.4.2. After the induced excitation has been applied then
a second exposure with equal time duration as the initial is recorded
on the same photosensitive medium. The holographic interferogram is
thus recorded and the chemical development follows [10]. Tab. 3.4.1: Recording procedure.
Tab. 3.4.2: Indicative thermally induced
excitation.
A
basic geometry to realize a holographic interference record is shown in
schematic arrangement of Fig. 3.4.1. The laser source L emits a
linearly polarized plane wave of wavelength l.
The beam is split by BS an optical element called Beam Splitter and two
beams are emerged in BS output. The reference beam RB is expanded by an
optical system for beam expansion which is usually consists of a
diverging and collimating lens BEXC or a SF spatial filter to diverge
and clean the beam, at a desired divergence depending on object size
and laser power, and by Mirror M1 is directed to fall in the
photosensitive medium where the hologram H is to be formed. The object
beam OB is expanded by a diverging lens or BEX or SF and is directed by
mirror M2 to illuminate the object and then to coincide in time and
space with the RB in order to form an invisible interference pattern
which is the hologram to be recorded at medium H. An object can be put
on the final branch of the beam path of Holographic
interferometry experiments require a stable ideally vibration isolated
table and magnetic holders for optic and mechanical components in order
to isolate any irrelevant movement which may affect the clear
displacement of the object. In case of pulsed laser as illumination
source for recording the requirement is minimized but the operator
should foreseen to isolate any extraneous rigid body motion during and
between the pulses. Formation of unwanted fringes is a characteristic
consequence effect of the sensitivity provided. Attention should be
given to keep an aspect ratio between the intensities of reference and
object beam ideally 1:1 or maximum The
same arrangement for double exposure holographic interferometry can
also be used for recording a single hologram [10]. 3.4.3.1 Optical Holographic Interferometry Holographic interferometry non destructive
testing (HINDT) has been well introduced as technique in a number of
applications in art conservation research [8,9,11-18]. Mostly known for
the facility provided to reveal invisible – to other
available practices – defects it was established as an
application of primal interest among art conservation and optical
metrology fields. In this context, a defect map extracted from a panel
painting is used to demonstrate the unique potential of the technique
as a structural diagnosis tool. The painting shown in Fig. 3.4.2 is Saint
Sebastian attributed to Rafael and belongs to the National Gallery of The painting was examined by the double
exposure technique in stable laboratory environment with controlled
thermal excitation and surface monitoring. The random high reflectivity
of the varnish layer caused localized contrast loss obstructing the
capturing of a normalized intensity photographic image. The interest
operator notes that the overall uniform bright and dark fringe
formation -or fringe pattern- due to its overall uniform response to
the temperature alteration is locally interrupted by smaller fringe
patterns generated by various subsurface discontinuities. The next step
is to locate and isolate by zooming in to those discontinuities in
order to extract those patterns so that at the final step only defects
are seen, some characteristic ones are shown in Fig. 3.4.3. After the defects have been isolated and thus
only the areas of the painting with a revealed anomaly are visualized,
as shown in Figure 3.4.4, the restorer has obtained a map with the
endangered areas. Defect location, size and morphology can be studied
in great detail. Not any other conventional or not method except
holographic recording can provide invisible information in such great
detail and fidelity. In x-ray investigation is simply uncovered the
existence of nails, cracks and holes but detailing and other defects
primarily due to detachments between the layers, voids in solids, loss
of material, or propagation of cracks and worms, are all remain hidden
from x-ray imaging. The colours have been extracted by mean fringe
value -measured by multiplying by half wavelength of the laser used the
total number of bright fringes and averaging over various viewpoints
starting from frame edge- from the overall painting displacement. From
green being equal to the overall displacement to red being more far
than the overall displacement. This attempt forms a priority risk map
as the fringes of holographic interferometry indicate. 3.4.3.2
Digital Speckle Holographic Interferometry Holographic interferometry can be also used for
outside laboratory applications due to the development of Speckle
Holographic Interferometry techniques which are using as a recording
medium a CCD instead of film as a photosensitive medium. In Tab. 3.4.3
characteristic features of CCD sensor for Speckle interferometry are
shown. The recording geometry follows the principles of as well as the
working procedure of double exposure holographic interferometry. Tab. 3.4.3: Employed specification of CCD as
sensor.
For in-situ applications, under the framework
of the EC project LASERACT coordinated from the author of the chapter
(EVK4-CT-2002-00096, coordinator IESL/FORTH-GR, partnership: BIAS-DL,
UNIPVM-IT, INFLRP-RO, ENVIROCOUSTICS-GR, ARTINNOVATION-NL, MMRI-ML,
LRMH-FR, PROOPTICA-RO), a digital speckle HINDT system was developed
using as laser source a custom made pulsed laser (INFLRP-RO). The
system allowed investigation of monuments in extreme outside laboratory
conditions such as Floriana fortifications in The results of the developed system both in
continuous wave and pulse mode of exposure have proven successful for
revealing defects inside conservation laboratories and on-field. An
example of numerically reconstructed detachment from an on-field
operation in a tomb in Costanza (INOE coordination in CULTURE
2000/Advanced on-site laboratories, and COST G7 member) is shown. The deleterious effects on the surface of the
wall painting of unknown or difficult to be extracted defects such as
extended detachments in multilayer structure became apparent at the
discontinuity of the fringe system. The remote capability of such
systems generally may vary but given the high coherent laser sources
used for this application can be at least at 0.5 m distance from the
interesting target, thus with a recording procedure lasting few seconds
at each position and a beam divergence which also varies but even with
low energy lasers due to high sensitivity of recording mediums (both
films and CCDs) can be min 30 cm diameter, one can synthesize the whole
surface response of endangered wall paintings to reveal detached
regions without the need for scaffolding and without having to carry
heavy, massive or hazardous instrumentation. The development of lasers and the early
envisage of the non destructive and non contacting techniques of
holographic interferometry for structural diagnosis in art conservation
applications enabled the research on new tools and practices for
cultural heritage preservation and protection. The need for accurate,
repeatable and detailed analysis of structural condition opened the
field to optical and digital laser metrology methods which may
introduce a new era in art conservation practices and profession.
Nowadays expansion of holography counterparts allows also investigation
on monuments and sites. The adaptability of the fundamental principles,
geometries and procedures is very promising for standardization
protocols for world wide application of laser metrology tools in art
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[15]
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Tornari V., Zafiropulos V., Fantidou D., Vainos N. A., C. Fotakis:
Discrimination of photomechanical effects after laser cleaning of
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Georgiou S., Zafiropulos V., Anglos D., Balas C., Tornari V., Fotakis
C.: Excimer laser restoration of painted artworks: procedures,
mechanisms and effects. Applied Surface Science 5048 (1998) Systems based on holography principles and
specially developed and tested for structural diagnostics in art
conservation applications are not yet found on the market. A lot of
practical information and sources for material for holography can be
found in the book of Graham Saxby [10]. For further information on the
technical and theoretical issues of this brief introduction any
interest reader may contact directly the author (vivitor@iesl.forth.gr). http://www.forth.iesl.gr/programs/Laseract Vivi
Tornari Applied
Optical and Digital Holography Laser
Applications Division Institute
of Electronic Structure and Laser Foundation
of Research and Technology Vassilika
Vouton Voutes GR
- 71110 Heraklion E: vivitor@iesl.forth.gr |