MULTIFUNCTIONAL ENCODING SYSTEM FOR ASSESSMENT OF
MOVABLE CULTURAL HERITAGE
∗
V Tornari1, E. Bernikola1, W Osten2, R, M, Grooves2, G. Marc3, G. M. Hustinx4, E Kouloumpi5,
S Hackney6
1. Foundation for Research and Technology-Hellas (FORTH), Institute of
Electronic Structure and Laser (IESL), Vassilika Vouton-Voutes, 71110
Heraklion, Crete, Greece.
2. ITO Institut für Technische Optik, Universität Stuttgart, Pfaffenwaldring 9,
70569 Stuttgart, Germany.
3. Centre Spatial de Liège, Liege Science Park, 4031, Angleur Liege, Belgium
4. OPTRION, Spatiopôle – Rue des Chasseurs Ardennais, B-4031 Liege, Belgium
5. Conservation Department, National Gallery – Alexandros Soutzos Museum,
1 Michalacopoulou Street, 11601, Athens, Greece.
6. Conservation Department, Tate, Millbank, London SW1P 4RG, UK.
ABSTRACT
This is an introductory paper of a recent EC project dealing with research in cultural heritage and aiming to
communicate new fields of application for optical metrology techniques. The project is in its initial state and more
conclusive information is expected to be available at the time of the perspective conference. Nowadays safety,
ethical, economical and security issues as well as the increase demand for loaning of art objects for exhibitions in
transit, are forcing the Conservation Community to undertake strong initiatives and actions against various types of
mistreatment, damage or fraud, during transportation of movable Cultural Heritage. Therefore the interest directs to
the development of innovative methodologies and instrumentation to respond to critical aspects of increased
importance in cultural heritage preservation, among which of prior consideration are: to secure proper treatment,
assess probable damage, fight fraud actions in transportation.
The MULTIENCODE aims to create a new standard way of monitoring the condition of artworks by using the latest
holographic technology. It will produce innovative methods and tools which will allow conservators to assess the
conservation state of an object and the need for any treatment; illustrate any new damage; monitor the impact of
transport; and confirm a piece's originality. An object would undergo transient alterations to extract its distinct
holographically encoded patterns before being digitally classified and archived. The coded data can then be easily
retrieved to be compared with past and any future entries to assess changes to the object.
It is thus proposed in this project the development of a novel Impact Assessment Procedure (IAP) based on
interference-signal direct encoding nature by exploiting and further providing to the conservation community the
holographic technology advances and innovative tools for a highly secure encoding-decoding system of objects’
features required in many critical aspects for sustainable preservation of movable artworks.
It may apply in many functional and strategic decision-makings in museums operation, from routine seasonal
examination of conservation state, to periodic assessment of conservation treatments and materials compatibility, to
deterioration control and definition of early-induced damage, to continuous monitoring of transportation impact, to
direct confirmation of originality and control of maintenance for any art object in transit, etc.
The effective proposed method relies on the original coded extraction of distinct features from the artwork under
conservation, transportation and loan that characterizes the state of conservation of the artwork and its originality.
The coding and decoding of such characteristic features can be performed holographically before and after have
been optically and numerically transformed for digital archiving. The object features or the archived coded data
forming the “signature” of the object can be recollected and compared through repeatable IAP at any later time to
∗
corresponding author and project coordinator vivitor@iesl.forth.gr
6th FWP, EC project MULTIENCODE SSPI 00064
provide indication of induced alterations. The project advances the state of the art elaborating in synergy with existing
methods and practices and concludes with specific novel instrumentation and standards for universal application and
worldwide exploitation.
The proposed method advances the state of the art elaborating in synergy with existing methods and practices and
concludes with specific novel instrumentation and standards for universal application and worldwide exploitation. It
would enable us to make further progress on our previous research into the deterioration of objects through handling,
transport and environmental conditions.
Introduction
Europe's works of art, such as paintings and sculptures, are the lifeblood of Europe's cultural heritage. Museums put
them on display and, increasingly, loan them out to other institutions. However, exhibiting art and moving it from place
to place causes problems. Repeated handling, the need for conservation treatments and exposure to sudden
environmental and climatic changes can all take their toll on old and delicate objects. Art in transit is also under threat
from mishandling and fraud. Conservators need to monitor the condition of artwork in a way that responds to these
issues. Structural and mechanical properties of artworks are an important factor of deterioration causing slow but
steady disintegration of the artwork. The thermal and moisture related degradation processes, transportation and
handling, various conservation and restoration actions, as well as the display and arrangement may systematically or
rapidly influence the condition of the concerned artwork, monument or antique. A novel tool to help conservation
researchers/conservators to visualize the invisible defects, constant disintegration processes and interventions has
been introduced [1-3] through the principles and targeted adaptation of holographic interferometry principles [4,5 ].
The visualization of small or inborn discontinuities in the bulk and their consequences on the mechanical instability of
the artwork construction can be optically and digitally obtained [6, 7]. The holographic technology is not based on light
penetration but on reflection of diffused laser beams from the artwork surface. Holography and related techniques
involved in structural diagnosis do not require any sample removal or surface preparation and are safe for use on
varnishes and pigments. In this context, the techniques can be classified as non-destructive, non-contact and noninvasive.
The methodology to visualize the defects of interest is based on differential displacement provoked in time by two
slightly different positions of the artwork reflecting surface of interest through relevant to material properties external
excitation. The displacement results in a relative optical path change of the reflected beam significantly modulated by
the excited bulk defects influencing the surface, which can be studied after have been optically or digitally converted
to visible interference patterned signal. The procedure is repeatable and can be formulated to generate slight
alteration at each record that would result in a thorough study of defect dynamics. Each patterning is an “encoded”
response of the examined artwork and indicates its conservation state correlated with the known induced excitation
and rest investigation parameters to a unique data termed IAP (Impact Assessment Procedure).
The holographic instrumentation allows investigation of a variety of recording media incorporating distinct properties
as well as variety of optical combinations to repeatedly reveal the required data [8-10]. Techniques based on
reflected wavefield recording and specklefield imaging are explored through experimentation of artwork simulated
samples and conservation protocols provided by conservators. At the time of writing encouraging results have been
obtained by the application schemes and the overall research is still on progress.
In the following paragraphs are presented some preliminary results of each implemented technique in an effort to
illustrate the concept of the project and the IAP.
Experimental
Holographic techniques are being under test for examination and classification of provided properties and advantages
in regards to the aim of multi-task sensor and development of IAP. The starting stage of the project at the moment of
writing does not allow any clear conclusion for the final decision which is expected at a later time according to the
work plan. However the techniques under exploitation and indicative result is presented.
Digital Speckle Shearography
Shearography is a double exposure speckle interferometry which has sensitivity to the displacement
gradient.. The sensor optically differentiates light from the object in a shearing Michelson interferometer and does not
require a separate reference. This is important because the interferometer is effectively common path and is
therefore much more insensitive to environmental disturbances when compared with other full-field interferometric
sensor types. Also the optical differentiation means the sensor is sensitive to displacement gradient, a parameter
closely related to the strain field, rather that the displacement sensitivity of the other sensor types.
Digital holography
The technique allows for the measurement of dynamic events with submicron accuracy and is therefore well
suited for the investigation of surface deformations of thermally loaded art works (icons, canvas). Hundreds of
holograms of the object that has been subjected to dynamic deformation are recorded. The acquisition speed and the
time of exposure of the detector are adjustable to allow for surfaces deforming at different rates. The phase of the
wave front is calculated from the recorded holograms by use of a two-dimensional digital Fourier-transform method.
The deformation of the object is calculated from the phase. The software developed allows the visualisation of the
deformation as a function of the time and for the detection of eventual defects.
Digital Speckle Holographic Interferometry
The technique allows the subtraction of the sequential alterations of a reflected by the artwork speckle field.
An off-axis holographic interferometry arrangement is adjusted to capture via a cube beam splitter in front of the
camera lens the interfering beams with angular separation of few degrees and moderated spatial frequencies
resolvable by the CCD sensor characteristics. The technique permits extended field of view (≈30 cm) of the object to
be examined in real time and micrometric resolution of flaws be directly evaluated.
Photorefractive holographic interferometry
Photorefractive crystals of the BSO family (Bi12SiO20) have been proved to be promising recording materials
for holographic interferometry in the visible wavelength range (blue-green). In these crystals charge migration
appears, under the photoconduction effect, between illuminated and dark zones that result from the interference
between two mutually coherent incident beams (an object and a reference). After charge trapping in crystal defects of
the dark zones, a local space charge field is created and this modulates the refractive index, through the linear
electro-optic effect, creating a phase hologram. This process is dynamic and reversible. The hologram is created
within a certain time constant and then reaches a saturation level. The two important figures of merit to consider in
applications are the diffraction efficiency and the recording energy density.
First, the diffraction efficiency is simply the ratio between the diffracted beam intensity and the readout beam
entering the crystal. Photo-refractive crystals (PRCs) have relatively small efficiencies (10-3 to 10-4) compared to other
recording media. Nevertheless this is not a problem if one considers the special properties of the crystals such as the
coupling effect or anisotropy of diffraction. Second, the energy density of recording is a constant of the material when
several experimental parameters are fixed (e.g. wavelength, angle between beams, value of external electric field).
The energy density is the quantity of luminous energy by unit surface that is necessary to reach the saturation of
holographic recording. For a BSO crystal with a 50° angle between recording beams and no electric field, its value is
2
about 10 mJ/cm . This quantity is the product of the response time of the crystal and the total intensity of the
recording beams. An important consequence is that the hologram can be recorded either with continuous or pulsed
lasers. At the present state-of-the-art, one can say that PRCs have practically no limit in the response speed since
they (at least in theory) respond at the nanosecond scale.
Besides this dynamic behaviour, PRCs present many interesting features. The first is that we can distinguish
between two separate configurations, by the polarization state of the diffracted beam. These are anisotropy or
isotropy of diffraction. In the function for a specific application we can choose the most suitable diffraction behaviour.
Each one of these processes is controlled by a suitable choice of the direction of the grating wave vector with respect
to the crystallographic axes, the polarization states of the recording beams and the direction of an external electric
field. In the applications described here, no external electric field is applied. The reason is that, without an applied
field, the diffraction is maximized at large angle between recording beams: This is more advantageous if one wants to
use short focal lengths objectives in front of the crystal to observe large objects.
The typical methodology is to record a hologram within the crystal when the object is at rest (unstressed
state). Once recorded, the hologram is readout and the object is stressed. The CCD camera of the holographic
system then simultaneously captures two images. The first one is light diffracted by the PRC and is the replica of the
unstressed object while the second is directly the current image of the stressed object. Both these images interfere,
giving rise to an interferogram which shows the surface behaviour of the object. Local variations of the object
response reveal particular local behaviour that may result from the presence of defects.
Experimental methodology
The common methodology for data retrieval by using the above mentioned techniques is to generate a transient
sequence of comparatively minute displaced object point positions which can be effectively recorded with the optical
metrology instrument in use. The plethora of the object point positions makes fringe patterns from defected regions
and each fringe pattern relatively different re-arrangements as compared to previous one to be surely and adequately
visualised. Thus each defect is retrieved and recorded with its dynamics of deformation and a databank of fringe
patterns representing each traced defect is secured. In checking the same defect in any later times is simplified by
the databank retrieval mechanism which can identify the defect in various fringe pattern shapes. This is a very
important feature of IAP since defects are tend to deteriorate and thus fringe patterns are not steady over time.
Experimental conditions to be satisfied
-
Uniform excitation
Excitation (thermal IR lamps) time of 10 sec controlled via the PC
Laser power of about 250 mW
Holographic camera on tripod
Environmental condition monitoring
The measurement procedure and the IAP development take into account other specific parameters of the
experimental system and components effects, such as:
•
Determine the type of defects that can be studied using the sensors
•
Sample holder evaluation
•
Excitation power evaluation
•
Excitation position evaluation
•
Investigation of known defect size
•
Repeatability study
•
Reproducibility study
•
Sensor performance – steady defects
•
Sensor performance – unsteady defects
The following tables are initially formed to fulfil the set parameters and the first data collection from developers.
Thermal excitation direction
Thermal excitation duration
Lamps-object distance
Defect appearance
Temperature difference
Front side
10sec
15cm
First after 180 s
6,9 oC
Thermal excitation direction
Thermal excitation duration
Lamps-object distance
Defect appearance
Temperature difference
Back side
60sec
20 cm
First after 190 sec
3,4 oC
Defect
Loss of ground layer
Wooden knots
Detected/Not detected
OK (database: coordinates ..)
OK (database: coordinates ..)
Fringe number measurement providing estimation of each defect magnitude is also performed eg graph shown, in
order to fully describe each defect by position in artwork, coordinates of defect, defect magnitude, defect pattern, etc.
Results
Exemplary signatures of defect classes for IAP development
Representative fringe patterns can be generated from a variety of structural discontinuities being natural eg wood
knot, or artificial eg nail, or degenerated effect eg detachment. The critical aspect in the IAP implementation is the
initial discrimination of type of defect termed here stable and unstable which allows isolating each of the variety of
fringe pattern keeping its origin of cause; which determines the expected dynamical range of change. Hence, by
keeping data of fringe patterns generated by an unstable flaw such as a detachment; the fringe pattern may change
considerably, whereas by a steady discontinuity, as a knot, a steady reference area of discontinuous fringe pattern is
always expected and an identification mapping is possible at any later instance even if appearance of some
discontinuities have changed. Further on the same databank can be used to assess the Impact that has been
provoked to an artwork either positively eg consolidation of detachment or negatively eg expansion of detachment.
In the following example a sample simulating a Byzantine icon with stable and unstable defects purposefully induced
for the aim of the experiments has been constructed by the specialist conservator figure 1a-d. Inspection and
isolation of defects has been achieved and a databank of artwork features or “signatures” has been obtained. Further
on the project foresees cycling of the sample under controlled conditioning according to conservation protocols which
will allow enriching the databank with potential fringe patterns.
a
a
b
c
d
Fig.1a-d. In a) The photo of icon sample, in b) the topographic map, in c) a PRC holographic
interferogram and in d) the extraction of fringe pattern from which defect isolation is performed
(blue: stable, red:unstable).
Defect extraction is shown in figure 2 from selection of few images from the same sample over time to form the
“signature” database. The recording is on PRC-based dynamic holography camera ∗ .
Fig 2. Exemplary routine applied on multiple images for “signature” extraction from
defected patterns.
Series of known samples were constructed and defect features as witnessed by fringe patterns were extracted
following the concept described above. Thereafter a cycling of artificial aging simulating longterm deterioration would
follow and the defected regions will be re-examined to trace fringe pattern alterations.
∗
CSL, Centre Spatial de Liege on NGA National Gallery of Athens sample
Another example is given with a canvas 1 with induced crack (fig 3) as was traced through the appearance of fringe
patterns with implementation of a digital speckle pattern holography interferometry system (DSHI) developed
specially to suit in the demands of art conservation investigation issues as they are declared and termed by the
endusers
and
a
digital
speckle
shearography
system
(DSS).
Field of view
defect
Fringe pattern with DSS
Fringe pattern with DSHI
Fig 3. The cracked area is clearly traced with specific distribution of fringe density and feature under fully
controlled parameters of inspection. Induced aging and damage is expected to change relatively or
considerably their appearance which under the same controlled parameters can be compared and
assessed.
1
IESL, Institute of Electronic Structure and Laser, ITO Institute Technishe Optic, Tate Gallery sample
Conclusions
Generally, the holography and speckle techniques examined are very promising methods and in regards to artwork
inspection show good defect traceability. There is no method so far that combines all this pictorial for the state of the
artwork structural information. Speckle-based systems (DSS and DSHI) have no or small sensitivity to
perturbations. Additionally they both show lower resolution results. Photorefractive dynamic holography (without
active phase control) is strongly sensitive to perturbations but shows a very good fringe quality and which makes
possible observation of small signatures.
The concept of “signature” of artwork based on its own defect-fringe pattern assembly is a coded operation by
nature. It was indeed seen in the investigation that most of the time, even if is difficult to obtain the same overall
fringe pattern, at the same time, the signature stays visible at the same place.
The Impact Assessment procedure can provide a multi-task data source involved in very demanding conservation
applications. The consortium expects this development in a more advanced state of the project.
Acknowledgement
From this position the authors wish to acknowledge the EC project MULTIENCODE SSPI 00064 for the opportunity to
study and develop this research.
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