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 Hellas (FORTH), Heraklion, Greece

 

Method

Non-destructive

Information obtained

Structural integrity

Type of object

Any (layered) structure

Sample size/amount needed

Size: several m2

Sampling type

Surface/subsurface layers

Portable/transportable version available

Yes

 

Contents

3.4.1     Introduction

3.4.2     Principles of Laser Holography

3.4.3     Case Studies

    3.4.3.1    Optical Holographic Interferometry

    3.4.3.2    Digital Speckle Holographic Interferometry

3.4.4     Conclusion

3.4.5    Supporting Information

    3.4.5.1     References

    3.4.5.2     Providers

    3.4.5.3     Useful Websites

3.4.6     Contact Information

 

3.4.1          Introduction

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 Venice and in fifteenth-century panel painting [8,9], introducing the optical coherent interference measurement as a novel and alternative information source in analysis of structural condition. Structural subsurface information in terms of visually exhibited systems of fringe patterns qualitatively and quantitatively evident could be produced by complex surfaces and three-dimensional shapes. Defected regions become revealed in isolated discontinuous regions in contrast to overall continuous distribution of the interference fringe system and then could be accurately located, sized at one to one scale, and restored.

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.

Steps

Procedure

1st step

Align the artwork to reflect the laser beam to the photosensitive medium where also the reference beam is directed in an angle

2nd step

Place a white card or a photometer to balance the intensity between the beams

3rd step

Block off the beams and allow the system to settle from vibrations

4th step

Unblock the beams for the 1st exposure for time relevant to the total brightness estimated (the lower the brightness the higher the exposure time required)

5th step

Turn on the thermal lamp or any device to induce transient displacement

6th step

Unblock the beams for the 2nd exposure for equal time as in 1st exposure

 

Tab. 3.4.2: Indicative thermally induced excitation.

Thermal excitation with IR lamp, sec

Initial Temperature To, oC

Final Temperature T1, oC

Temperature Difference 

ΔT=T1-T0

1

24.9

25.7

0.8

2

25.2

26.4

1.2

4

25.6

27.7

2.1

5

26.2

28.6

2.4

7

27.10

30.8

3.4

9

26.2

30

3.8

12

26.5

31.3

4.8

15

26.3

32.1

5.8

 

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 OB either in a transmitted as shown or reflecting way by an additional reflecting mirror. In either case the total beam path between OB and RB should be equal. The procedure to form a hologram can be repeated twice and in between a thermal gradient cause by any thermal emission device e.g. thermal lamp or a hot-air gun has affected the object producing a displacement at all object points- The displacement is very small of the order of same multiples of wavelength but efficient to reveal hidden discontinuities in the bulk of the object exactly because the measurement scale is of the order of half wavelengths.

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 1:10. The intensities are measured by a photometer or visually estimated at the recording plane.

The same arrangement for double exposure holographic interferometry can also be used for recording a single hologram [10].

3.4.3       Case Studies

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 Athens.

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.

Sensor Size (WxH) mm

10.2 x 8.3

Pixels (WxH)

1392 x 1040

Pixel Size (WxH) μm

6.45 x 6.45

 

 

FOV:

30 cm

 

80 cm

 

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 Malta (COST G7 partner and partner of LASERACT project) with the aim to assess the condition and age differentiation of stone. In the Fig. 3.4.5 the laboratory transportable prototype at the restoration unit is shown that was transferred to examine El Greco paintings for detachment effects on surface, and the final developed system on action in Malta which is currently under pre-industrial construction by consortium developers (ARTINNOVATION-NL, INFLRP-RO, PROOPTICA-RO).

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.

3.4.4          Conclusion

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 conservation field.

3.4.5       Supporting Information

3.4.5.1    References

[1]         Vest C. M.: Holographic interferometry. John Wiley & Sons 1979

[2]         Juptner W.: Non destructive testing with interferometry. Physical Research, Fringe Academie Verlag (1993) 315-324

[3]         Osten W.: Active optical metrology - a definition by examples”. Proc. SPIE Vol. 3478 (1998) 11-25

[4]         Hung Y.Y.: Image-shearing camera for direct measurement of surface strains. Applied Optics, Vol. 18 (1979) 1046-1051

[5]         Oliver D. E.: Scanning Laser Vibrometers as Tools for Vibration Measurement and Analysis. Test Engineering & Management (1991) 18-21

[6]         Castellini P., Revel G.M., Tomasini E. P.: Laser Doppler vibrometry: a review of advances and applications. The Shock and Vibration Digest 30, No.6 (1998) 443-456

[7]         Hung Y.Y.: Shearography for non-destructive evaluation of composite structures. Optics and Lasers in Engineering, Vol. 24 (1996) 161-182

[8]         Amadesi S., Gori F., Grella R., Guattari G.: Holographic methods for painting diagnostics. Applied Optics 13 (1974) 2009-13

[9]         Asmus J. F., Guattari G., Lazzarini L., Wuerker R. F.: Holography in the conservation of statuary. Studies in Conservation 18 (1973)

[10]      Saxby G.: Practical Holography. Prentice Hall Int. UK last edition 2003

[11]      Tornari V., Bonarou A., Zafiropulos V., C. Fotakis, FORTH/IESL and Michaelis Doulgeridis NGA: Holographic applications in evaluation of defect and cleaning procedures. Journal of Cultural Heritage 1 (2000) 325 - 329

[12]      Tornari V., Zafiropulos V., Bonarou A., Vainos N. A., Fotakis C.: Modern technology in artwork conservation: A laser based approach for process control and evaluation. Journal of Optics and Lasers in Engineering 34 (2000) 309-326

[13]      Tornari V.: Non invasive laser measurement for diagnosing the state of conservation of frescoes and wooden icons. Invited paper for the 4th European Commission Conference for the Research on protection, conservation and enhancement of Cultural Heritage, Strasbourg, 22-24 Nov. 2000, session B, pp 74-80

[14]      Mieth U., Osten W., Juptner W.: Investigation on the appearance of materials faults in holographic interferograms. Fringe 2001, Elsevier ed., 163-173 (2001)

[15]      Tornari V., Tsiranidou E., Orphanos Y.: Holographic interferometry in research of structural diagnosis. ITECOM, EC Conference, 2003

[16]      Osten, W.: Active optical metrology - a definition by examples. Proc. SPIE Vol. 3478 (1998) 11-25

[17]      Tornari V., Zafiropulos V., Fantidou D., Vainos N. A., C. Fotakis: Discrimination of photomechanical effects after laser cleaning of artworks by means of holographic interferometry. OWLS V: Biomedicine and Culture in the Era of Modern Optics and Lasers, Heraklion, Crete 13-16 October 1998, 208-212

[18]      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)

3.4.5.2          Providers

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).

3.4.5.3          Useful Websites

http://www.holoworld.com

http://www.berkley.phys.com

http://www.forth.iesl.gr/programs/Laseract

3.4.6          Contact Information

Vivi Tornari

Applied Optical and Digital Holography

Laser Applications Division

Institute of Electronic Structure and Laser

Foundation of Research and Technology Hellas (FORTH)

Vassilika Vouton

Voutes

GR - 71110 Heraklion

Crete

Greece

E: vivitor@iesl.forth.gr
W: http://www.forth.iesl.gr/programs/Laseract