A preliminary investigation has taken place employing Digital Holographic Speckle Pattern Interferometry (DHSPI) in order to assess the effect of handling and transportation on canvas paintings. Canvas dummies were used on a series of measurements on a transport simulator which allows reproducible simulation of any transport logs in the laboratory. A number of cycles of controlled vibrations were applied on the samples and after each cycle a measurement with DHSPI was taken to monitor the behavior of the samples while increasing the vibration loading and also to record the conditions under which the first crack appears. The transport simulations in combination with DHSPI monitoring revealed the amplitude of oscillation where the first cracks appear on new canvas paintings and also the way these cracks grow. During the tests it was also feasible to locate areas at risk of future deterioration.
Keywords: Holographic interferometry, canvas, transportation, crack propagation, transport simulator
1. INTRODUCTION Transportation of artworks causes unwanted and harmful exposure to shock and vibration of usually delicate and fragile objects. Handling of freight at airports, as well as transfer on bumpy roads on the way to delivery are associated with considerable risks for the artworks. Several approaches to this topic have been published in the 1980ies and early 1990ies providing mainly acceleration data during transport1-5. Commercial sensors have been developed in order to record the oscillation characteristics of vibration and shock during transportation and others simpler to prove the event of a mishandling or the application of a critical frequency6. These sensors indeed monitor the vibration of an artwork providing useful data for research, but they don’t record the impact of the vibrations or the reaction of the artwork itself to these events. Conventional methods commonly used by conservators such as visual examination, raking light or microscopy are used to document the effect of transport. A non conventional method developed to predict crack creation and growth is finite element analysis using computer simulated models7. The main problem though remains that the impact of vibration of composite objects like artworks, under real travelling conditions is very difficult to be predicted. Other full field techniques used in the topic of structural documentation of canvas, from high to moderate resolution, are photorefractive holography and shearography8,9,10 but they were not used for assessing the impact of vibration loadings so far.
To date there is no known publication assessing the destructive potential of repeated deformation (i.e. due to transportation) on fragile painting structures. The critical level of tolerable strains induced by the vibration levels quoted in the literature are based on fatigue research dealing mainly with modern construction materials11,12. The problem of recording and assessing the impact of transport and handling directly from the object with high precision is fronted in this study by employing Digital Holographic Speckle Pattern Interferometry (DHSPI), widely used up to date in structural documentation and diagnosis of artworks13-18. The combination of DHSPI with a new transport simulator19 that
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allows reproducible simulation of any transport logs on sample paintings in the laboratory, in the framework of CTI Project: “Transporting fragile paintings”20, has enabled the research to develop a basis for predictive risk assessment.
Coherent interferometric techniques are based on the holographic principle and record phase variations of mutually coherent laser beams represented by beams carrying an object (O) and reference (R) field. The holographic process of wave superposition at recording plane may be represented mathematically by the general intensity equation (Eq.1)
RRRR UUUUUUUUI *0
*0
**00 +++∝ (1)
In optical holographic interferometry the superposition of holographic wave fields gives rise to visible interferometric fringes with the number of fringes multiplied by ½λ corresponding to the magnitude of surface displacement, expressed in μm or fractions of micrometers. The technique is directly quantitative while the measurement unit of half of the laser wavelength employed allows the recording of microscopic surface motion with high precision. More analytically, it implies that any object point P scatters laser light in all directions and to the recording plane and after the object point is displaced by L, the point moves from P to position P', which is accompanied by displaced light scattering. In the two instances before and after displacement a recording plane with two waves U1(x,y) and U2(x,y) representing positions P, P' interferes with the plane of the reference wave UR(x,y) as given by
)],(exp[),(),(1 yxiyxyxU ϕα −= (2)
where φ(x,y) denotes the phase information related to object shape, features, depth etc.
)]},(),([exp{),(),(2 yxyxiyxyxU ϕϕα Δ+−= (3)
where Δφ(x,y) in the second term of the object wave equation encodes any known or unknown load induced in the object. The holographically formed object point waves U1(x,y) and U2(x,y) are reconstructed simultaneously with their superposition visualising the amplitude term and an interference pattern. The irradiance of the reconstructed wave is given by
where the term 2α2(x,y) is the amplitude of the image of the object and Δφ(x,y) the interference pattern of phase difference due to the displacement of the object following displacement. The fringe intensity distribution is described by the cosine term {1+cos[Δφ(x,y)]}.
Fringes are isotopic curves with constant values of Δφ with dark fringes being odd and bright fringes even multiples of π. If a lens is used to image a speckle pattern field for digitisation and numerical analysis21 with a reference, the object beam intensity varies locally and is considered coherent only locally and not in the whole field area, as in the lensless optical processes. This is due to restrictions of the autocorrelation function in speckle fields determined by the intensity distribution and size of the speckle22. In this case the intensity distribution in the recording medium is varied for each individual speckle region, defined by pixel and speckle size of the system, so that the object field before displacement for each region is calculated as
)],(),(cos[),(),(2),(),(),( 2121 yxyxyxIyxIyxIyxIyxI ORO Φ−Φ++= (5)
And after displacement as
)],(),(),(cos[),(),(2),(),(),( 2121' yxyxyxyxIyxIyxIyxIyxI OR ϕΔ+Φ−Φ++= (6)
the numerical subtraction before and after displacement of the intensity field after averaging of intensity speckled region is calculated as
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( ) ( )[ ]ϕΔ−=−= cos14 212
122 IIIII (7)
The contour lines of interference fringes describe in a three dimensional way the deformation of the object’s surface. Taking advantage of the fact that the surface is deformed according to the content of the internal bulk of the object, the surface deformation topology is studied in order to acquire information about the internal structural condition. The areas of internal alterations are detected by the aberrations they cause to the homogeneity of the interference fringes, such as abrupt change of the fringes curvature or changes in fringes density (Table 1).
Table 1. Fringe Patterns in regards to material cracks 23,24,25
Fringe Patterns
Curved Fringes Dead-end Fringes
Features Open curves, smooth
direction change, continuous curves
Non continuous curves
Possible cause
Internal crack or detachment Surface crack
2.2 Digital Holographic Speckle Pattern Interferometry (DHSPI)26
For the purposes of this study a laboratory prototype DHSPI set up was used to monitor the structural responses of the canvas dummies. The experimental set up uses a Nd:YAG Elforlight G4 laser as a light source with special characteristics: 250 mW at 532 nm, DPSS (Diode Pump Solid State), high spatial-temporal coherence with TEM:00 SLM (Single Longitudinal Mode) and a coherent length of 30 m for far access illumination to the target, and a CCD detector Basler A102f with resolution 1392H x 1040V and pixel size 6,45 μm x 6,45 μm as high resolution digital recording medium. The captured images are transferred to a PC using the Firewire 1394 protocol. The object’s surface is recorded using the 5-frame algorithm, which uses two sets of five captured images separated at temporal windows of 10 sec at each set. The first set of images is captured using the π/2 phase difference in a relaxed state of the sample. The second set of images is captured again using the π/2 phase difference but in a deformed (or displaced) state following an induced surface displacement of the canvas, with unknown phase difference. Every set of 5-images is captured and is compared to the first initial set. The metrological data provided by DHSPI is of the order of 266nm (λ/2, where λ is the laser wavelength) for the out of plane surface deformation.
2.3 Transport simulator
The transport simulator (shaking machine) is built to simulate linear movement along a single axis with a maximum displacement of 70 mm. It is driven by four parallel voice-coil motors mounted perpendicular to the axis of movement. A maximum weight of 20 kg can be accelerated up to 50 m/s2. Sample paintings can be mounted along the x, y or z axis on the slider. This allows performing the simulation sequentially along each axis to achieve every translational degree of freedom. For this study the movement direction perpendicular to the sample painting (revealed as the most relevant concerning the canvas oscillation) was used (Fig. 1). The control element (dSpace, DS1103) is capable of reproducing any logged vibration profiles captured during real transport monitoring as well as harmonic vibrations and bandwidth limited white noise. The movements on the sample painting are logged by a triaxial accelerometer (PCB 356A16) attached to the stretcher and a uniaxial accelerometer (PCB 352A73) mounted in the centre of the back. The placement of the uniaxial sensor was based on the ideal behavior of membranes. The highest amplitudes are expected in the centre of the canvas. The actual canvas displacement can be derived from the acceleration signals by appropriate numerical computations.
2.4 Vibration loading
In order to intentionally cause damage to a sample painting, it has to be exposed to appropriate vibrations. Based on the results of preliminary measurements concerning the frequency spectrum of transport vibrations, a random white noise with limited bandwidth (1 to 50 Hz) and variable amplitude was chosen as vibration loading. In contrast to real transport vibrations it does not possess any dominant frequencies or shock events. This allows better controllability when
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approaching the critical values of the vibration amplitude and duration. The vibration amplitude is thereby characterized by the root of the quadratic mean (rms-level) in m/s2 of the according acceleration signal.
2.5 Frequency response measurements
Additional measurements were carried out to determine the characteristics of the frequency response of each sample painting before and after applying the vibration loading. The sample painting was therefore exposed to sinusoidal oscillations and their frequency response was logged. Frequency response is defined by the ratio of the excitation amplitude to the response amplitude and their phase difference, measured over a frequency range. The ratio is commonly expressed in dB (decibels). Resonant frequencies of canvas thereof lead to maxima in the frequency response plot. These are expected to change its characteristic during the experiment, which leads to an additional method of visualizing the relative health of the canvas.
Figure 1. Newly developed transport simulator.
2.6 Sample Description
For the experiments the following painting structure was produced. Support is a linen canvas, which was sized by brush with warm skin glue. Two layers of gesso serve as vulnerable paint layer. A partial black layer of acrylic paint was applied for optical contrast. Shellac and dammar were used for varnish. On the structure “weak” spots were integrated. In order to produce adhesion gaps between the sized support and the gesso layers Tricyclen- Camphen was used. (Tricyclen-Camphen sublimes very fast. It was heated to 70°C and applied with a brush. Because it is non-polar adhesion
(a) (b)
Figure 2. (a)Principal areas of the dummy systems used. Circles indicate the location of weak spots. (b) Principal construction of the sample painting.
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support (shellac/ethanol) was necessary to apply the gesso layers). The sample has a size of 60x 80cm (Fig.2) and it was attached on a tensional frame illustrated in Figure 4. The position of the weak spots is shown in Figure 2 and they are described in detail in Table 2.
Table 2. Description of dummy and induced alterations
Vibration loadings applied by the transport simulator and followed by DHSPI measurements took place as described in Table 3. The applied vibration loading was noise of 1-50Hz and 10 seconds of duration and variable acceleration starting at 1m/s2 (rms). Twelve vibration cycles were applied with increasing acceleration of +1m/s2 (rms) per step, but since the limit of the instrumentation was 10m/s2, the 11th and 12th cycle were applied by 9m/s2 (rms) of acceleration.
Table 3. Experimental Procedure
STEP 1 1. DHSPI measurement before any vibration cycle, by thermal loading
2.Vibration cycle
STEP 2 3. DHSPI measurement by thermal loading
4. Raw data check for visible crack creation and propagation
5. Vibration cycle
STEP 3 6. Repeating of 3, 4, 5 until crack creation and propagation conditions are clarified
To detect the flaws of the canvas with DHSPI thermal loading was applied, by two infrared lamps, placed in front of the dummy. The maximum of the temperature increase of the dummy was about +0,7 oC. The measurements with the thermal loadings were taken while there was a backboard support fixed at the back of the dummy in order to reduce the high movement of canvas. The head of DHSPI was at a distance of 1,20m from the sample and the IR lamps at 0,70m from the centre of canvas. The measurements took place in lab environment, with stable conditions, to ensure that the recorded differences of the reaction of the dummies would only be due to the vibrations.
2.8 Measurement Methodology
The measurement methodology to provide a crack map and measure the crack growth is presented schematically in Figure 3. The fringe patterns concerning cracks are extracted in regards to the classification Table 1. If a crack appearance is intermittent then higher excitation and/or longer monitoring should be applied in order to define better the crack shape. In case the appearance of a crack is continuous the procedure is to set the most convenient coordinates according to the objective of the measurement (i.e. for scalar measurements of plane objects x,y is efficient). The crack
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map is then possible to be produced and crack length to be measured. Finally, after the repetition of this procedure successively with vibration loadings, it is possible to import the data of the crack length into a diagram and observe the propagation rate of the crack growth.
Figure 3. Measurement methodology.
3. RESULTS The test painting used was the one illustrated in Figure 4. The investigated area (about 250x330mm) was in the centre of the painting as it is the point of the highest movement when oscillating.
Figure 4. Test painting clamped onto the tensionable frame and investigated area marked with green line.
The first crack appeared after the fifth vibration cycle (1-50Hz, rms=5m/s2, 10sec duration). The interference patterns revealing it, overlapped on the photo of the dummy, are illustrated in Figure 5. The indication of the internal crack noted on the left side of the image with the blue dashed line, was evident already after the third vibration cycle. After the fourth cycle nothing changed until the fifth.
Center of painting. A sensor is attached on the back side
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Figure 5. Crack map of the canvas dummy after the 5th vibration cycle. Red line marks the surface cracks and blue dashed line the
internal one.
After the sixth cycle the first crack propagated from both edges having a length of about 40% longer of its initial, while the seventh cycle created the second crack and caused further propagation of the first one. The eighth cycle caused the third crack and the propagation of the other two, while the ninth cycle caused the creation of another three cracks and the propagation of the previously created. Finally the crack map after the 12 vibration cycles is shown in Figure 6, while the measured length of all cracks and the way they grow is illustrated in Table 3 and Figure 7.
Table 3. Crack length in mm after each vibration cycle
It must be clarified that the length measured only within the field of view, which means that for the cracks propagating out of it, the real length could not be known. The crack length was measured via software in pixels and was then converted to mm. The error bar for the crack maps is ±1mm. For every crack map about three interferograms where used. This is essential because on the moment an interferogram is recorded not all the cracks have an influence to the surface distinguishable from the healthy areas. Some cracks or even some parts of a crack react as one with the healthy
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parts when monitored in time, usually when relaxing from a loading (i.e. intermittent appearance of cracks, Figure 3). In general they react much different from healthy parts but not always. One crack is not reacting the same way from one edge to the other, also because everything is laying on textile which moves a lot comparing to other more bulky artworks. If a loading is not the appropriate or it is not high enough to force an altered area to move in a different way from healthy parts, then the altered area will never be detected. In this case, either the type of loading or its intensity should be reconsidered.
Figure 6. Crack map of the canvas dummy after the 12th cycle of noise, superimposed on the photo of the investigated area of the
painting and the interferogram. Red lines mark the surface cracks while the blue line mark the internal ones. The red dot is pointing the place where the sensor is attached on the back side and in the centre of the canvas.
Figure 7. Crack creation and growth by increasing the amplitude of oscillation.
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The centre of the canvas has received an extra stress apart from the inevitable higher oscillation because of being the centre. A sensor measuring the characteristics of the canvas oscillation was attached on the back side and in the centre. Though the sensor was very light it has probably influenced the crack formation around the centre as shown in Figure 6.
Figure 7 illustrates that the slope of crack growth is high when the first cracks appear, until the ninth cycle and then it is minor.
As expected, the characteristic of the frequency response changes with decreasing health of the canvas. After the vibration loading the main resonant appears at a significant lower frequency but with a higher damping (i.e. peak is lower). The reason for this might be a decrease of the canvas tension due to cracks. Figure 8 shows frequency response plots of before and after vibration loading. The main resonant frequency of the canvas dummy changes from 15 Hz to 11 Hz.
Figure 8. Frequency response plots of canvas dummy before and after the vibration cycles.
The indication of the internal crack marked with blue dashed line in Figure 5 (crack number 3 in Figure 6) is illustrated in detail in Figure 9 after the 3rd and 9th vibration cycle, together with the three dimensional depiction of it. Cracks under the surface, since they have a continuous material surface above them, they provide the curved fringe pattern of Figure 9a23-25. After the crack has reached the surface the topography changed and this time the interference fringes describing it are “broken” like in Figure 9c, proving the discontinuity of the surface. The “broken” fringe pattern is the limit to define the edges of a superficial crack. The same interpretation stands also for the internal crack of Figure 6 marked with the blue dashed line. The curved fringes which define the crack prove that it has not reached the surface yet, but in the future under specific conditions it might do.
(a) (b) (c) (d) Figure 9. Part of interferogram at the area of crack number 3 after (a) the 3rd vibration cycle and (c) the 9th vibration cycle. (b) and (d)
Three dimensional depiction of interferograms (a) and (d) correspondingly.
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One more thing worth noting about the location of the crack appearance is illustrated in Figure 10.
(a) (b)
Figure 10. Part of interferogram overlapped with the canvas photo at the central vertical sector (a) before any vibration cycle and (b) after the 12th vibration cycle.
The interferogram acquired from the canvas before any vibration cycle has described the central vertical sector, along the axis separating the white from the black painted part, as well as the horizontal area pointed by the yellow arrow, with a sudden higher fringe density than the rest of the canvas (Fig. 10a). These areas are characterized as stress zones and observing the same part after the 12th cycle (Fig. 10b) provoking of cracks is evident. Furthermore, as illustrated in Figure 10a the sensor attached in the centre of the canvas on the back side, has proved its presence probably by the bending fringes as pointed by the red arrow. The last observations enhance significantly the foreseeing capability of the method.
4. CONCLUSIONS The preliminary tests carried out at the Hochschule der Kuenste Bern / TI-Burgdorf Berner Fachhochschule on the risk assessment for shock and vibration emissions with new prevention strategies in the transport of fragile paintings has been successful in providing crack maps and also monitoring the crack propagation with precision. Lab simulations of transporting in combination with DHSPI monitoring has revealed the amplitude of oscillation where the first cracks appear on new canvas paintings and also the way these cracks grow. During the tests it was also feasible to locate areas at risk of future deterioration. A systematically study could also successfully define the threshold limits of transport conditions in order to develop a safe transportation protocol.
ACKNOWLEDGMENTS The project was partially funded by the Swiss Innovation Promotion Agency CTI (Commission for Technology and Innovation) and partially by the Institute of Electronic Structure and Laser of the Foundation for Research and Technology-Hellas (IESL-FORTH) in the premises of the holography laboratory. The authors would like to thank Anita Hoess, Giovanna di Pietro, Claudia Bäschlin, Christian Wasserfallen, Kostas Hatzigiannakis and Michalis Andianakis for their assistance during the project.
Center of painting. A sensor is attached on the back side.
Stress zones
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