holography in the conservation of statuary

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Holography in the Conservation of StatuaryAuthor(s): J. F. Asmus, G. Guattari, L. Lazzarini, G. Musumeci and R. F. WuerkerSource: Studies in Conservation, Vol. 18, No. 2 (May, 1973), pp. 49-63Published by: Maney Publishing on behalf of the International Institute for Conservation ofHistoric and Artistic WorksStable URL: http://www.jstor.org/stable/1505458 .

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Studies in Conservation, 18 (1973), 49-63 49

HOLOGRAPHY IN THE CONSERVATION OF STATUARY

J. F. ASMUS, G. GUATTARI, L. LAZZARINI, G. MUSUMECI and R. F. WUERKER

Abstract-During the Spring of 1972 an experimental study was conducted in Venice, Italy, to demonstrate the feasibility of in situ holography of life-sized statuary and wood-carvings. Emphasis was on holographic subjects that were in a state of accelerating disintegration resulting from environmental processes. Archival image records were produced with pulsed ruby-laser holography of the wood-carving 'San Giovanni Battista' by Donatello and the marble statue'Madonna con Bambino' by Nino Pisano. In addition, interferometric techniques were found to be of potential utility in statue restoration through the location of hidden patches, cracks and flaws.

INTRODUCTION

A little more than ten years ago an entirely new science became practical through the development of highly coherent lasers [1]. This science is known as holography, and it allows for the first time the three-dimensional photographic recording of the detailed shape of an object with a single two-dimensional photographic plate [2]. Moreover, the image formed can be many times more precise than the conventional photographic image. Until very recently, limitations in laser performance restricted holographic recording to subjects of no more than a few centimeters in size. Further, it was necessary to perform holographic recording in an interferometrically stable laboratory environment (viz., free of acoustic vibration). Recently, through the development of short-pulse ruby laserswvith both high energy and high coherence, it has become routine to make short-exposure holograms of rather large objects (up to 9 meters in size) in a vibrationally dynamic environment [3]. This advent of pulsed holography facilitates numerous new holographic techniques. Examples include vibration analyses, non-destructive materials testing, detonation and flame propagation studies, flight ballistics analyses, and medical diagnostics [4]. A most important consequence of this development is that holography is no longer a science con- strained to vibrationally stabilized locations. During the period 1969-1970 numerous holographic testing techniques were developed in the laboratory to analyse the response of various materials to thermal, chemical and mechanical stress [5]. Many of these laboratory results suggested possible applications of holography in the preservation and restoration of artistic statuary. During the Fall of 1971 Professor Walter H. Munk, Director, Institute of Geophysics and Planetary Physics, University of California, together with Mrs Munk, proposed a feasibility study to determine whether these sophisticated laboratory techniques could be applied in the field to actual artistic materials and problems. For this experimental demon- stration Venice, Italy, was chosen. Reasons for this choice include the rich diversity of artistic statuary, the very grave local deterioration problems, the availability of financial support from Ente Nazionale Idrocarburi, offers of technical support from local laboratories, and the existence of vigorous restoration programs there. The principal goal of the program was to demonstrate that high-quality pulsed holograms



50 J. F. Asmus, G. Guattari, L. Lazzarini, G. Musumeci and R. F. Wuerker

of large objects can be produced routinely without the aid of a sophisticated optics laboratory. In addition, it was hoped that archivally useful holograms could be produced to provide standards of comparison for the determination of possible future deterioration, damage, or erosion of important statuary. Further, there was the question of the applicability of non-destructive testing laboratory techniques to the location of flaws or damage in actual artistic objects now that laboratory simulations had been performed. Finally, as literally hundreds of applications of laser technology have been identified spanning almost every facet of human endeavor [6], it was deemed propitious to facilitate the close collaboration of restoration scientists and laser scientists for an interchange of ideas and techniques.

A BRIEF DESCRIPTION OF HOLOGRAPHY

By 1968 over one thousand articles on holography had appeared in scientific and technical journals [7]. Consequently, it is impossible to cover very much of the field without resorting to a monograph on holography [8, 14]. A brief but readily understandable introduction to the subject is provided by Reference 4. For the convenience of the reader who is unfamiliar with the principles of holography, a non-mathematical discussion follows. A number of laser types now exist that are sufficiently monochromatic and coherent to enable the formation of interference fringes from extended sources. In contrast, prior to the advent of the laser, optical interference fringes could only be formed by passing the light from a spectral lamp through a microscopic hole or series of holes whose distances from the interference plane differ only by a distance of less than a millimeter. To produce a hologram the beam from a highly coherent laser source is now used to illuminate the object to be recorded. The light scattered from the object is allowed to interfere with another light beam coming from the same laser source. This is called the reference beam. The resulting interference pattern is recorded on a sensi- tized photographic plate following the principles of ordinary photography. However, the developed plate (the hologram) does not resemble the object at all. Observed with the unaided eye, it shows a uniform darkening, sometimes with circular and/or irregular fringes. These are due to the non-uniformity of the reference beam, and they do not contain any information about the object. However, observed on a microscopic scale, the hologram is seen to be a series of fringes, recording the original interference between the object and the reference beam. These fringes contain all the information about the position and the brightness of every point of the object [2]. In other words, it would be possible from the fringe characteristics (spacing, direction, contrast) to calculate all the visible information about the object. Of course, the calculations would be very complicated, and fortunately there is a much simpler way to reproduce a faithful three-dimensional image of the original object. For this it is sufficient to illuminate the hologram with a laser beam similar to the reference beam originally used in constructing it. Some emerging light is diffracted from the hologram in such a way that it reconstructs a virtual image of the object in the same place where the object was during the recording. If an observer looks through the hologram, he observes an image of the original object with all its three-dimensional characteristics. For instance, by moving his head from one edge of the plate to the other, the observer can verify the parallax effect and can look at different sides of the object. Also he can focus on different planes of the object, thus verifying the depth of field.

Studies in Conservation, 18 (1973), 49-63



Holography in the conservation of statuary 51

Since holography is based upon the recording of an interference pattern, some extreme and rather stringent stability conditions result. Simply stated, the interference pattern must be stationary during the exposure of the plate. Motions of the object or mirror by as much as one-tenth wavelength of light shift the interference pattern enough to degrade the quality of the hologram reconstruction. As already noted, the pulsed ruby laser offers the only means of recording holograms without recourse to vibration and air current isolation equipment. A hologram requires typically one joule of light. A ten milliwatt gas laser would thus require a 100-sec exposure. A pulsed ruby laser achieves the exposure in a single pulse, either a fraction of a millisecond or a fraction of a microsecond. The above briefly describes holography and the nature of a hologram. Before concluding this summary, it is appropriate to describe some peculiar characteristics of this image formation process that make it so important. First, the photographic record (hologram) is not the image but a system of microscopic fringes. Any degradation or deterioration of the hologram does not correspond to an analogous deterioration of the image recorded in it as in ordinary photography. Also, if a hologram is broken into smaller pieces, the image contained in it is not lost because every piece can independently reconstruct the object (with reduced resolution). Second, no lens is needed to record the hologram. Consequently, large high-quality optical components of high cost are not necessary to reconstruct a faithful image. Third, many objects can be recorded on the same hologram; and each of them can be reconstructed alone with all of its three-dimensional information. Finally, the holographic image can be analysed in much the same way as the original object. For instance, it can be viewed in totality, or a particular spot can be examined with a microscope.

CHARACTERISTICS OF THE LASER

Holograms of the Venetian art objects were recorded with a ruby laser. Ruby was chosen because it emits high-energy light pulses in the visible portion of the spectrum where high- resolution photographic plates are sensitive. The short duration of the ruby laser pulse enables one to record holograms without vibration isolation equipment needed with low- power gas lasers. Ruby lasers have two modes of operation. These are the 'long pulse' or 'regular lasing mode' and the short pulse or 'Q-switch mode'. In the former the laser oscillates throughout a fair fraction of the flashlamp's pumping pulse. Typical emission time is 0-1-0-5 milliseconds, depending on the initial capacitor bank energy. Near threshold, the energy output is 1-2 joules for an oscillator such as the one used in this program (see Figure 1). In the 'Q-switch mode' energy is stored in the metastable chromium ions in the ruby rod. Oscillation is thwarted by an absorber placed within the oscillator cavity. In the present laser chlorophyll in mineral oil was used as the Q-switch. Increased pumping by the flashlamp causes the laser rod gain to exceed intercavity ab- sorption loss with the result that oscillation commences. The absorber bleaches abruptly, with the result that the excess energy stored in the excited chromium ions in the ruby crystal is converted into light within times measured in terms of the light transit time between the end mirrors of the oscillator. When operated in the Q-switch mode, a ruby oscillator, such as the one used in the present program, produces light pulses of < 0.1 microsecond duration. Energy content is typically 1/2-1 joule. The Q-switch mode enables one to record holograms under any condition of vibration or motion. Portrait holograms of living people, for example, require a Q-switch laser. Q-switching in general is an explosive phenomenon that results in oscillation of many modes.





Dye cell Q-switches, however, are employed as mode-controlling elements. The chlorophyll- mineral oil dye cells used to Q-switch the laser under the present program were chosen for their unique mode-selecting features. Whether one uses the regular lasing mode or Q-switching mode depends on ambient vibration conditions under which the hologram is recorded. As mentioned, portraiture of live subjects necessitates operation of the laser in the Q-switch mode. Holography of moderately stable subjects permits in many cases operation in the regular lasing mode. In holography one must first satisfy the condition that the object and mirrors of the holographic set-up are stable to < X/10 during the exposing time of the hologram (e.g., 10-7-10-3 sec). Next, one has to ensure that the coherence of the laser is sufficient to record the depth of the scene. Finally, one has to have enough energy returned to the hologram or plate for adequate exposure. The 'long pulse' or regular lasing mode has the feature that the process of stimulated emission keeps the rod gain down to low values with the result that few modes oscillate; as a result, coherence is fairly good. One is then concerned only with the shift in wavelength due to heating of the rod by the flashlamp. In general, one obtains more moderately coherent energy in the regular laser mode than in the Q-switch mode. Coherence lengths of > 10 cm are possible depending on how close to threshold the laser is operated. Since the statues were basically static and floor-mounted, the laser was operated in the regular lasing mode for the greater number of holograms recorded in this study. The regular lasing mode is, further, not as prone to damage the optical components of the laser. The ruby laser head was brought by air from TRW SYSTEMS in Redondo Beach, California. The head was a copy of one developed [15] for USAF/RPL. Its main features are its porta- bility, the inclusion of an HeNe spotting laser, aligning autocollimator, pumping uniformity, and mode control. The laser head is shown schematically in Figure 1. All components are mounted on a common plate which may be enclosed in covers. The laser contains two ruby rods identical in size (1-28 x 9-5 cm). One is in the oscillator, the other serves as an amplifier. Each is mounted in watertight housings which also contain the helical flashlamps and silver lamp reflectors. Rubies and flashlamps are cooled by flowing water in and out of the housing. Coaxial cables connect the flashlamps to external capacitor banks. The oscillator portion is formed by the one ruby rod and a pair of aligned external mirrors. In this laser the mirrors were mounted on three-point aligning plates attached to an invar frame. Mirror separation was 1 m. Feedback by the mirrors in concert with the gain of the ruby, when pumped, causes the combination to oscillate over the band of frequencies permitted by the R, transition of the ruby rod (~ 1/4 Angstr6m). One of the mirrors of the laser resonator is a 99 % dielectric mirror. The other is a sapphire plate of 3-2 mm thickness. This functions both as an output mirror and as a mode-discriminating element in the laser cavity. The output beam from the laser is reflected by two 90' prisms. These turn the beam through 1800 and direct it into the second rod in the amplifier housing. One expects an energy gain of about three from such a rod when pumped near saturation. For Q-switching the dye cell is placed before the 99 % dielectric mirror. Two chlorophyll dye cells were brought to Italy, one of 23 cm in length and the other of 53 cm [3]. Also mounted on the base plate was a dark field autocollimator and a small one-milliwatt helium neon laser. Retractable mirrors permitted one to sight down the ruby oscillator with the autocollimator. Reflection of the light from the collimator's internal are enabled one to align the dielectric mirror and sapphire reflector of the ruby oscillator to high precision. The telescope further permitted alignment of the helium neon laser so that its




Holography in the conservation of statuary Retractable To Capacitor

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FIG. 1 Schematic diagram of the ruby oscillator-amplifier assembly used for holographic illumination.

beam was parallel to and coincidental with the pulsed ruby beam. The helium neon laser could then be used to align the other optics of the holographic arrangement. Operation of the laser head required a power supply with individual capacitor banks to drive the two flashlamps in the laser heads. Such a unit can weigh as much as 400-500 kilo- grams. Rather than bring a power supply from California, it was decided to improvise and assemble one from components procured in Italy. Six 3-5 kV/250 pF energy-storage capacitors were loaned by CNEN, Frascati, along with a cart and a 5 kV/1A high-voltage charging supply.

HOLOGRAPHIC ARRANGEMENT

The pulsed holograms were exposed with the two-beam arrangement depicted in Figure 2. The ruby laser main beam is first split into the main illumination beam and a reference beam. The beam splitter directs 4 % of the energy into the reference beam. The negative lens and positive mirror in the reference leg expand the reference beam to the diameter of the holographic plate. The negative lens (which in some instances was a cylindrical lens and a ground glass diffuser) in the primary beam expanded the radiation

SUBJECT

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,

HOLOGRAPHIC PLATE

SUBJECT REFERENCE ILLUMINATION BEAM

RUBY PRIMARY RISM LASER BEAM BEAM

SPLITTER

FIG. 2 Schematic diagram of the arrangement of components in the holographic set-up.





so as to illuminate the subject. The reference beam mirror was positioned so that the optical ray paths for the primary and reference beams were equal. This served to maximize the coherence of the interference at the holographic plate. In addition, it was discovered that the subjects were strongly depolarizing to the red laser radiation so that a sheet polarizer was required in front of the holographic film. A photograph of the holographic apparatus is shown in Figure 3. The laser head (under- going adjustment) occupies the left half of the table. The holographic plate (obscuring the face of the statue) is above the right end of the table. The reference beam folding mirror is supported by a tripod behind the table. The scene is in the Restoration Laboratory of the Superintendency of Galleries and Works of Art of the Veneto located in San Gregorio.

HOLOGRAPHIC SUBJECTS

The original holographic subject selection criteria that were established to guide the project were the following: - The subject should be on the outside of a building, by a known artist or architect, and

contribute to the quality and frame of the building; - The subject should have a 'medical' history including the origin of the stone, when

carved, restoration and history of exposure; - The subject should be deteriorating but not yet totally obliterated, and - The subject should be at the limit of holographic technology (,-;

2 m in size) and too fragile and/or expensive to transport for the purpose of restoration.

FIG. 3 Photograph of the holographic apparatus as assembled in the Restoration Laboratory of the Superintendency of Galleries and Works of Art of the Veneto located in San Gregorio. Studies in Conservation, 18 (1973), 49-63




Two examples were chosen in Venice that best met the above conditions. The first was the statue 'Temperanza' which is in storage in the Palazzo Ducale. It is made of Carrara marble that is badly encrusted and eroded. The second was the capital under the statue of Solomon which is out of doors at the corner of the Palazzo Ducale. The goal of holographing these subjects had to be set aside midway through the project. There were several reasons for this change in plans. The major reason concerned the fact that the relocation of the laser from Laboratorio San Gregorio to the Palazzo Ducale would require an inordinate amount of time considering the limited duration of the project. In addition, the holographic work in Laboratorio San Gregorio revealed that the statuary subjects are very poor reflectors of laser radiation. Consequently, it appears that a larger laser will be required to produce satisfactory holograms of a large dark statue such as the 'Temperanza'. Consequently, the primary subjects that were holographed were two important statues that were fortuitously available in Laboratorio San Gregorio. These were 'San Giovanni Battista' by Donatello and 'Madonna con Bambino' by Nino Pisano. They are shown in Figures 4 and 5. The statue 'San Giovanni Battista' is a polychromed wood-carving by Donatello. It was completed in 1453-54 prior to the master's return to Florence. The carving measures 1-40 x 0-42 meters. It is normally kept in the Cappella dei Fiorentini within the Chiesa di Santa Maria Gloriosa dei Frari in Venice. The second subject was the 'Madonna con Bambino' by Nino Pisano. It dates from the fourteenth century and is made of Carrara marble. Its dimensions are 1-40 x 0-45 x 0-32 meters. It is housed in the Chiesa dei San Giovanni e Paolo in Venice.

EXPERIMENTAL RESULTS

1. Holography Approximately fifty holograms were produced during the experimental phase of the program (three weeks). In about 80 % of these the image is clearly visible with good contrast and definition. Most of these are close-ups of the 'San Giovanni Battista' and the 'Madonna con Bambino'. The usual approach involved first making 10 x 12-5 cm close-up holograms (AG 8E75 and 10E75 emulsions) in order to optimize laser operation and subject illumination. The subject was then moved back from the holographic plate on successive shots in order to obtain an ever even wider field of view (a greater portion of the statue). This was all accomplished with the 10 x 12-5 cm plates, as a shutter was available in this size so that the work could be done in normal light. Then at night the shutterless 20 x 25 cm plates could be exposed yielding the widest subject perspectives. Due to the unanticipated very dark hue of both statues, no single hologram was produced that yielded an image of an entire statue. However in some instances as much as 90% of the statue was recorded. It is probable that with about twice the laser energy that was available these dark subjects could be recorded in totality. The TRW laser might normally have been capable of such an output energy; however, the borrowed capacitors had a very high inductance, and reduced performance resulted. Also, if these subjects had not produced such severe depolarization there would have been little difficulty in obtaining complete views.





Fig. 4 Fig. 5

FIG. 4 Photograph of the polychromed wood-carving 'San Giovanni Battista' by Donatello (1453-54). FIG. 5 Photograph of the fourteenth-century Carrara marble statue by Nino Pisano, 'Madonna con Bambino'.

Alternatively, if the subjects could have been dusted with a fine-grained light-colored powder, then the available laser energy would have been sufficient. Figure 6 displays a photograph of a holographic reconstruction of the face of the Donatello statue of 'San Giovanni Battista.' In this instance the illuminating laser beam is visible along with the 'speckle' effect that emerges when a hologram is viewed from a small subtended angle. In Figure 7 the subtended angle has been increased with a much shorter focal-length camera. Here the observable 'speckle' has almost disappeared.

2. Contouring Mode locking was observed when the laser was fired above threshold by 10-20 % in voltage. This mode of operation yields a train of laser pulses that are separated in time by a period equal to twice the optical transit time through the sapphire etalon. Illuminating both statues in this manner produced a series of holograms whose images contained a series of alternately bright and dark contour rings [9]. The contour spacing should be about 5 mm; however, no microscopic measurements were performed on the holographic images to verify this figure. Studies in Conservation, 18 (1973), 49-63




FIG. 6 Photograph of the holographic image of the 'San Giovanni Battista' with laser reference beam showing on the left.

FIG. 7 Large-aperture photograph of the holographic image of the 'San Giovanni Battista' illustrating the reduction of 'speckle' over Figure 6.

3. Holographic Interferometry Several double-exposure interferometric holograms were produced during the course of the experimental program. Those attempted with the wooden Donatello statue proved successful, whereas those attempted with the marble Nino Pisano statue did not yield any visible fringes.





Fig. 8

Fig. 9

FIG. 8 Photographic reproductions of the interferometric holographic image of the right leg of the 'San Giovanni Battista' showing the effects of humidity on the unhomogeneous materials of the carving. FIG. 9 Photograph of the reconstruction of a double-exposed holographic interferogram of the 'San Giovanni Battista.' Between the two exposures, the right side of the face was warmed thermally with a lamp. The fringes show the thermal distortion of the order of a wavelength of light. Note that the fringes accentuate the visibility of cracks in the finish, particularly in the area of the cheek.

The first interferometric tests involving the Donatello statue were concerned with thermally induced distortion. In the interval between the two exposures one side of the head was raised slightly in temperature. This was accomplished by pointing an incandescent light at one side of the statue from a distance of about 1 m. Heating times of the order of one to two minutes produced observable distortions of at least ten fringes. Studies in Conservation, 18 (1973), 49-63




Similar tests with the marble Nino Pisano statue did not produce observable fringes. This is probably a consequence of the relatively high specific heat of marble and the resul- tant very minimal temperature rise obtainable with the light. Late in the experimental phase of the program the Donatello statue was placed within a transparent box. In this manner holograms could be made under various conditions of local relative humidity. Specifically, a desiccant material was placed in the box so that the relative humidity within the container continually decreased. A first holographic exposure would be made before the desiccant had removed any significant amount of moisture. Then after an hour or more when the relative humidity had decreased a few per cent (e.g., from 60 % to 580% R.H.), a second holographic exposure was taken on the same plate. Successful interferograms were produced in this manner. Invisible subsurface cracks and patches were identified on the wooden statue by this technique. Figure 8 shows two photographs of an interferometric image made in this fashion of the right leg of the 'San Giovanni Battista.' Figure 9 is a photograph of a thermally induced interferometric image. Double exposure interferograms were also attempted with the marble statue. A slight dimensional change was sought through surface cleaning between exposures. No interference fringes were observed on these resulting holographic images that were conceived as an erosion measurement simulation. Possible explanations for the non-appearance of fringes are (1) inadvertent statue movement while cleaning, (2) highly unhomogeneous material removal during cleaning, (3) chemical surface change without physical change during cleaning, (4) excessive material removal, or (5) insufficient material removal.

CONCLUSIONS

There were several areas of accomplishment in the feasibility study reported here. The most significant of these was undoubtedly the insight gained into the potentialities and limitations of the holographic technique in the art conservation field. In addition holograms of potential archival utility were produced of the two important statues. Further, the collaboration of laser scientists and conservation scientists led, rather unexpectedly, to the development of what appears to be an economical, rapid, and high-quality statuary- cleaning technique using laser radiation [10]. The results of the holographic tests indicate that high-quality pulsed holograms can be made with a minimal investment of time and funding. Support facility requirements are also not great. An improvised power supply was adequate for laser excitation. A borrowed plastic aquarium pump and ice cubes from a nearby bar and nunnery facilitated laser cooling. A wooden table was sufficiently stable for supporting the laser and the holographic plate. Finally, the transportation and reassembly of the laser did not present any insur- mountable problems. The probative work described in the preceding sections suggests a series of potential applications of holography in aiding the conservation of statuary.

1. Recording In many instances the recording capability itself might be of considerable significance. For example, a unique holographic museum could provide a visual record of the appearance of art treasures for posterity. In addition, such an institution could also be useful for classification purposes. This is an especially important problem in Italy where a large number of artistic works are still unclassified due to the extensive and diverse artistic patrimony. An extension of this concept involves the holographic image as a standard of





comparison or template. In this context it could be useful in detecting and locating damage sustained through transportation or other causes. Similarly, the holographic image might serve as a template in repairing such damage.

2. Non-destructive Testing Undoubtedly, the most dramatic aspect of the study was the ease with which sub-surface cracks and patches in statuary could be located through double-pulse holographic interferometry. This should not be especially surprising, as non-destructive testing is probably the major application of holography in industry at this time. In a similar vein this capability could be of enormous usefulness to the restorer of statuary.

3. Teaching Aid It has been suggested that holograms might be quite useful as a visual aid to supplement normal photographs in teaching art history, techniques, and conservation [11].

4. Erosion Measurement One of the principal goals of the study was that of measuring the erosion rate of a marble surface. The few hours of experimental work that were devoted to this problem yielded a negative result. One of the authors of this paper [12] has suggested that this type of measurement requires an interferometric laboratory arrangement. Holographic interferograms could then be made of individual marble crystals through a microscope.

5. Speculative Applications It would be of great interest to art scholars and restorers, as well as laymen to be able to see certain masterpiece sculptures as they appeared prior to the onset of erosion and deterioration. In principle this original image-recovery should be possible by combining holographic and high-capacity (and high-speed) numerical data-processing techniques. In essence this is an inversion of the holographic technique for measuring erosion. However, the realization of such a goal will require major advances in computer technology, computer software, and holographic technology. The non-linear character of typical erosion will also provide a major obstacle. Another advanced concept involves 'machining' a replica statue from any desired material by utilizing a high-energy laser and a holographic template. In the infrared portion of the spectrum sufficiently powerful lasers exist to remove through evaporation any desired quantity of material [13]. The hologram itself might be used to focus the light three-dimen- sionally, assuming it was able to pass such fluxes without damage. The problems of converting the holographic virtual image into a perfectly focused three- dimensional real image to implement localized 'machining' will obviously require further development.

ACKNOWLEDGEMENTS

The work discussed in this paper resulted from an idea developed by Professor and Mrs Walter Munk in collaboration with Dr John Asmus during the 1971 JASON Summer Study supported by the Advanced Research Projects Agency and the Executive Office of Science and Technology. The initial funding for the investigation was provided by Ente Nazionale Idrocarburi with the assistance of Ing. R. Girotti, Dr E. Ottier, and Ing. Gervasio. Ad- ditional financial support was provided by Science Applications, Inc., and TRW Systems Group. The Istituto Centrale del Restauro, Rome, which had been instrumental in initiating Studies in Conservation, 18 (1973), 49-63




earlier work at the University of Rome on the applications of holographic interferometry in the field of the conservation of works of art (particularly deformations of paintings on canvas), was responsible for interesting ENI in the project. The initial plans for the project were formulated by Professor and Mrs Munk, Dr E. Ottier (ENI), Dr G. Urbani (Istituto Centrale del Restauro, Rome), Dr R. Frassetto (Laboratorio Grandi Masse, Venice), Professor F. Valcanover (Soprintendente alle Gallerie ed Opere d'Arte del Veneto, Venice), and Professor F. Gori and Dr G. Guattari (University of Rome). Soprintendente Valcanover kindly provided space in the Laboratory of San Gregorio for the work. The experimental sequence was planned by Dr J. Asmus (SA1) and implemented by Dr R. Wuerker (TRW) with the assistance of Dr J. Asmus, Dr G. Guattari, Dr L. Lazzarini (Laboratory of San Gregorio), and Mr Matthew C. Wuerker (Palos Verdes Estates). The laser was supplied by TRW. The staffs of Laboratorio San Gregorio (particularly the photographic staff) and Laboratorio per lo Studio della Dinamica delle Grandi Masse, under the direction of Soprintendente Francesco Valcanover and Dr Roberto Frassetto respec- tively, provided extensive assistance. This work could not have been accomplished without the generosity, co-operation, and assistance of a host of individuals and organizations that were not directly connected with the project. These include Professor Romano Toschi (Laboratori Gas lonizati, EURATOMCNEN, Frascati), Count Paolo Marzotto and Ing. Similian Cibic (Marzotto e Figli, Valdagno-Vicenza), Mr Kenneth Hempel (Victoria and Albert Museum, London) and Dr Oswald Ganley (U. S. Embassy, Rome). Special thanks are due to ALITALIA for superb handling of the transportation of the laser.

GLOSSARY

Coherence - refers to the regularity or spectral purity of an optical wave.

Feedback - the portion of the laser beam reintroduced into the laser in order to maintain laser emission.

Flashlamp - a gaseous electrical discharge lamp (usually containing xenon) used to supply optical energy (excitation) to a solid transparent laser material such as ruby.

Interference fringe - the loci of constructive or destructive interference between two coherent beams of light.

Interferometric - generally pertains to an optical apparatus whose components are vibrationally stable to a fraction of the wavelength of light (e.g., 10-7 meter). Laser cavity - a system of mirrors that permits a portion of a laser beam to repeatedly traverse the laser material so as to maintain the laser emission.

Mode discriminating element - an optical component introduced into a laser cavity in order to suppress certain (longitudinal) laser cavity resonances so that the laser beam is more monochromatic and higher in coherence.

Pumping pulse - the pulse of light from the flashlamp that transmits energy to the laser material in order to excite it into laser emission.

R, transition - the spectroscopic designation of a very narrow red emission line in the ruby spectrum that is usually excited in a ruby laser.





REFERENCES

1 LEITH, E. N., and UPATNIEKS, J., 'Wavefront Reconstruction with Continuous-Tone Objects', J. Opt. Soc. Amer., 53 (1963), 1377-1381.

2 GABOR, D., 'A New Microscopic Principle', Nature, 161 (1948), 777. 3 WUERKER, R. F., and HEFLINGER, L. O., 'Ruby Laser Holography', Presented at the SPIE

Conference, 14-17 September 1970, Anaheim, California. 4 GABOR, D., KOCK, W. E., and STROKE, G. W., 'Holography', Science, 173 (1971), 11. 5 LEITH, E. N., and VEST, C. M., Investigation of Holographic Testing Techniques, University of

Michigan Report 2420-12-P, September 1970. 6 STICKLEY, C. M., and GRINGRANDE, A., Bibliography of Laser Applications, Report AD655774,

Air Force Cambridge Research Laboratories, April 1967. 7 LATTA, J. N., 'A Classified Bibliography on Holography and Related Fields', SMPTE Journal,

(April 1968), 540-580. 8 STROKE, G. W., An Introduction to Coherent Optics and Holography, Academic Press, New

York, 2nd ed., 1969. 9 HEFLINGER, L. 0., and WUERKER, R. F., 'Holographic Contouring Via Multifrequency

Lasers', Applied Physics Letters, 15 (1969), 28-30. 10 LAZZARINI, L., ASMUS, J. F., and MARCHESINI, L., 'Lasers for the Cleaning of Statuary: Initial

Results and Potentialities', Presented at ler Colloque International sur la Deterioration des Pierres en (Euvre, La Rochelle, France, 11-16 September 1972.

11 CHAMBERS, C. M., Associate Dean, George Washington University, private communication, June 1972.

12 WUERKER, R. F., TRW Systems, private communication, April 1972. 13 AsMus, J. F., 'Potential Commercial Applications of High-Power Lasers', AIAA Conference

on Plasma Dynamics, Palo Alto, California, June 1971. 14 COLLIER, R. J., BURCKHARDT, K., and LIN, L. H., Optical Holography, Academic Press, 1971. 15 WUERKER, R. F., Instruction Manual for Ruby Laser Holographic Illuminator, TRW Report

11709-6003-RO-00, February 1970.

First received 10 September 1972 Received in revised form 1 November 1972

JOHN F. ASMUS, B.S. 1958, M.s. 1959, Ph.D. 1964, all at the California Institute of Technology. Between 1954 and 1965 he held positions with the u.s. Naval Ordnance Laboratory and Tektronix, Inc., where he performed research on ionospheric phenomena, electro-luminescent devices, and high-temperature plasma phenomena. Between 1965 and 1969 he managed the Laser Department at General Atomic, San Diego, California. From 1969 to 1972 he provided co-ordination and analyses of Defense Department laser programs while with IDA, Washington, D.C. He has approximately twenty-five publications in the scientific literature and is now a Vice-President of Science Applications, Inc., with headquarters at La Jolla, California.

Author's address: Science Applications, Inc., 122 La Veta Dr., N.E., Albuquerque, New Mexico 87108, U.S.A.

GIORGIO GUATTARI, born 1943. Received the Italian degree of 'Doctor in Physics' from the University of Rome (1966). Presently, Assistant Professor in Experimental Physics at the Institute of Physics, Faculty of Engineering, University of Rome. He has worked in the field of optics with particular interest in image processing and holography and is a member of the Optical Society of America.

Author's address: Universita& di Roma, Facolta di Ingegneria, Istituto di Fisica, Rome, Italy.

LORENZO LAZZARINI, born 1947. Graduated in Industrial Chemistry (1967) and is taking a Doctorate in Geology with a specialization in Mineralogy at the University of Padua. Since 1967 he has been working at the Scientific Department of the 'Laboratorio del Restauro di S. Gregorio' for the Soprintendenza alle Gallerie e alle Opere d'Arte del Veneto. Experience and publications in technical and scientific examination of paintings and in problems of conservation of stones and marbles.

Author's address: Laboratorio del Restauro di S. Gregorio, Dorsoduro 170, 30123 Venice, Italy





GIULIA MUSUMECI, born 1948. Specialized in restoration of marble and stone sculpture. Graduated from the Licea Artistico of Venice in 1968. Intensive study sponsored by Italian Art and Archives Rescue Fund at the Conservation Department of the Victoria and Albert Museum of London. Presently, working in the Chemical Laboratory of the 'Laboratorio del Restauro di S. Gregorio' for the Soprintendenza alle Gallerie e alle Opere d'Arte del Veneto. Her work keeps her in contact with the Victoria and Albert Museum (London), Istituto Centrale del Restauro (Rome), Centro Internationale di Studi per la Conservazione e Restauri dei Beni Culturali (Rome), and Centro per la Conservazione delle Sculture all'Aperto (Bologna).

Author's address: Laboratorio del Restauro di S. Gregorio, Dorsoduro 170, 30123 Venice, Italy.

RALPH R. WUERKER, B.s., Occidental College 1951, Ph. D., Stanford University 1960. Presently, a member of the Systems Group Research Staff of the Applied Technology Division at TRW, Redondo Beach, California. His thesis was written under the direction of Paul Kirkpatrick, one of the pioneers of holography. He is a member of the American Physical and Optical Societies, and the American Association of Physics Teachers. He is the author of over fifteen scientific papers and has nineteen patents granted. Honors are Sigma Xi (1952), the Research-Engineering-Scientific Applications Award (1966), and guest lecturer at universities for the Midwest Mechanics Seminar (1971). He taught a holography course at UCLA Extension.

Author's address: Systems Group of TR W, Inc., Building R1, Room 1144, One Space Park, Redondo Beach, California 90278, U.S.A.

Abstrait-Au printemps de 1972, a Venise, une 6tude exp6rimentale a Wt6 men6e pour d6- montrer la possibilit6 d'holographie in sivu de statues et de sculptures sur bois de grandeur naturelle. L'accent a Wt6 mis sur les sujets holographiques en 6tat de d6sagr6gation de plus en plus prononc6e, r6sultant de processus provoqu6s par l'environnement. Il a Wt6 produit des enregistrements d'images archivales de la sculpture sur bois 'Saint Jean Baptiste' de Donatello, et de la statue en marbre 'Madonne avec Enfant' de Nino Pisano, a l'aide d'holographie pul- sative au laser a rubis. De plus, les techniques interf6rom6triques holographiques se trouvaient Wtre d'une utilit6 potentielle dans la restauration de statues, du fait de localisation de taches non visibles, de fissures et de f6lures.

Kurzfassung-Im Frtihjahr 1972 wurden in Venedig (Italien) experimentelle Studien durch- geftihrt, um die Anwendung der in situ Holographie in der Aufzeichnung lebensgrosser Bild- und Schnitzwerke nachzuweisen. Im besonderen wurden Hologramme von Kunstwerken hergestellt, die durch Umweltprozesse einem beschleunigten Zerfall ausgesetzt sind. Holo- graphische Bildaufzeichnungen wurden mittels eines gepulsten Rubin-Lasers von dem Schnitzwerk 'St. Johannes der Tiufer' von Donatello und dem Marmorbildwerk 'Madonna mit Kind' von Nino Pisano gemacht. Ausserdem zeigte es sich, dass die interferometrischen holographischen Techniken m6glicherweise bei der Restaurierung von Bildwerken von Nutzen sein k6nnen, da man versteckte Flickstellen, Risse und Brtiche damit aufdecken kann. Riassunto-Nella primavera del 1972 uno studio sperimentale fu svolto a Venezia per dimo- strare la praticabilith di olografia sul luogo di statue in marmo ed intagli in legno a grandezza naturale. L'interesse principale era rivolto ad oggetti da olografare che si trovassero in uno stato di accelerata disgregazione a causa di processi ambientali. Documenti-immagine da per archivio furono ottenuti tramite olografia (laser ad impulsi a rubino) della statua in legno 'S. Giovanni Battista' di Donatello, e della statua in marmo 'Madonna con Bambino' di Nino Pisano. Inoltre si soprl che la tecnica di interferometria olografica

, di possibile utilith per il

restauro di statue, tramite la localizzazione di rattoppi, incrinature e crepe nascoste. Extracto-Durante la primavera del afio 1972 se efectu6 en Venecia, Italia, un estudio experimental para demonstrar la posibilidad de holograffa in situ de estituas y esculturas de madera en tamanio natural. El acento se puso en sujetos hologrificos que estaban en un estado de desintegraci6n accelerada resultando de procesos del ambiente. Se produjeron relaciones de estatua para el archivo mediante holografia pulsada por medio de un 'ruby-laser' de la escultura de madera 'San Juan el Bautista' por Donatello y de la estatua de marmol 'Madona con Nifio' por Nino Pisano. Ademis se encontr6 que t6cnicas hologrificas inter- ferom6tricas tienen utilidad potencial para la restauraci6n de estatuas mediante la locali- zaci6n de parches, grietas y defectos escondidos.




holography in the conservation of statuary

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