Frontier, innovative research performed by the “PHOTONICS FOR HERITAGE SCIENCE (HS)” group for over twenty years has positioned IESL-FORTH among the leading centers worldwide for scientific research and technological development of cutting-edge laser and optical tools that offer new and efficient approaches to the diagnostics and conservation of works of art and antiquities. Emphasis is placed on: a) laser ablation methodologies for cleaning and restoration, b) laser spectroscopy for compositional analysis, c) multi spectral imaging and d) holographic metrology techniques for structural diagnosis.
The vision and the mission of this activity has been to develop state-of-the-art laser-based methods in order to a) enhance our understanding about cultural heritage (CH) materials and processes and b) provide versatile and effective tools for the analysis, diagnosis, preservation and restoration of archaeological/historical objects, works of art, and monuments.
The principal axes and the main developmental objectives of the individual activities are highlighted in the following:
The main direction of the optical imaging activity is to develop new methods for the non-contact, non-invasive, in-situ examination and continuous inspection of CH objects. Applications include mapping of varnish and paint layers, stratigraphic analysis and visualization of under-drawings, assessment of past conservation treatments, real time monitoring of cleaning interventions etc., on the basis of reflectance imaging and spectroscopy. The goal of this research is to investigate, quantify and expand the potential applications of spectral imaging on CH objects, while continuous technical developments both in hardware and software target to improve analytical capabilities.
Recently, a novel technique, Photoacoustic Imaging, predominantly developed in the context of biomedical research, has been applied on CH objects. The photoacoustic signal can overcome limitations of light and offer substantially improved detection sensitivity at high spatial resolution. The technique has been proved to be capable of uncovering “hidden” features in multi-layered CH objects such as paintings and determining the thickness of thin layers thus providing micrometric precision stratigraphic information. Finally, the potential of applying the photoacoustic signal for the in situ and real-time monitoring of laser cleaning interventions is currently investigated.
Different spectroscopies, relying largely on laser sources but not ignoring conventional ones, are employed in order to characterize the chemical composition of materials and study photo-physical and photochemical processes in condensed phase systems. Emphasis is placed on a) the applications of spectroscopic tools in art and archaeology (LIBS, LIF, Raman, SERS etc.) and b) the development of compact/portable laser-based analytical instrumentation for in-situ and field deployable applications, which combine different spectroscopic capabilities in hybrid arrangements.
Laser Interferometry for Structural Diagnostics. The main direction of this activity is to investigate deformation, deterioration and fracture mechanisms in order to evaluate the structural condition of materials and systems. The remote optical sensing of almost invisible defects and alterations, which allows their location, measurement and exact positioning within the structure of the object, as well as their monitoring through environmental and climate changes, conservation treatments, natural or provoked ageing, transportation or handling are among the objectives of this research group. This is achieved by the effective transfer of interferometric techniques and know-how to CH applications (artworks and monuments) and the continuous inspiration, design, development and implementation of new experimental processes, methodologies and instruments in order to exploit the full potential of laser metrology.
Cleaning Artworks with Lasers
Research at the Laser Cleaning (LC) group is developing novel laser ablation methodologies in order to meet a number of diverse conservation and cleaning challenges. Highlights of this activity are:
- The initiation of a Bilateral collaboration between IESL-FORTH and the Conservation department of the PALACE MUSEUM in Beijing, CHINA with the aim to organize a common Laser Technology Joint Laboratory on Cultural Heritage with the name “NIKI”.
- The development of a prototype laser system and methodology for the removal of pollution encrustation from the Parthenon Sculptures, which ensures safe and controlled cleaning without discoloration side-effects. This methodology is being applied since 2002 in a number of sculptures at the Athenian Acropolis Monuments (i.e. the West Frieze, metopes and pedimental sculptures of the Parthenon, the Frieze of the Athena Nike Temple, and the roof of the Caryatids’ porch in the Erechtheion). Since 2011 it has been also included in the regular conservation practice of the Acropolis Museum and along these lines an advanced laser laboratory has been set up on the visitors’ floor where the Caryatids are exhibited. Removal of pollution accumulations from their surface takes place in this laboratory while visitors can follow live the cleaning process being carried out. Further synergies between the Acropolis Museum and IESL-FORTH have been planned in the context of an “open common laboratory” in which modern laser based diagnostic and imaging techniques will be used for expanding the knowledge and deciding the best conservation practices for the exhibits.
Current research and activities aim at:
- elucidation of laser induced side-effects in Cultural Heritage applications e.g. yellowing of stonework and pigments’ discoloration,
- optimisation of laser assisted removal of polymeric over-layers from paintings,
- monitoring and control of the cleaning intervention through optical and laser based techniques (spectral, optoacoustic, non-linear imaging, laser spectroscopies, holographic interferometry, etc.),
- realization of laser cleaning projects on important CH objects and monuments in the lab or in-situ.
Additionally, the implementation of laser technology to other challenges in Heritage Conservation is investigated. Recently research efforts have been focused to the restoration of cracks and other pathologies found on the surface of glazed ceramics using laser irradiation; cracked glazed surfaces have been restored on the principle of laser-induced local and controlled melting of the existing glaze material.
a unique analytical and monitoring tool for Cultural Heritage objects
IRIS-II is a complete, flexible instrument for imaging the compositional and structural information of layered surfaces. It is fully portable and independent enabling thus the examination of objects in situ (museums, conservation laboratories, archaeological areas etc.).I This Infrared imaging system, provides detailed information related to the chemical composition of materials, based on reflection and fluorescence spectroscopy.
- Paintings (under-drawings, over-paintings, varnishes, biological alterations)
- Archaeology (traces of pigments, biological attack)
- Food (quality characterization)
- Industry (analysis, quality control)
- Medicine (optical biopsy)
The main elements of the multi-spectral imaging system include a camera, an imaging monochromator (i.e. filter wheel), the objective lens, adaptation optics, electronics and a computer that integrates all the components. The most important part of the spectral imaging system is the imaging monochromator. In our case a novel filter wheel is used that includes 28 band-pass filters. The camera used on the system is a monochrome digital CMOS camera. The spatial resolution is 5MPixels, while the dynamic range applied is 8bit. This sensor is sensitive from 350nm up to 1200nm. With our filter set, we can exploit the whole sensor sensitivity range and in some cases, we can even push it to its limits (350nm and 1200nm). The whole system is design to be portable. It is lightweight and small and can be carried in a small case and a laptop bag. It is also mobile, with enough battery to operate for up to 2 hours in places where electricity is not an option. In those cases, we have developed battery operated monochromatic light sources with LED technology. Finally, custom made software, entirely developed in LabView is employed. This software enables the control of the system and the acquisition of measurements. Additional processing software for calibration and analysis is also developed and used.
A prototype laser cleaning instrument customarily developed for the removal of pollution encrustation from the Athens Acropolis Monuments. This is a specially modified Nd:YAG system, emitting at the fundamental (1064nm) and third harmonic (355nm) frequencies. The two beams are spatially and temporally overlapped while they can be used separately or in combination (in varying Fluence ratios, FIR/FUV).