Domenica Paoletti

Displacement measurement using the Talbot effect with a Ronchi grating

In this study the Talbot effect with a Ronchi grating is applied to measurement of shift. It is shown that by using a measurement procedure based on the Fourier transform a cosine grating can be replaced by a Ronchi grating. Two different methods for data reduction and some experimental results are given.

Contouring of artwork surface by fringe projection and FFT analysis

In the present work, we propose a simple optical method to perform profilometry on works of art. The method is based on the projection of a Ronchi grating onto the surface to be analyzed. When viewed at an angle different from the projection angle, the grid pattern appears deformed by the surface shape. This pattern is digitized, by a high-resolution CCD camera, and then processed using a Fourier transform analysis. The technique is free from the errors caused by higher harmonic components of the grating pattern. Furthermore, the method relies on very simple equipment and it is therefore suitable for in situ measurements. Theoretical details and examples of the technique in operation are given.

Artwork diagnostics with fiber-optic digital speckle pattern interferometry

The possibility of employing a digital speckle pattern interferometer with optical fibers for nondestructive testing of artwork has been evaluated. A simple and accurate fiber-optic interferometer, able to perform real-time measurements in a hostile environment, has been realized with a minimum of optical components and a minimum of adjustments. As a result a deformation map of wooden panel paintings or mural frescoes under thermal stress may be depicted. Experimental results, obtained by the application of this interferometer both to laboratory models and to mural frescoes in situ, are presented.

Diagnostics of panel paintings using holographic interferometry and pulsed thermography

Holographic and thermographic techniques have been recently applied in artwork diagnostics for the quantitative evaluation of defect size and depth in laboratory samples of artworks. The aim of this study is a comparison between holographic interferometry (both double exposure and real time), and pulsed thermography (PT) processing techniques such as differential absolute contrast (DAC) and pulsed phase thermography (PPT) for the detection of the subsurface flaws on wooden panel paintings. The performance of holographic techniques can be reserved for investigation of particular defects (cracks, detachments) at incipient stages, where high resolution/sensitivity is required, while PT can provide interesting quantitative results in situ.

IV Interferometric Methods for Artwork Diagnostics

Publisher Summary This chapter surveys different interferometric methods and their application to nondestructive artwork testing. The chapter analyzes the holographic methods used for artwork diagnostics and considers some optical arrangements. A measuring instrument should visualize micro deformation and/or displacement, perform real-time analysis, operate under field conditions, and require a minimum of adjustments. These requirements can be fulfilled with electronic speckle pattern interferometry (ESPI), a technique used as an industrial inspection tool. The interferometric techniques complement other methods of nondestructive testing used in artwork diagnostics but are not yet sufficient to give an optimal response to the many problems of the restoration field. A more effective collaboration with conservation scientists and restorers in the future might play an important role in familiarization with various problems of conservation and yield to the development of portable, inexpensive and easy to use instruments.

From the experimental simulation to integrated non-destructive analysis by means of optical and infrared techniques: results compared

In this work the possibility of modeling manufacturing ceramic products is analyzed through the application of transient thermography, holographic interferometry and digital speckle photography, in order to identify the subsurface defects characteristics. This integrated method could be used to understand the nature of heterogeneous materials (such as plastic, sponge simulating a void, wood, aluminum) potentially contained within ceramic materials, as well as to predict crack formation due to them. The paper presents the analysis of green ceramic tile containing defects of different types and sizes located at different depths. The finite element method is used for solving the problem of transient heat transfer occurring in experimental conditions. Unknown parameters of the numerical model (such as convective heat transfer coefficients and sample surface emissivity) were adjusted to obtain numerical simulation results as close as possible to those obtained experimentally. Similarities and differences between experimental and simulated data are analyzed and discussed. Possibilities for improving the results and further developments are proposed.

Subsurface defect characterization in artworks by quantitative pulsed phase thermography and holographic interferometry

In this study, experimental data from two artwork specimens was acquired and processed by pulsed phase thermography (PPT) and holographic interferometry. The first specimen was a wood painting with a variety of damages typical of this kind of pieces. A comparative study between thermography and interferometry results showed the potential complementarities of both techniques. The second inspected specimen was a fresco with fabricated inserts inspected by PPT to detect and characterize the subsurface defects. The well-known concept of Signal-to-Noise Ratio (SNR) is proposed for the selection of the proper phasegram frequency at which defect sizing is performed. A de-noising step was required prior to the application of the Canny edge detection algorithm. It is demonstrated with this investigation that PPT and holographic interferometry are valuable tools for the qualitative and quantitative assessment of artworks.

Holographic contouring method: application to automatic measurements of surface deffects in artwork

An electro-optical system for the detection and quantitative measurements of surface or subsurface defects is presented; it is based on a holographic contouring technique and digital image processing for an automatic analysis of fringe patterns. Several approaches have been followed on models, so that the defects can be unambiguously detected and possibly quantitatively analyzed. Some examples of applications in deterioration investigations of ancient artworks are reported.

Ceramics and defects

The feasibility of square-pulsed thermography nondestructive testing for the detection of defects in one ceramic material sample has been carried out by finite element (FE) analysis. In particular, a ceramic plate containing defects of different diameters, depths, locations, nature, and shapes has been numerically investigated by means of Comsol® Multiphysics computer program, taking into account the results coming from both a MATLAB™ script and the infrared thermography (IRT) technique. Indeed, the FE method simulates through a 3D model the heat transfer process induced into the ceramic material by two halogen lamps that have been applied in order to provoke an optimum thermal stress. Moreover, further defects like cracks arose beneath the surface of the plate due to the shrinkage process, have been discovered, and contrasted using a non-usual segmentation algorithm that when correlated in the time to IRT data simulates the thermo-elastic effect. Following the non-direct procedure proposed, both the depth of each defect and its main dimensions have been retrieved into a satisfactory accuracy.

Holographic and speckle methods for the analysis of panel paintings. Developments since the early 1970s

Abstract Of the methods of scientific investigation that are currently finding their place beside traditional arthistorical research, holographic and speckle methods are among those that can greatly contribute to the care and conservation of paintings on wooden panel. This paper aims to describe the state-of-the-art and the evolution of optical non-destructive analytical techniques, including holographic interferometry, electronic speckle pattern interferometry (ESPI) and speckle decorrelation. The various features of each technique are outlined.