Dario Ambrosini

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.

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.

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.

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.

Integrated approach between pulsed thermography, near-infrared reflectography and sandwich holography for wooden panel paintings advanced monitoring

The durability of an exterior finish is affected by the characteristics of the wood. Satisfactory finish life is usually more difficult to achieve on woods of higher density. All wood shrinks as it loses moisture and swells as it absorbs moisture, but some species are more stable than others. Species that shrink and swell the most cause more stress on paint films than woods that are more stable [1]. To this end, let us recall that a painting on wood can be considered as a layered structure: The wood support is coated with a number of superposed priming layers made from mixtures of gesso and glue. A frequent fault resulting from such a system is the formation of detached regions inside the layered structure caused by the shrinkage process of the wood support [2]. Obviously, wood deteriorates more rapidly in warm, humid regions with respect to cool or dry places [3]. The influence of wood conditions on surface coatings is a critical point that should be monitored and that depends on environmental parameters such as microclimate. To prevent and control the effects, keeping costs down, a non-destructive monitoring of wood support behavior under thermal stress is needed. In this work, an integrated approach based on traditional and innovative (HI, PT and NIR) techniques was conducted on a primed support of poplar wood with a complex-shape surface containing areas of artificial defects at several depths due to the influence of the support on the various layers. The obtained results could be arranged, if integrated into a multidisciplinary approach, in order to define and design the conservation of the wooden artifacts.

Diffractive optical element-based profilometer for surface inspection

We introduce a new 3-D sensor based on a diffractive optical element (DOE). Structured light is obtained by the interference of two beams. The design of the projection unit has the following features: the system is very simple, small and cheap and the fringe spacing is easily adjustable. Fringes generated by the DOE interferometer are analyzed by Fast Fourier Transform (FFT) method, which produced a 3-D per- spective plot. The technique and the experimental results of real surface profiles are given.

Vibration monitoring by fiber optic fringe projection and Fourier transform analysis

Abstract A fiber optic fringe projection interferometer is proposed to obtain local amplitude vibration of a diffuse surface. A sinusoidal fringe pattern is projected onto the vibrating surface by an optical fiber version of Youngs interferometer. An image of this surface is captured by a CCD camera and fed to a PC through a frame grabber and hence displayed onto a TV monitor. The visibility of the interference pattern is modulated by a function of the local amplitude of vibration. Thus loci of constant vibrational amplitude can be directly observed as regions of low fringe visibility in the image of the surface. A quantitative analysis is performed by a Fast Fourier Transform (FFT) algorithm. Experimental results in agreement with the theory are presented.

High-precision digital automated measurement of degree of coherence in the Thompson and Wolf experiment

We propose a high-precision digital automated quantitative determination of the modulus of the complex degree of coherence. The Thompson and Wolf experiment is repeated, using a CCD and a measurement method based on the fast Fourier transform. The experimental results agree very well with the predictions of the theory.