On development of portable digital holographic speckle pattern interferometry system for remote‐access monitoring and documentation in art conservation

Abstract

Artworks are delicate objects representative of the most high values of human kind that require constant control over the situation they are in and effective actions to preserve them and deliver them safely to next generations. As such, the non‐destructive or minimally destructive nature of operator–artwork interactions has been considered as essential in this field. The complete diagnosis of the status of the art object signifying a full knowledge of invisible defects and their morphology allows its timely and correct maintenance and ensures its durability. It is herein presented the basic technical information on the development of a portable system termed Digital Holographic Speckle Pattern Interferometry (DHSPI) for the structural diagnosis of works of art. DHSPI has been tested on a variety of conservation problems over the years. It is a completely portable and fully PC driven automated system for the detection and non‐destructive diagnosis of the structural alterations of an artwork, whether they are defects lying on the subsurface or between the layers below it, the fringe number information allows the depth estimation and the complete documentation. The system exploits the benefits of a highly coherent laser beam as essential presupposition for remote on‐field access—that is the main emphasis given here—to immovable/movable art objects. It takes advantage of a CCD camera as photosensitive medium to record the interference of the retro reflected object laser beam from the object under consideration with a twin reference laser beam. The device allows micro projections of the defects on the surface of the object to be detected with a resolution that corresponds to half the wavelength of the laser beam. Local disparities, in relation to the total response of the object, are directly correlated to the detected microdisplacement of object surface as structural defects. Special tailor‐made software allows full control of the system, processing of the data, and extraction of anomalies. Direct visual qualitative observation of the displacement processes is followed as captured in the monitor. A second custom‐made software—not presented in detail here—enables further processing of the results to improve their imaging, quantification, and dimensioning. The final result is a quantitative map of alterations, the coordinates, and the classification of their risks which can be also extracted if further information is necessary. The qualitative or quantitative documentation map can be used as a guide to the conservator for maintenance and restoration work. Further on, the system also allows observation of the object's response to climate changes in order to predict the formation of alterations or the evolution of existing ones providing the only known direct tool for scheduling preservation strategies.

Impact of Relative Humidity on Wood Sample: A Climate Chamber Experimental Simulation Monitored by Digital Holographic Speckle Pattern Interferometry

“…The processing (wrapped–unwrapped phase) and post-processing (measurement of surface displacement, 3D graph visualization) of the recorded sequential interferograms in full field mode of operation (the whole surface simultaneously) provided all the information regarding the surface displacement data related to this climate chamber experimental work. The measurement was performed with the special 5-phase shifted algorithm software [ 22 , 25 ] and processing of the captured differential images provided the number of fringes (N) and the surface displacement (Δφ), allowing for the calculation of the rate of displacement (RoD) as described in the previous sections. Figure 9 shows the RoD during day 7 [ 23 ].…”

“…In order to measure micrometric surface displacements on a wood sample due to fluctuations of the RH, the DHSPI system that is based on the holographic interferometry technology was employed [ 25 , 26 , 27 ]. Holography is a method to create a fully three-dimensional image of an object, which is called a hologram.…”

“…In this context, in the study of climate induced effects on movable and immovable CH pieces of art, a work-package related to the “experimental monitoring techniques for assessment of risk in the Climate Change era” was included, and the work-package was realized through: Surface inspecting technologies coupled with RH/temperature sensor readings, close-up 3D microscopy (3DM) determining surface damage phenomena, a new set of sensors being glass dosimeter (GD) providing synergetic environmental risk and the free-water-sensors (FWS) providing assessment of the available water content of the air together [ 24 ]. A nondestructive remote sensing system to record surface displacements as a response to the RH environment change was employed, making use of laser interferometry system termed digital holographic speckle pattern interferometry (DHSPI) [ 25 ]. The experimental results showed that mass and moisture content follow the RH change and produce subsequent surface and volume alterations [ 26 , 27 ].…”

A Combined Non-Invasive Approach to the Study of A Mosaic Model: First Laboratory Experimental Results

“…The simultaneous application of digital holographic speckle pattern interferometry (DHSPI) and stimulated infrared thermography (SIRT) is already a recognized combined approach for subsurface structural diagnostics of a wide range of cultural heritage objects [ 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 ]. Both methods are non-invasive, full-field, remote control and non-contact techniques, aimed to evaluate the structural condition of the examined objects.…”

“…The experimental set-up uses an Nd:YAG laser as a light source with the following characteristics: 250 mW at 532 nm, DPSS (diode pump solid state) of high spatial-temporal coherence with TEM:00 SLM (single longitudinal mode) and coherent length of 30 m. The high-resolution digital recording medium is a CCD detector with a pixel size of 4.4 μm. The detailed presentation of the system can be found in the relative literature references [ 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 ].…”