IPERION CHIntegrated Platform for the European Research Infrastructure ON Cultural Heritage
Type: 2D/3D Analysis

Digital Holographic Speckle Pattern Interferometry (DHSPI) is a non-destructive and non-contact optical technique which investigates deformation, deterioration and fracture mechanisms in order to evaluate the structural condition of materials and systems as a result of ageing, mechanical deterioration and materials’ failure. The technique is based on the principles of digital imaging and interferometry and thus it combines the ease of operation and high spatial resolution of digital cameras with the ultrahigh (in sub μm scale) displacement sensing.

The technique enables sensing, detection and documentation of invisible defects, allowing their location, measurement and exact positioning within the structure of the object. Moreover monitoring of structural changes on the basis of environmental and climatic changes, conservation treatments, natural or provoked ageing, transportation or handling, are among the potentials of this technique.

Technical details

The DHSPI-II instrument is a portable custom-made pre-industrial prototype. It has been developed at IESL-FORTH on the basis of experience acquired through the participation within several national and EU projects related with artworks diagnostics. It is a light-weight portable system comprised of an optical head and a control unit.

Optical Head

A 300mW DPSS (Diode Pumped Solid State) 532nm laser with a coherence length higher than 30 m is used for illumination. The output beam divergence is approximately 40cm at 1m. A C-mount lens is used to collect the backscattered light from the object. The field of view (FOV) depends on the lens selection (the FOV for a typical 25mm lens is ~30cm at 1m). The image is captured by a 2 Megapixel CCD and is further processed. This optical configuration provides a sensitivity of 114lines/mm and a displacement resolution of 266nm.

Peripherals: Control unit, software, excitation mechanism, scanning mechanism

The control unit includes all necessary power and control electrics and electronics, as well as a microcomputer that controls all the devices, acquires and processes data. The system is controlled remotely via an external laptop PC while tailor-made operation, processing and post-processing software applications have been developed in LabVIEW environment. The processing software utilizes a step-by-step procedure for easy adjusting the acquisition parameters, capturing and saving data from the camera and the environmental sensors, real-time processing and storage of data and results. The post-processing software contributes to the enhancement and quantification of measurement results, the dimensioning and positioning of structural defects to generate risk-maps.

Further readings
  1. “A new portable Digital Holographic Speckle Pattern Interferometry system for artworks structural documentation”, K. Hatzigiannakis, E. Bernikola, V. Tornari, Lasers in the Conservation of Artworks – LACONA IX proceedings, eds. D. Saunders, M. Strlic, C. Korenberg, N. Luxford and K. Birkholzer, Archetype publications Ltd, London, 210-212 (2013)
  2. “Interference fringe-patterns association to defect-types in artwork conservation: an experiment and research validation review”, V Tornari, E. Tsiranidou, E. Bernikola, Applied Physics A 106(2), 397–410 (2012).
  3. “Rapid initial dimensional changes in wooden panel paintings due to simulated climate-induced alterations monitored by digital coherent out-of-plane interferometry”, E. Bernikola, A. Nevin, V. Tornari, Applied Physics A 95, pp. 387-399 (2009).
  4. “Fully non contact holography-based inspection on dimensionally responsive artwork materials”. Tornari, E. Bernikola, A. Nevin, E. Kouloumpi, M. Doulgeridis, C. Fotakis, Sensors 2008, 8, DOI 10.3390/sensors (2008)
  5. “Laser Interference-Based Techniques and Applications in Structural Inspection of Works of Art”, V. Tornari, Analytical and Bioanalytical Chemistry; 387, 761-80 (2007).