Digital Holographic Speckle Pattern Interferometry in Structural Diagnostics of Paintings: Toward Displacement-Based Heritage Metrology

A central challenge in the structural diagnostics of paintings is the accurate, repeatable, non-invasive, and ideally non-contact documentation of subsurface defects such as detachments, cracks, and interlayer decohesion, which often precede visible surface deterioration. Beyond the visual detection of damage, preventive conservation requires methods capable of revealing how painted structures mechanically respond to environmental or induced stimuli before irreversible alteration occurs. Digital Holographic Speckle Pattern Interferometry (DHSPI) is a displacement-sensitive interferometric methodology developed to address the specific requirements of cultural heritage diagnostics. Based on holographic and speckle interferometry principles, DHSPI enables sequential full-field measurements of out-of-plane surface-normal transient displacements under controlled low-energy thermal excitation in a complete cooling-down cycle. In this framework, the painting is not treated only as an image-bearing surface, but as a mechanically responsive stratified system whose displacement field contains diagnostic information on structural response, hidden defects, deformation evolution, and structural stability. This review presents the conceptual foundation and methodological evolution of DHSPI, from classical optical holographic interferometry for defect detection to current digital sequential thermo-mechanical monitoring. This evolution resulted from examining the diagnostic information contained in interferometric results while progressively addressing earlier methodological limitations. As such, particular emphasis is placed on the strict boundary conditions, interferometric sensitivity, sequential wrapped fringe-pattern interpretation, unwrapped phase-map reconstruction, excitation strategy, and data complementarity with related non-destructive techniques such as Infrared Thermography. Relevant interferometry techniques such as Electronic Speckle Pattern Interferometry (ESPI) and Digital Speckle Shearography (DSS) are also examined and are briefly discussed. DHSPI is emphasised here for its combined use of sequential wrapped fringe-pattern interpretation and unwrapped phase-map reconstruction, allowing both long-term temporal tracking and quantitative displacement documentation. A key argument here is that sequential displacement-based documentation provides information that may not be fully accessible through static imaging or through unwrapped displacement maps alone. Wrapped interferometric sequences can preserve evidence of whole-field response, local defect localisation, defect interaction, propagation pathways, and transient mechanical behaviour during thermal relaxation. In this sense, DHSPI supports a transition from contrast-based inspection toward displacement-based heritage metrology. Emerging perspectives, including DHSPI–thermography multimodal integration, AI-assisted interpretation, and multi-channel holographic concepts, are also briefly discussed as future directions for preventive conservation of paintings and large-format painted heritage surfaces.