Multi-sensor evaluation of a wooden panel painting using terahertz imaging and shearography

Abstract

Cultural heritage objects are increasingly being investigated using advanced non-destructive optical measurement techniques. Holographic and speckle interferometry based instrumentation allow dimensional measurement of objects at the tens of nanometer scale. For the structural diagnostics of artwork, double-exposure techniques are often used to locate defects, delaminations, voids and other structural features. Shearography is a speckle interferometry configuration that uses a close-to-common-path shearing interferometer configuration to give a direct sensitivity to displacement gradient at the object surface. This configuration is particularly useful for measurements outside the optical laboratory, as the stability requirements are much reduced compared with holography techniques. Terahertz imaging is a new category of sensor, used to investigate materials using electromagnetic radiation in the 0.1 to 10 THz frequency range. At these frequencies many materials become semi-transparent, so bulk structural diagnostics can be performed. Typically terahertz imaging is performed using a scanning pixel, or multi-pixel, sensor. In this manuscript shearography is first used to identify areas of interest of possible structural anomalies in the artwork. These regions of interest are then studied in more detail using the terahertz imaging instrument. Together the two instruments provide an analysis of both the surface and bulk structural features. The approach is demonstrated experimentally using a wooden panel painting.

Classical Holography in the Terahertz Range: Recording and Reconstruction Techniques

“…Fresnel approximation can be used for a hologram recorded at a large distance (12) when the path difference between any point of the object and any point of the hologram differs by a value significantly smaller than the wavelength. Mathematically, this allows replacing the radius vector with the coordinate .…”

Application of shearography with thermal loading for the structural inspection of Rembrandt’s Night Watch

“…Detectable defects include delaminations, debonding, cracks and fiber breakage. In the recent decade, shearography has raised increasing interest in the inspection of artworks including paintings [8][9][10][11][12] , where it can reliably and repeatedly reveal delaminations and cracks and also exclude the human factor when compared with traditional inspection techniques (e.g., visual inspection and tapping test) 12 .…”

“…These structural features normally affect the in-plane strain distribution, e.g. as cracks in wooden panels 6,9 .…”