ABSTRACT
A modified 3D colour digital image correlation method (3D cDIC) is proposed for efficient displacement measurements of colour objects with natural texture. The method is using a separate analysis of correlation coefficient (sigma) value in the RGB channels of CCD cameras by utilising local information from the channel with the minimum sigma. In this way, merged U, V and W displacement maps are generated based on the local correlation quality. As the proposed method applies to colour filter array cameras, the images in RGB channels have to undergo a demosaicking procedure which directly influences the accuracy of displacement measurements. In the paper, the best performing demosaicking methods are selected. The metrological analysis of their influence on the results of canvas paintings investigations obtained by unmodified and modified 3D cDIC processing is presented.
ABSTRACT
We demonstrate the application of a fully non-contact and non-invasive photoacoustic (PA) imaging system integrating a high sensitivity spherically focused air-coupled ultrasonic transducer, for the uncovering of hidden underdrawings in paintings. By selectively transforming optical absorption information into ultrasonic waves which propagate virtually unobstructed through the paint layers, PA signals provide specific imaging of underlying pencil sketches even for paints presenting high optical scattering and absorption properties. The developed system could be employed for case studies involving the investigation of paintings with historical significance, considerably complementing the capabilities of existing methods.
ABSTRACT
In this Letter, we present a combined photoacoustic imaging method, based on consecutive excitation using either the fundamental or the second-harmonic wavelength of a pulsed Nd:YAG laser for the stratigraphy of painted artworks. Near-infrared excitation was employed for the imaging of hidden underdrawings in mock-up samples, whereas visible light was used for the thickness mapping of the overlying paint through the detection of photoacoustic signal attenuation. The proposed methodology was proven effective in measuring thick and strongly absorbing layers, which would not be possible by means of other pure optical techniques, while also enabling the visualization of features underneath the painted surface. Such an implementation expands significantly the applicability of the previously presented photoacoustic technique, which was limited to point-measurements, and paves the way for novel application in historical and technical studies, as well as in documenting restoring operations.