Non-invasive photoacoustic detection of hidden underdrawings in paintings using air-coupled transducers.

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.