1 Scientific RepoRts | 7: 747 | DOI:10.1038/s41598-017-00873-7 www.nature.com/scientificreports photoacoustic imaging reveals hidden underdrawings in paintings George J. tserevelakis 1 , Ilianna Vrouvaki 2 , panagiotis siozos 1 , Krystallia Melessanaki 1 , Kostas Hatzigiannakis 1 , Costas Fotakis 1,3 & Giannis Zacharakis 1 A novel, non-invasive, imaging methodology, based on the photoacoustic efect, is introduced in the context of artwork diagnostics with emphasis on the uncovering of hidden features such as underdrawings or original sketch lines in paintings. photoacoustic microscopy, a rapidly growing imaging method widely employed in biomedical research, exploits the ultrasonic acoustic waves, generated by light from a pulsed or intensity modulated source interacting with a medium, to map the spatial distribution of absorbing components. Having over three orders of magnitude higher transmission through strongly scattering media, compared to light in the visible and near infrared, the photoacoustic signal ofers substantially improved detection sensitivity and achieves excellent optical absorption contrast at high spatial resolution. photoacoustic images, collected from miniature oil paintings on canvas, illuminated with a nanosecond pulsed Nd:YAG laser at 1064 nm on their reverse side, reveal clearly the presence of pencil sketch lines coated over by several paint layers, exceeding 0.5 mm in thickness. By adjusting the detection bandwidth of the optically induced ultrasonic waves, photoacoustic imaging can be used for looking into a broad variety of artefacts having diverse optical properties and geometrical profles, such as manuscripts, glass objects, plastic modern art or even stone sculpture. Visible optical imaging, including optical microscopy, ofen enhanced with multi-spectral resolution, represents a powerful approach in the investigation of various types of artworks, particularly those bearing color such as easel or wall paintings and polychromies. Spectrally-resolved imaging enables mapping of diferent paints across the surface of a painting and in many cases the recovered spectral information leads to pigment identifcation. However, paintings typically consist of several successive strata of paint, which are optically opaque, primar- ily because of strong scattering arising from the pigment particles. As a result, visible optical imaging is rather limited to a superfcial analysis and thus features, which have been painted over, and can be signifcant for eval- uating the previous history of a painting or even the making of it, may escape detection. Tis problem is directly related to the nature of light transport through opaque media, which is governed by scattering and difusion that scramble any spatial information the same way fog blurs our vision. Terefore, there is an evident need for the development of novel, high precision, non-destructive imaging tools that will surpass the limitations imposed by light scattering and hence, extend the depth from which accurate information can be extracted. In this context, a challenge art conservation scientists ofen face relates to the detection and mapping of under- drawings in paintings. Underdrawings typically represent the original rough sketch used by the artist to por- tray the theme of the painting and guide the development of the artwork. Such kind of preparatory sketching is generally applied directly on the prepared canvas using charcoal, graphite or carbon black paint. Unavoidably, the covering of the preliminary drawing with several layers of paint, obstructs locating and retrieving the infor- mation related to the initial part of the artistic process, namely the original sketch, and furthermore the degree to which the artist followed faithfully this sketch or introduced subsequent alterations in the artwork (penti- mento). Underdrawings or underpaintings have been of exceptional interest to scholars for long time, because they contain information about the working practice of a particular artist or workshop. More specifcally, the non-destructive detection of such hidden features has the potential to shed light to the social, historical, geo- graphical or even psychological framework of the artistic creation, providing thus invaluable knowledge not only to art historians, but also to cultural heritage scientists who are interested in the determination of artworks authenticity. It is worth mentioning that there are numerous bibliographical references stating case-studies of 1 institute of electronic Structure and Laser, foundation for Research and technology Hellas, Heraklion, crete, Greece. 2 Department of chemistry, University of crete, Heraklion, crete, Greece. 3 Department of Physics, University of crete, Heraklion, crete, Greece. correspondence and requests for materials should be addressed to G.J. t. (email: tserevel@iesl.forth.gr) Received: 3 January 2017 Accepted: 20 March 2017 Published: xx xx xxxx opeN