Abstract
A novel, non-invasive, imaging methodology, based on the photoacoustic effect, 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 offers 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 profiles, such as manuscripts, glass objects, plastic modern art or even stone sculpture.
Highlights
Visible optical imaging, including optical microscopy, often enhanced with multi-spectral resolution, represents a powerful approach in the investigation of various types of artworks, those bearing color such as easel or wall paintings and polychromies
The photoacoustic signal is detected by a spherically focused ultrasonic transducer immersed in a carboxymethyl cellulose (CMC) gel layer (Methods section) applied on the front surface of the painting and serving as an immersion medium for the effective ultrasound wave propagation from the substrate to the detector
Photoacoustic imaging takes advantage of light scattering that enables excitation of a broad volume within the investigated medium, both axially and laterally, in a manner which is entirely independent of the directions of the interacting photons
Summary
Visible optical imaging, including optical microscopy, often enhanced with multi-spectral resolution, represents a powerful approach in the investigation of various types of artworks, those bearing color such as easel or wall paintings and polychromies. 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 extend the depth from which accurate information can be extracted In this context, a challenge art conservation scientists often face relates to the detection and mapping of underdrawings in paintings. The state of the art and the technique most exploited to date for detecting and mapping underdrawings in paintings, is Near Infrared (NIR) Reflectance Imaging[10,11,12], typically employing radiation in the wavelength range of 0.8–3 μm The principle behind this methodology relies on the reduced absorption and scattering of pigment materials in the NIR, which permits light to penetrate deeper within the painting and provide information about features situated immediately underneath the top paint layers, albeit within a range no deeper than 200–300 μm. The reflected intensity distribution in the NIR image of the painting is drastically affected by the presence of the highly absorbing underdrawing material, which permits mapping the underlying sketch by use of a NIR sensitive CCD camera
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