Revealing Hidden Features in Multilayered Artworks by Means of an Epi-Illumination Photoacoustic Imaging System.

George J Tserevelakis,Kristalia Melessanaki,Jana Striova,Evgenia Klironomou,Antonina Chaban,Giannis Zacharakis

doi:10.3390/jimaging7090183

Abstract

Photoacoustic imaging is a novel, rapidly expanding technique, which has recently found several applications in artwork diagnostics, including the uncovering of hidden layers in paintings and multilayered documents, as well as the thickness measurement of optically turbid paint layers with high accuracy. However, thus far, all the presented photoacoustic-based imaging technologies dedicated to such measurements have been strictly limited to thin objects due to the detection of signals in transmission geometry. Unavoidably, this issue restricts seriously the applicability of the imaging method, hindering investigations over a wide range of cultural heritage objects with diverse geometrical and structural features. Here, we present an epi-illumination photoacoustic apparatus for diagnosis in heritage science, which integrates laser excitation and respective signal detection on one side, aiming to provide universal information in objects of arbitrary thickness and shape. To evaluate the capabilities of the developed system, we imaged thickly painted mock-ups, in an attempt to reveal hidden graphite layers covered by various optically turbid paints, and compared the measurements with standard near-infrared (NIR) imaging. The obtained results prove that photoacoustic signals reveal underlying sketches with up to 8 times improved contrast, thus paving the way for more relevant applications in the field.

Highlights

In an emerging position among imaging technologies during the last decade, we find photoacoustic (PA) imaging, a novel methodology developed mainly in the context of contemporary biomedical research studies
We present here a novel PA imaging apparatus developed in an epi-illumination geometry, integrating laser excitation and respective ultrasonic detection on one side, aiming to provide universal diagnostic information in objects of arbitrary thickness and shape
We demonstrate that the effectiveness of PA imaging can complement substantially the existing state of the art methods for this purpose, including visible and NIR optical imaging [12,13,14,15], optical coherence tomography (OCT) [16,17], multiphoton microscopy [18], THz imaging [19,20,21], and X-ray-based techniques [22,23,24,25]

Summary

Introduction

In an emerging position among imaging technologies during the last decade, we find photoacoustic (PA) imaging, a novel methodology developed mainly in the context of contemporary biomedical research studies. During the incidence of a short light pulse, a portion of the absorbed optical energy is converted into heat, inducing a rapid thermoelastic expansion of the medium and the subsequent generation of an initial pressure that propagates in space in the form of ultrasonic waves. These acoustic waves, typically found in the MHz frequency regime, are usually recorded in time using the same detection equipment (i.e., piezoelectric elements) as in conventional ultrasound imaging. The PA technique provides optical absorption imaging contrast with 100% relative sensitivity (i.e., a given percentage change in the optical absorption coefficient yields the same percentage change in the PA amplitude) [4]

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Discussion

Conclusion