History of Mexican Easel Paintings from an Altarpiece Revealed by Non-invasive Terahertz Time-Domain Imaging

Terahertz pulsed imaging has attracted considerable interest for revealing the stratigraphy and hidden features of art paintings. The reconstruction of the stratigraphy is based on the precise extraction of THz echo parameters from the reflected signals. Several historical panel paintings and wall paintings have been well studied by THz reflective imaging, in which the detailed stratigraphy has been successfully revealed. To our knowledge, however, the stratigraphy of oil paintings has not been clearly uncovered by THz imaging, since the paint layers in an oil painting on canvas, especially for the 16 th and 17 th century art works, are usually very thin (~10 m) in the THz regime. Therefore, in order to improve the performance of THz imaging, advanced signal-processing techniques with higher depth-resolution are still needed. In this study, THz reflective imaging is employed to reveal for the first time the detailed stratigraphy of a 17th century Italian oil painting on canvas. The paint layers on the supporting canvas are very thin in the THz regime, as the THz echoes corresponding to the stratigraphy totally overlap in the first cycle of the reflected THz signal. THz sparse deconvolution based on an iterative shrinkage algorithm is utilized to resolve the overlapping echoes. Based on the deconvolved signals, the detailed stratigraphy of this oil painting on canvas, including the varnish, pictorial, underdrawing, and ground layers, is successfully revealed. The THz C-and B-scans based on the THz deconvolved signals also enable us to reveal the features of each layer. Our results thus enhance the capability of terahertz imaging to perform detailed analysis and diagnostics of historical oil paintings on canvas with foreseen applications for the study of the artist's technique and for authentication.

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Section: Resultsmentioning

confidence: 99%

Terahertz pulsed imaging reveals the stratigraphy of a seventeenth-century oil painting

Locquet

Melis

Citrin³

2018

Unconventional Optical Imaging

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“…3 ), and the depth resolution based on the raw THz signals is not high enough to resolve the various paint layers. The available THz spectrum in our system extends to ~3 THz; this corresponds to ~100 μ m. Accounting for a representative refractive index of 1.85 10 , this limits us to a depth resolution, conventionally defined, of 100 μ m/1.85 ≈ 54 μ m. In other words, the absence of useful information from the raw THz is not merely due to the overlapping echoes; the minimum optical thickness that can be resolved is limited to ~100 μ m. Nonetheless, as we shall see, due to the nature of the painting’s layer structure, we can reconstruct the stratigraphy on a scale significantly below ~100 μ m. This is also the reason the time-domain THz C-scan in Fig. 2(a) contains both the surface and subsurface features.…”

Section: Resultsmentioning

confidence: 99%

“…THz B- and C-scans clearly reveal the features of the ground, imprimatura, and the underpainting layers, which are fairly uniform across the painting. Assuming the refractive index of gesso is about 1.52 10 , the average physical thickness of the ground layer is about 132.7 μ m. For other oil-based paint layers, although the difference of refractive index between each layer is sufficient to produce the THz reflections, we assume a mean refractive index of 1.85 10 in order to estimate the physical thickness of each paint layer. Based on this assumed refractive index, we estimate the average thickness of imprimatura is ~34 μ m and the average thickness of the underpainting layer is ~38 μ m. The presence of these three preparatory layers is confirmed by optical microscopy near the painting’s edge, shown in Fig.…”

Section: Discussionmentioning

confidence: 99%

“…Due to the penetrative capability of terahertz (THz) electromagnetic radiation in broad classes of nonmetallic material 7 , THz reflectometry based on THz time-domain spectroscopy (THz-TDS) systems has attracted considerable interest for revealing the stratigraphy in situ , as well as hidden features of art paintings 8 , such as the artist’s signature 9 , under-drawings 10 , and modifications due to the reuse of an earlier painting or canvas 11 . THz reflectometry may provide information in depth by analyzing the reflected THz signal with an incident approximately single-cycle THz pulse (which means that the spectrum of the THz radiation is broadband-from ~100 GHz up to ~3 THz for a typical THz-TDS system) 12 .…”

Section: Introductionmentioning

confidence: 99%

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Global mapping of stratigraphy of an old-master painting using sparsity-based terahertz reflectometry

Dong

Locquet

Melis³

et al. 2017

Sci Rep

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The process by which art paintings are produced typically involves the successive applications of preparatory and paint layers to a canvas or other support; however, there is an absence of nondestructive modalities to provide a global mapping of the stratigraphy, information that is crucial for evaluation of its authenticity and attribution, for insights into historical or artist-specific techniques, as well as for conservation. We demonstrate sparsity-based terahertz reflectometry can be applied to extract a detailed 3D mapping of the layer structure of the 17th century easel painting Madonna in Preghiera by the workshop of Giovanni Battista Salvi da Sassoferrato, in which the structure of the canvas support, the ground, imprimatura, underpainting, pictorial, and varnish layers are identified quantitatively. In addition, a hitherto unidentified restoration of the varnish has been found. Our approach unlocks the full promise of terahertz reflectometry to provide a global and detailed account of an easel painting’s stratigraphy by exploiting the sparse deconvolution, without which terahertz reflectometry in the past has only provided a meager tool for the characterization of paintings with paint-layer thicknesses smaller than 50 μm. The proposed modality can also be employed across a broad range of applications in nondestructive testing and biomedical imaging.

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“…Such shaped single- or few-cycle THz pulses may permit new functionalities in time-of-flight noninvasive imaging for various applications, including biomedicine, 23 security, 24 and art. 25,26 The ability to generate and manipulate the THz radiation on a single surface has many advantages as it circumvents some fundamental issues that are encountered by conventional THz generation and manipulation schemes that need to be compensated for, such as materials dispersion, 27 phase matching, 28,29 and phonon absorption. 30 Indeed, a variety of methods to overcome these issues exist; however, they usually require a careful design and are not applicable to all materials.…”

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confidence: 99%

Nonlinear Metasurface Fresnel Zone Plates for Terahertz Generation and Manipulation

2019

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We introduce a nanoengineered nonlinear metasurface based optical element that acts as an emitting Fresnel zone plate of terahertz (THz) waves. We show that the nonlinear zone plate generates broadband THz radiation and focuses each generated frequency on a different focal point along the optical axis. Therefore, a narrow beam waist and spectral selectivity of both the bandwidth and central frequency are achieved. Furthermore, we measure and analyze the temporal structure of the focused THz electric field and show that it comprises of few cycles with an axially varying carrier frequency in agreement with the calculated dispersion of the zone plate. This demonstration of controlled emission and focusing of THz waves opens the door for the development of a wide variety of additional holographic metasurface-based THz emitters and can lead to the development of efficient, active, integrated, and ultracompact optical devices for the THz spectral region.

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History of Mexican Easel Paintings from an Altarpiece Revealed by Non-invasive Terahertz Time-Domain Imaging

Cited by 21 publications

References 24 publications

Terahertz pulsed imaging reveals the stratigraphy of a seventeenth-century oil painting

Terahertz pulsed imaging reveals the stratigraphy of a seventeenth-century oil painting

Global mapping of stratigraphy of an old-master painting using sparsity-based terahertz reflectometry

Nonlinear Metasurface Fresnel Zone Plates for Terahertz Generation and Manipulation

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