M. Bellini1, C. Corsi2, R. Fontana1, M. Materazzi1, M. Mastroianni1, L.Pezzati1,

A.Tortora1

1 CNR-INOA Istituto Nazionale di Ottica Applicata (Florence)2LENS - University of Florence

M. Bellini1, C. Corsi2, R. Fontana1, M. Materazzi1, M. Mastroianni1, L.Pezzati1,

A.Tortora1

1 CNR-INOA Istituto Nazionale di Ottica Applicata (Florence)2LENS - University of Florence

OCT applied to artworks: results

CNR-INOAIstituto Nazionale di Ottica Applicata

ADG - Artwork Diagnostic Group

Optical techniques in artwork conservation

Optical Coherence Tomography

Optical Coherence Tomography (OCT) is a low coherence optical interferometry method providing high resolution and cross-sectional imaging of partially transparent or scattering tissue on a micrometer scale in situ and in real time.

Based on broadband interferometry: depth profiling is obtained through the measurement of optical path length or phase difference between the sample and the reference arm.

OCT is an efficient diagnostic imaging tool in the biomedical applications (ophthalmology, dermatology). Image resolution is one or two order above the conventional ultrasound-based methods

UNDERDRAWING

GROUND

VARNISH

PAINT LAYER

PANEL

0.5÷1 mm

50÷100 m

UVVISIR RX

50÷200 m

•RX•IR-reflectography•false-colour•UV fluorescence•multispectral imaging•3D survey

Optical techniques in Artwork:

Effectiveness and safety for frail and precious objects

In the last decade science has offered a fundamental contribution to conservation of paintings and the use of optical non invasive analysis is becoming widespread in the monitoring and restoration phases.

So far, optical techniques have been of great support for the characterization of painting layers

Reflectography Underdrawing, underpaintings

Multisdpectral imaging. Spectral and colorimetric characterization of the painting surface

Nevertheless none of them is suitable for a quantitative characterization of the varnish layer.

Role of the varnish layer

The varnish film was generally spread on the painting both to protect it and to make it brilliant. Surface cleaning is one of the most important and sometimes controversial stages of the conservation process: decisions have to be made regarding partial or complete removal of varnish..Restorers’ and conservators’ work would be considerably helped by the monitoring of varnish removal.

Raffaello’s Madonna del Cardellino: detail of the painting

Restored region

Opaque region

Varnish thickness: old technique -> stratigraphy Varnish thickness: new technique

-> optical coherence tomography

•A painting’s micro-sample is cut and embedded in a

transparent liquid resin to form a small block.

•After solidification, the block is polished so to highlight the cross-section of the fragment

by means of microscope examination. Drawbacks:

•invasive and destructive analysis •localized information referred to fringe areas

• non-invasive and non-destructive analysis

• can be performed everywhere on the painting surface.

Drawback: •Up to now, we are not able to make a complete painting stratigraphy

IVUSOCT

Brezinski, et al., Am. J. Cardiology 77 (1996): delineation of arterial layers

• Future improvement can be very useful to achieve a better resolution and a better optical depth to detect also the painted layers under the varnish film. In a new configuration the setup should become cheap and compact enough to allow also in situ analysis.

•The OCT has the potential to become an alternative non invasive tool for painting stratigraphy diagnostics.

How to reach us

INOANational Institute for Applied

Optics Largo Enrico Fermi 6, Firenze,

Italyhttp://www.ino.it

ADGArt Diagnostic Group

http://arte.ino.itbeniculturali@ino.it

Optical Coherence Tomography (OCT) for painting diagnostics

Our experimental setup

lens mirror

translation stage

photodiode

Broadband SLED source

Fiber coupled= 1550 nm 100nmz 10nm

oscilloscope

Michelsoninterferomet

erbased on a

fiber coupler

sample

lenspersonal computer

to translation

stages

translation stage

20)2ln(2

z

Depth resolution:

By adjusting the length of reference arm, it is possible to detect the signal from specific depths within the sample and thus build up a cross-sectional image.

The interferograms have been obtained by acquiring a series of OCT images with a step of 5 min the x direction.

Conclusions

ACKNOWLEDGMENTSACKNOWLEDGMENTS We are indebted to Roberto Bellucci and Cecilia Frosinini (Opificio delle Pietre Dure) for giving us the possibility to study an ancient painting and for the microscope analysis for the successive comparison.

3mm 3mm 2mm

z

x

step1step2step3

Spectral reflectance factor before varnish removals

Spectral reflectance factor after varnish removal

Signal processingThe software performs a data smoothing and recovers the envelope of the signal for each depth scan (Z direction)

The measurement is repeated for each transversal step (X direction)

We obtain the reflectivity map of the sample vs. depth and transversal position.

x

z

Thanks to the collaboration with the Opificio delle

Pietre Dure in Florence we have had the possibility to

perform OCT analysis on the Ritratto Trivulzio, by

Antonlello da Messina, in order to monitor the

different steps of varnish layer removal during the

restoration

The OCT analisys

was accompanied

by the

multispectral

analysis of the

artwork aimed to

monitor the

evolution of

spectral reflectance

and colorimetric

coordinate of the

three Region Of

Interest (ROI)