Visible-induced luminescence imaging: A user-friendly method based on a system of interchangeable and tunable LED light sources

    Visible-induced luminescence imaging: a user-friendly method based on a system of interchangeable and tunable LED light sources A. Daveri, M. Vagnini, F. Nucera, M. Azzarelli, A. Romani, C. Clementi PII: DOI: Reference:

S0026-265X(15)00289-1 doi: 10.1016/j.microc.2015.11.019 MICROC 2318

To appear in:

Microchemical Journal

Received date: Revised date: Accepted date:

17 July 2015 6 November 2015 11 November 2015

Please cite this article as: A. Daveri, M. Vagnini, F. Nucera, M. Azzarelli, A. Romani, C. Clementi, Visible-induced luminescence imaging: a user-friendly method based on a system of interchangeable and tunable LED light sources, Microchemical Journal (2015), doi: 10.1016/j.microc.2015.11.019

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Visible-induced luminescence imaging: a user-friendly

LED light sources

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method based on a system of interchangeable and tunable

C.Clementi(2)

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A.Daveri (1), M.Vagnini (1) , F.Nucera (1), M.Azzarelli (1), A.Romani (2) and (1) Associazione Laboratorio di Diagnostica per i Beni Culturali, piazza Campello 2, 06049 Spoleto (PG), Italy

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(2) Dipartimento di Chimica Biologia e Biotecnologie, Università degli Studi di Perugia, via Elce di Sotto, 8,

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06123 Perugia, Italy

Keywords: luminescence imaging, LED light sources, spectrofluorimetry, near-infrared luminescence pigments, fluorescence, art polychromy

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Abstract

Imaging techniques represent an unavoidable analytical tool for the non-invasive investigation of cultural heritage objects providing useful information for the identification and distribution of materials on the investigated surface. In particular, photo-induced

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luminescence imaging can give important indications as to the distribution of organic (binders, resins, dyes and lakes) and inorganic (Egyptian blue, Han blue, Han purple, zinc oxide, cadmium based pigments) constituting materials. Recently the use of LEDs, generating a narrow emission band in the Visible range, has been introduced opening up a new perspective in the detection of inorganic luminescent pigments. In this work we propose a user-friendly tool for luminescence acquisition based on the use of a custom made interchangeable and tunable LED light source system, that allows us to select both the power and the wavelength of the excitation light. The study involved the analysis of panel painting replicas prepared applying several pigments and lakes commonly used by artists in different historical periods. The luminescence images were recorded by a high sensitivity CCD camera and a Vidicon camera in the visible and NIR range, under three excitation narrow bands (red LEDs at 630 nm; green LEDs at 517 nm and blue LEDs at 465 nm). The reliability of the 1

ACCEPTED MANUSCRIPT developed experimental setup has been validated by spectrofluorimetric measurements. A very high correspondence between imaging and spectral response has been found concerning emission band position, emission intensity and excitation wavelength effect. Furthermore, the

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NIR luminescence of a modern yellow pigment, nickel titanate, has been for the first time documented. The defined methodology has been applied to the study of works of art of

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different historical periods where it has been proved to successfully detect luminescent inorganic and organic pigments, whose presence has been confirmed also by elemental

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analyses and spectrofluorimetry.

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1.Introduction

Over the last decades, the importance of imaging technique for mapping and occasionally identifying constituting materials of precious works of art has been extensively proved, constituting today a reliable diagnostic tool broadly acknowledged by the scientific

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community, art historians and conservators [1, 2]. In this context photo-induced luminescence

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imaging technique plays an important role in the characterisation and localization of luminescent organic (binders, resins, dyes and lakes) and inorganic (Egyptian blue, Han blue, Han purple, zinc oxide, cadmium based) compounds [3, 4, 5, 6, 7, 8, 9]. This technique has been conventionally performed with the use of Wood’s lamps as

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ultraviolet excitation source, to detect the fluorescence of organic binders and colorants in the Visible range. [3, 10]. A suitable lamp filtration has also been proposed to obtain a maximum

output at 365 nm avoiding any interference due to parasite Visible light [11]. A number of studies have been carried out in order to understand the nature of the recorded luminescence images through cross validation with spectrofluorimetry and spectrophotometry, highlighting that UV radiation, being scarcely absorbed by coloured compounds, whose absorption maximum fall in the Visible range, and preferentially absorbed by binders and resins, is not always an efficient excitation source for the best detection of their luminescence. This not only leads to difficulties in detecting low luminescent materials but also to serious distortion of the emission features due to self-absorption, multiple scattering and inner filter effect with a consequent detection of false emission signal [11, 12, 13]. 2

ACCEPTED MANUSCRIPT In order to overcome the issues related to the use of UV lamps, in recent years imaging techniques have resorted to the use of the emerging LEDs technology that allowed a more sensitive and selective detection of luminescence signals therefore enhancing the strengths of

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luminescence-based imaging methodologies [4, 5, 14]. In this work we propose a user-friendly method based on the use of a custom made

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interchangeable and tunable LED light source system that allows us to select the intensity and the wavelength of the excitation light. The system is composed of red LEDs (emission peak at 630 nm), green LEDs (emission peak at 517 nm) and blue LEDs (emission peak at 465 nm)

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which can be individually used as a narrow emission band chromatic source or simultaneously applied as a white light source. A manual switch ensures the purity of the

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excitation source output avoiding any frequency mixing among the different LEDs. The Visible-induced luminescence images have been collected combining the described system

with an appropriately filtered Mamiya camera, which allows luminescence signals from 380 nm to 1100 nm to be recorded, and a Vidicon Camera for the 1100-2200 nm detection range.

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The first part of the study concerned the analysis of panel painting replicas, prepared

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applying a variety of pigments and lakes commonly used in works of art of different historical periods, on a black ground preparation layer to minimize any interference due to eventual reflected light [15]. In order to collect the intrinsic luminescence of the selected

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colorants a non-luminescent binder was chosen. The images were collected in emission mode with excitation produced by respectively red, green and blue LEDs, either in the Visible and near-infrared region. Spectralon ® nonluminescent grey scale target (99%, 50%, 25%, 12% reflectance in the UV-VIS-NIR range), manufactured by Labsphere, was used as reference standard. The observed emissions were explained on the basis of spectrophotometric and fluorimetric results obtained on the same test panels under similar excitation conditions. The developed procedure was successfully applied on modern and contemporary easel paintings and a XVI century illuminated manuscript. The obtained results, confirmed by means of non-invasive spectroscopic techniques, promote this new application of imaging technique as a suitable and easy tool for the non-invasive characterisation and spatial distribution of luminescent materials on artworks. 3

ACCEPTED MANUSCRIPT 2.Material and Methods 2.1 Panel replicas

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The test panels were prepared applying selected pigments and lakes onto a black ground preparation consisting of three layers of ivory black and Primal AC 61 (1:1 wt/wt) spread on

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wood support. The materials and their description are summarized in table 1. The pigments were applied from a minimum of three to a maximum of seven layers, according to their covering ability, using rabbit-skin glue as a binding medium (1:1 wt/wt). Some pigments and

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dyes were diluted in different proportions with a white pigment in order to have a less saturated colour. The replicas were organized following a chronological order relating to the

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period of use. All the pigments were analysed by XRF spectrometry (the results are summarized in table 1), Mid-FTIR spectroscopy and X-ray diffraction.

Pigments

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Code

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Table 1: test panel description and XRF results on the constituting pigments. Chemical elements are reported in decreasing order of abundance. The elements present as traces are included in brackets. Manufacturer

XRF

Yellow panel

YELLOW OCHRE

ZECCHI

Fe, Ca, Sr (K, Ti, Zn)

Y2

NAPLES YELLOW

KREMER

Pb, Sb, Sn

Y3

SAFFLOWER YELLOW + gypsum*

KREMER

K, Ca, Fe, Sr (Mn)

WELD LAKE

KREMER

Ca, K (Fe, Zn, Sr)

STIL DE GRAIN

KREMER

Ca, K, Fe

STIL DE GRAIN + gypsum (1:10)

KREMER

LEAD TIN YELLOW

KREMER

Pb, Sn (Ti)

Y8

ORPIMENT

KREMER

As

Y9

GAMBOGE + gypsum (1:5)

KREMER

(Ca, Mn, Fe)

Y10

CHROME LEMON

WINSOR&NEWTON ( 1st series)

Ti, Ni, Sb (Pb)

Y11

CHROMIUM YELLOW

MAIMERI

Pb, Cr

Y12

LEMON YELLOW (nickel titanate)

WINSOR&NEWTON (2st series)

Ti, Ni, Sb (Pb)

Y13

ZINC YELLOW

MAIMERI

Zn, Cr, Pb (K)

Y14

CROCOITE

KREMER

Pb, Cr (Fe)

Y4 Y5 Y6 Y7

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Y1

4

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KREMER (21010)

Zn, Cd

Y16

CADMIUM YELLOW

KREMER (21020)

Zn, Cd

Y17

CADMIUM YELLOW

KREMER (21030)

Cd, Zn (Ba, Sr)

Y18

CADMIUM YELLOW

KREMER (21040)

Cd, Zn, Ba, Sr

Y19

CADMIUM YELLOW

KREMER(21050)

Y20

CADMIUM YELLOW

KREMER(21060)

Y21

CADMIUM YELLOW + lithopone (1:1)

KREMER + ZECCHI

Y22

CADMIUM YELLOW DEEP

WINSOR&NEWTON 3st series

Y23

CADMIUM YELLOW PALE

WINSOR&NEWTON 3st series

Cd, Zn (Se)

Y24

CADMIUM YELLOW

WINSOR&NEWTON 3st series

Zn, Cd

Y25

CADMIUM YELLOW PALE

MAIMERI

Zn, Cd (Sr)

Y26

CADMIUM YELLOW LEMON

MAIMERI

Zn, Cd, Sr (Ba)

Y27

CADMIUM YELLOW DEEP

MAIMERI

Cd, Se, Sr, Ba (Zn)

Y28

AUREOLIN

WINSOR&NEWTON 3st series

Co, K (Ti, Al)

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Y15

Cd, Zn, Se

Cd, Se, Zn

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Cd, Zn, Se, (Ba, Sr)

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Red panel

RED OCHRE

ZECCHI

Fe, Ca, Sr (Zn, Ti, Si)

R2

NATURAL HEMATITE

ZECCHI

Fe, Ca (Ti, Sr, Cu, Zn, Cr)

R3

SAFFLOWER DYE **

KREMER

K, Ca, Fe, Sr (Mn)

R4

MINIUM

KREMER

Pb

MADDER LAKE

KREMER

K, Ca, Fe (Zn, Al)

MADDER LAKE + gypsum (1:1)

KREMER+ZECCHI

MADDER LAKE

ZECCHI

Ca, Fe (Cl)

ALIZARIN

KREMER

Ca, Sr, Fe, Cu (K, Ti)

R9

REALGAR

KREMER

As (Fe, Ca)

R10

FUSTIC + gypsum (1:3)

KREMER+ZECCHI

K ( Ca, Fe, Cu, Sr)

R11

CINNABAR MINERAL

ZECCHI

Hg

R12

CINNABAR DARK

MAIMERI

Hg

R13

LAC DYE ***

KREMER

(Cl, Ca, Fe)

R14

SANDAL WOOD

KREMER

Ca (K, Fe, Sr)

R5 R6 R7 R8

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R1

5

ACCEPTED MANUSCRIPT CARMINE NACCARAT

KREMER

Ca, Fe (K)

R16

CARMINE NACCARAT + gypsum (1:3)

KREMER+ZECCHI

Ca, Fe (K)

R17

MARS RED

MAIMERI

Fe (Cr, Cu, Zn, Pb)

R18

CHROMIUM ORANGE

WINSOR&NEWTON

Pb, Cr, Ba (Zn, Cd)

R19

CADMIUM ORANGE

WINSOR&NEWTON

R20

CADMIUM RED

KREMER (21080)

R21

CADMIUM RED

KREMER (21090)

Se, Cd, Zn (Ba, Sr)

R22

CADMIUM RED

KREMER (21100)

Se, Cd, Zn (Ba, Sr)

R23

CADMIUM RED

KREMER (21110)

Se, Cd, Zn

R24

CADMIUM RED

KREMER (21120)

Se, Cd, Zn (Ba, Sr)

R25

CADMIUM RED

KREMER (21130)

Se, Cd, Zn

R26

CADMIUM RED

KREMER (21140)

Se, Cd, Zn (Ba, Sr)

R27

CADMIUM RED

KREMER (21150)

Se, Cd, Zn (Ba, Sr)

R28

CADMIUM RED + Lithopone (1:1)

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R15

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Cd, Se, Zn (Ba, Sr, Ca) Se, Cd, Zn (Ba, Sr)

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KREMER

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Blue panel

EGYPTIAN BLUE

ZECCHI

B2

EGYPTIAN BLUE+gypsum

ZECCHI+ZECCHI

B3

AZURITE

KREMER

Cu, Fe (Ca, Ti, Mn)

INDIGO

ZECCHI

Fe, Ca, Sr (Ti, K, Zn)

INDIGO + gypsum (1:1)

ZECCHI+ZECCHI

INDIGO + gypsum (1:10)

ZECCHI+ZECCHI

HAN BLUE

KREMER

B8

HAN BLUE + gypsum (1:1)

KREMER+ZECCHI

B9

VIVIANITE

KREMER

Fe, (Si, Ca, Zn, Sr)

B10

LAPISLAZULI (pure)

KREMER

Ca, Fe, Sr, K (Si, Ti, Mn, Cu, Cr, As, Cl)

B11

LAPISLAZULI (medium quality)

KREMER

Ca, Fe, Sr, K (Si, Ti, Mn, Cu, Cr, As)

B12

LAPISLAZULI (grayish blue)

KREMER

Fe, Ca, Sr , K, (Si, Ti, Mn, Cu, Zn, As, Rb, Cl)

B4 B5 B6 B7

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B1

Cu, Ca (Zn, Si, Fe, Pb, Sr)

Cu, Ba, Sr (Sn, Fe)

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ULTRAMARINE BLUE (natural afghan)

ZECCHI

B14

ULTRAMARINE BLUE (natural afghan) + gypsum (1:1)

ZECCHI+ZECCHI

B15

ULTRAMARINE BLUE (natural chile)

ZECCHI

B16

ULTRAMARINE ASH

KREMER

B17

MAYA BLUE

KREMER

B18

MAYA BLUE+ gypsum (1:1)

KREMER+ZECCHI

B19

SMALT

ZECCHI

Co, K (Si, Fe)

B20

PRUSSIAN BLUE

ZECCHI

Fe (Ti, Ca, Zn)

B21

COBALT BLUE (light)

ZECCHI

Zn, Co (Fe, Pb)

B22

COBALT BLUE TURQUOISE DARK

KREMER

Cr, Co, Zn, Ba, Sr (Ca)

B23

ULTRAMARINE BLUE (dark artificial)

ZECCHI

Fe, Ti, Si , K (Sr, Rb, Zn, Ca, Pb/As)

B24

CERULEAN BLUE

MAIMERI

Co, Sn, Zn, Cr (Pb)

B25

ULTRAMARINE BLUE ARTIFICIAL

KREMER

Fe, K, Si, Sr, Rb (Ti, Cu, Pb/As)

B26

MANGANESE BLUE

WINSOR&NEWTON II series

Ba, Sr, Mn

B27

MANGANESE BLUE+ gypsum (1:1)

WINSOR&NEWTON II series+ZECCHI

B28

BLUE PHTHALO

ZECCHI

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Ca, Sr,Fe, (Si, Ti, As, Mn, Cu, K, Pb)

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Ca, Fe, Sr, K, (Si, Ti, Mn, Cu, As) Fe, Ca, K, Ti, Mn, (Si, Sr, Zn)

Cu, Ca

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Fe, Ca, Cu, K, Si, Sr, Rb (Ti, Au, Pb, Mn, Zn)

* The safflower dry petals were soaked in distilled water for 1 h at room temperature and then filtered. The yellow extract was used to swell rabbit-skin glue (3:1 wt/wt) by storing it for 12 h at room temperature in the dark. The obtained solution was mixed with gypsum (1:1 wt/wt) and directly applied on the panel [16]. ** The safflower dry petals were soaked in distilled water for 12 h at room temperature and then filtered. The petal residue was dissolved in alkaline solution (lye made from wood ashes) for 12 h at room temperature in order to extract the red dyestuff. The red extract was used to swell rabbit-skin glue (3:1 wt/wt) by storing it for 12 h at room temperature in the dark. The obtained solution was mixed with gypsum (10:1 wt/wt) by immersing it in a hot bath to favor the evaporation of the water excess and then achieving a suitable consistence and the desired colour shade [16].

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ACCEPTED MANUSCRIPT *** The dye powder was dissolved in turpentine essence (1:1 wt/wt ) by adding some drops of clarified siccative oil. 2.1 X-ray fluorescence (XRF)

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XRF spectra were collected using the Bruker ARTAX400. It is equipped with a low-power metal-ceramic-type X-ray tube and a Mo anode as the excitation source, which operated at 50

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kV and 700 mA. The X-ray fluorescence is revealed by a Peltier-cooled silicon drift detector (SDD) with an active area of 10 mm2 and a Be window. The typical energy resolution at 5.9 KeV is <155 eV. The distance between sample and detector is about 10 mm and the X-ray

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beam is collimated on the analysed surface with a spot diameter of 650 µm.

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2.2 Imaging techniques

Radiation source: custom-made interchangeable and tunable LED light source system composed of two identical units of red LEDs (emission peak at 630 nm), green LEDs (emission peak at 517 nm) and blue LEDs (emission peak at 465 nm) which can be

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individually used as chromatic source or simultaneously applied as a white light source. A

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manual switch allows us to select the specific excitation source by ensuring its spectral purity thus avoiding any wavelength mixing. The system also allows the power (white LEDs, Irradiance (Irr) from 224 to 2330 lux; blue LEDs, Irradiance from 110 to 1175 lux; green

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LEDs, Irradiance from 87 to 918 lux; red LEDs =from 75 to 760 lux) and the wavelength of the excitation light to be selected. Each radiation source is 30x14 cm, and mounted 360 LEDs thus ensuring, at a distance of about 50 cm from the analysed surface, a homogenous illumination in a 1 m2 area.

The homogeneity of the illumination on the analysed panels and works of art was checked by moving a radiometer along and very close to the whole irradiated surface.

Recording system Range 380-1100 nm Digital images were collected by means of a camera body Mamiyaleaf IXR with a 80 mm lens, equipped with a digital back LEAFCREDO 60 megapixel WS (Wide Spectrum) which allows luminescence signals from 380 nm to 1100 nm to be collected. 8

ACCEPTED MANUSCRIPT Range 1100-2200 nm Images were acquired with a High Performance Vidicon Camera HAMAMATSU C2400-07

Emission filter :In Table 2 the filter set used is listed.

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N2606-26, sensitive up to 2200 nm, equipped with an objective computar 25 mm.

Description

Spectral range (nm)

Transmittance

B+W 486 UV IR CUT

Glass UV-IR blocking

395-690

97%

B+W 092 IR 695

Glass Visible blocking

735-1100

90%

B+W 040

Glass cut-on filter

585-1100

95%

B+W 090

Glass cut-on filter

633-1100

95%

850-1100

90%

1100-2200

85%

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Emission Filter

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Table 2 : description of the filters used in this study. The spectral range corresponding to the maximum of filter transmission is also reported.

Gelatin Visible blocking

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HOYA filter 1000

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KODAK WRATTEN IR filter Gelatin Visible blocking 87C

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Reference standard: Spectralon ® non-luminescent grey scale target (99%, 50%, 25%, 12% reflectance in the UV-VIS-NIR range), manufactured by Labsphere, has been used.

2.3 UV-Vis –NIR absorption and emission spectroscopy Reflectance and luminescence measurements were carried out by a portable prototype described in detail in a previous paper [17]. The excitation sources used in this study are: a Deuterium-Halogen lamp (Avalight-DHc) for reflectance measurements and two ultracompact diode laser sources (Toptica Photonics AG, DE; excitation wavelengths 445 and 640 nm) together with a Neodymium:YAG laser (532 nm) for steady-state luminescence measurements. A CCD spectrometer (Avaspec 2048 USB2, 200-1100 nm range, spectral resolution 8 nm) was used for reflectance measurements (integration time was set at 800 ms). High sensitivity CCD spectrometers, Avaspec-ULS2048 XL-RS5 USB2 (300-1150 nm 9

ACCEPTED MANUSCRIPT range, spectral resolution 8 nm) and AvaSpec-NIR256-1.7TEC (950-1600 nm range) detected the steady-state luminescence. Longbandpass filters having zero transmittance in the excitation spectral range and constant transmittance in the emission range were placed in

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front of the detector in order to avoid the second order of the excitation light (GG475, OG590 and RG695 Newport filters were used under 445, 532 and 640 nm excitation wavelength

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respectively). Emission spectra were collected by setting for a certain panel the same integration time (800 ms) and the same power of the excitation source (1 mW for the 445 nm blue line, 2 mW for the 532 nm green line and 0.5 mW for the 640 nm red line) so that

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eventual differences in emission intensity among colorants are independent of the experimental conditions used but only related to a diverse luminescence behaviour under a

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certain excitation wavelength. The high emission intensity of manganese, Egyptian and Han blue required both lower power intensity and integration times (500 ms) to be used. 2.4 Artworks

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A painting by Gerardo Dottori, conserved at the Museo della Penna in Perugia, an oil and

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tempera on canvas by Pietro Consagra, exhibited at Palazzo Collicola in Spoleto and a XVI century illuminated manuscript stored at Biblioteca Comunale in Spoleto, were investigated. 2.5 Methodological approach

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The two interchangeable and tunable LED light sources were symmetrically oriented at approximately 45° to the camera focal axis. The system was tested on each panel replica with excitation produced by red (λ=630 nm), green (λ=517 nm) and blue (λ=465 nm) LEDs respectively:

● Simultaneously applied as a white light source for the Visible reflected images (VIS) with B+W 486 UV IR CUT filter placed in front of the camera (fig.1a); ● Blue LEDs for blue-induced Visible luminescence images (BIVL) with B+W 090 joint B+W 486 UV IR CUT filters in front of the camera, blue-induced infrared luminescence images (BIL) with B+W 092 IR 695 or KODAK WRATTEN IR filter 87C (fig.1b);

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ACCEPTED MANUSCRIPT ● Green LEDs for green-induced Visible luminescence images (GIVL) with B+W 040 joint B+W 486 UV IR CUT filters in front of the camera, green-induced infrared images (GIL) with B+W 092 IR 695 or KODAK WRATTEN IR filter 87C (fig.1c);

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● Red LEDs for red-induced infrared luminescence images (RIVL) with B+W 092 IR 695 filter placed in front of the camera, red-induced infrared images (RIL) with

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KODAK WRATTEN IR filter 87C (fig. 1d);

● White LEDs for the induced-Visible infrared luminescence images (VIL) with filter HOYA 1000;

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● All images were collected with reference standard, Spectralon® non-luminescent grey scale target (99%, 50%, 25%, 12% reflectance in the UV-VIS-NIR range). The

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eventual contribution of spurious light on photo-induced luminescence images, that is any source of light other than luminescence coming from the analysed surface (ambient light, light emitted by other materials present in the room, parasite light coming from the irradiation source), was evaluated and taken into account by using

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the Spectralon ® 99% reflectance standard following a published procedure [4,5]. For this study, all images were acquired as raw images and transformed into 8,984X6,732 pixel resolution images in 16 bit.tiff format with Capture one 7 software.

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The sectors that show luminescence were subsequently analyzed by UV-VIS-NIR spectroscopy selecting a suitable excitation wavelengths, to validate the observed luminescence signals.

3. Results and discussion

The first part of the study concerned the analysis of panel painting replicas. The excitation LED source was selected as a function of the absorption maximum of the coloured materials constituting each panel. The 99% reflectance standard content in Spectralon ® grey scale target was considered as reference for the evaluation of eventual spurious light in the luminescence images [4,5]. The observed signals were then interpreted in terms of intensity and validated on the basis of the luminescence spectra collected with the portable spectrofluorimeter by exciting at wavelengths very close to those emitted by the LEDs. As 11

ACCEPTED MANUSCRIPT the last step the developed procedure was extended and successfully applied on real artworks of different nature and historical periods.

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3.1 Test panel results Concerning the yellow panel, the blue-induced luminescence image in the 585-690 nm range

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(BIVL, fig.2b) displays an intense luminescence corresponding to the safflower (Y3) and weld lake (Y4) sectors; a much less intense but still evident signal is instead observed for the sector containing the stil de grain lake diluted with gypsum (Y6). Conversely only the sector

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of safflower lake exhibits luminescence when excited with the green LEDs (GIVL, fig.2c). This behaviour was explained with the support of spectrofluorimetric results.

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The emission spectra recorded on sector Y3, Y4, Y5 and Y6 by exciting at 445 nm show in fact a higher fluorescence intensity for safflower and weld lakes compared to stil the grain pigment (fig.3a). Contrary to expectations, the emission intensity of the latter, almost undetectable via imaging in the sector containing the pure lake (Y5), slightly increases when

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the lake is diluted with gypsum (Y6). An analogous phenomenon has been already observed

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on carmine lake painting layers prepared with increasing ratio of lead white as a scattering extender. It has been demonstrated that this behaviour is due to a combined effect of reduced self-absorption and higher multiple scattering of the emitted light. These physical phenomena

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can produce important modifications of the intrinsic fluorescence spectrum of the lake, leading to difficulties in interpreting spectra and therefore images [13]. All the yellow lakes of the panel show, under both 445 and 532 nm excitation wavelength, an emission at 666 nm with a shoulder at 720 nm (insert fig.3a) due to the presence of chlorophyll that remains in the pigment as a residue after the vegetal matter extraction [18]. This emission makes the fluorescence spectra so broad that their tail extends also beyond 700 nm being therefore responsible, particularly for Y3 and Y4 sectors, of the luminescence phenomena observed in the BIL image in 735-1000 nm range (fig.2d). This tail emission is not detected by imaging when exciting with the green LED (image not shown) being this wavelength scarcely absorbed by chlorophyll whose absorption maxima are mainly located in blue and red portion of the Visible spectral range [19]. The emission of safflower sector is instead so high and broad that it can be detected also in the Visible range by exciting with the green LED (fig.2c). 12

ACCEPTED MANUSCRIPT As far as yellow inorganic pigments are concerned no luminescence is observed either with imaging or spectrofluorimetry under the three Visible LED excitation lines, with the exception of cadmium based compounds that exhibit a peculiar behaviour. The blue-induced

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luminescence image in the 585-690 nm range (Fig.2b) highlights in fact that only some cadmium pigments show a detectable luminescence, whereas no signal is observed in the

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green-induced luminescence image in the 633-690 nm range (Fig.2c). Furthermore, as already extensively reported [7, 8, 20, 21, 22], all the cadmium yellow series proves to be luminescent in the Near Infrared range under both blue (Fig.2d) and green LED excitation.

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Once again the recorded luminescence images show a good correspondence with spectrofluorimetric measurements carried out on the panel. Spectrofluorimetric results

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highlight in fact that the analysed cadmium pigments are characterised by two main luminescence bands with variable relative intensity: one is located in the Visible, with maxima centred at about 715-750 nm, and one in the NIR with maxima at about 900-990 nm (Fig.3b). These bands, due to different trap emission states of the semiconductor pigment,

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show a variable relative intensity which is responsible for the behaviour observed in the

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Visible induced luminescence images, whereas exciton emission is so weak that its contribution may be considered negligible [23]. By exciting at 445 nm, some sectors, as for example Y15, Y16 and Y17, show high emission intensity in the 585-690 nm range thus

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appearing luminescent in the blue-induced Visible luminescence images (fig. 2b). For other sectors (as for example Y22 in fig. 3b) the luminescence is so low as to be almost undetectable by imaging technique. When the cadmium yellow series is excited with the green LEDs no emission is observed in the green-induced Visible luminescence image except sector Y15 that shows a weak halo (fig. 2c). This phenomenon is probably ascribable to the lower intensity of the green LEDs compared to the blue but also to the fact that the green excitation wavelength has an energy slightly lower than the band gap energy of the studied pigments, making the absorption and thus the excitation less efficient. However, the Y15 sector excited at 532 nm shows an emission intensity greater than the other yellow pigments (fig. 3c) thus explaining the weak luminescence detected by imaging [23]. In the green induced luminescence image of the red organic pigments in the 633-690 nm range (fig. 4b) the luminescence of safflower red (R3) is evident and, although less intense, the fluorescence from sandalwood (R14), lac dye (R13) and carmine lake both pure (R15) 13

ACCEPTED MANUSCRIPT and diluted (R16). In the blue induced luminescence image in the 585-690 nm range only the R3 and R14 fluorescence is detectable (fig. 4c). By exciting these pigments at 445 nm in the same experimental conditions, safflower red (R3

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sector) and sandalwood lake (R14 sector) display an emission intensity approximately ten times higher than lac dye (R13) and carmine lake (R16) (fig. 5a) despite its lower absorption

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(fig. 5b). This explain why in the BIVL image (fig. 4c) collected in the 585-690 nm range only the R3 and R14 sectors are visible. The lakes of animal origin, instead, may be better detected by imaging technique under green LEDs excitation (GIVL image in the 633-690 nm

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range in fig. 4b) because, even though less intense, the emission wavelengths of these LEDs fall exactly in the absorption maximum of the lakes (fig. 5b). It is worth noting that the

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emission of safflower red, centred at 550 nm, does not correspond to the maximum reported in literature (590 nm) [24] but rather to the emission observed for the yellow pigment obtained from the same vegetal source. This may be due to a partial removal of the yellow dyestuff whose fluorescence completely covers that of the carthamin red component.

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Similarly to that observed under 445 nm excitation, by exciting at 532 nm both safflower red

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and sandalwood lakes, show high fluorescence intensity (Fig.3a insert); their emission tail is, in fact, detected in the green induced luminescence images in the 735-1100 nm range (GIL image, fig.4d).

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As for cadmium red pigments, those having an orange hue (R19, R20 and R21) show a detectable luminescence in the VIS by exciting with blue LEDs (fig.4c) whereas those with a red hue (R24-R28) exhibit luminescence under Green LEDs excitation (fig 4b). The optical properties, and therefore the behaviour observed by imaging technique, is strictly related to the chemical composition of these pigments. It is known in fact that in sulfoselenide pigments an increase in selenium content with respect to cadmium, results in a deeper red shade due to a decrease of the band gap energy. This leads to a consequent shift of the absorption, exciton and trap emissions to longer wavelengths. [8, 20, 22]. In figure 6a are reported some XRF spectra collected on cadmium pigment series where the different content of the Se element is highlighted (R19, R21, R24, R27). The relative amounts of the Se and Cd elements calculated by normalizing the counts of the maximum Kα line in the cadmium pigments are illustrated in figure 6b [20]. The pigments containing a lower amount of selenium (R19, R20, R21) have in fact a detectable luminescence under blue LED 14

ACCEPTED MANUSCRIPT excitation whereas those showing the highest amount of this element are more efficiently excited by green LEDs. All the red cadmium series present instead an intense luminescence signal in the 735-1100 nm range (GIL, fig.4d).

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The blue panel luminescence images in the infrared range are shown in figure 7. The redinduced luminescence images collected either in 735-1100 nm (RIL695) and 830-1100 nm

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ranges (RIL830) prove the intense luminescence of Egyptian blue (B1 and B2 sectors) and Han blue (B7 and B8 sectors) pigments (fig.7). X ray diffraction measurements confirmed the cuprorivaite structure for the Egyptian blue (B1) and effenbergerite structure concerning the

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Han blue (B7 and B8) [25]. As already reported in literature, the emission spectra recorded under Visible excitation show maxima centered at about 910 and 940 nm for Egyptian blue

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[26] and Han blue [5] respectively. No further luminescence signals have been highlighted for

blue pigments by the optimised imaging procedure in the whole explored range (585-1100 nm) under the three different excitation wavelengths used as also confirmed by spectrofluorimetry.

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Since it has been reported that some pigments, as for example manganese blue [14], show

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luminescence features at wavelengths longer than 1100 nm and therefore not detectable with the conventional cameras, the custom made LEDs were used as excitation source to probe the eventual luminescence of the test panels in the 1100-2200 nm range by a filtered Vidicon

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camera. In order to guarantee the maximum intensity of the excitation source, the red, blue and green LEDs were employed simultaneously. The VIL images recorded in 1100-2200 nm range (fig.8b), exhibit the luminescence of the cadmium red and yellow whole series. Furthermore, the sectors containing manganese blue (B26 and B27) show an intense luminescence now detectable by the new excitation setup and recording system. Very interestingly, two sectors of the yellow panel (Y10 and Y12) painted respectively with chrome lemon (WINSOR&NEWTON 1st series) and lemon yellow (nickel titanate, WINSOR&NEWTON 2st series), exhibit a well-defined luminescence. Spectrofluorimetric measurements carried out in both sectors by exciting at 445 nm and 532 nm confirm in fact an emission band centred at 1470 nm with a broad shoulder at about 1580 nm (fig.9). Figure 9 shows that this emission may not be detected by a CCD having the same sensitivity range as the camera used to collect the Visible-induced luminescence images, being the emission signal completely flat under the same excitation conditions. X-ray fluorescence analysis 15

ACCEPTED MANUSCRIPT reveals the presence of nickel and titanium together with antimony in both sectors, whereas X-ray diffraction pattern confirms a rutile-like structure for both pigments. It is evident that chrome lemon results from a mislabeling. The nickel rutile yellow (colour index PY53) is in

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fact an antimony oxide-nickel oxide-titanium oxide compound prepared by calcining the three oxides [27]. The pigment is synthesized dissolving the oxide chromophores, in an

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oxidation state different to +4, in the crystal lattice of rutile. Titanium dioxide in rutile crystalline form does not have luminescence properties in the infrared range when excited with Visible radiation, however it becomes luminescent under the doping with nickel and

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antimony. Photophysical and structural studies are in progress to understand more in depth the nature of this luminescence. It is worth mentioning that, although less intense, the same

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luminescence features in the 1100-2200 nm range, may be detected via imaging by exciting with single narrow band LEDs highlighting that the intensity of the setup excitation source is so high that the luminescence signal may be detectable even by a Vidicon camera.

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3.2 Case studies

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In this section the developed procedure was applied on two paintings, Primavera (1912) by Gerardo Dottori, conserved at the Museo della Penna in Perugia (Italy) and Untitled (1985) by Pietro Consagra exhibited at the Palazzo Collicola in Spoleto (Italy) and finally on a XVI

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century illuminated manuscript stored at Biblioteca Comunale in Spoleto (Italy). The Visible induced luminescence imaging study of Primavera highlighted some peculiarities concerning the use of red pigments. Comparing the Visible (VIS, fig.10a) and the luminescence image in the 735-1100 nm range (GIL, fig.10c) a whitish luminescence is observed on localised painted areas that appear to be homogeneous to the naked eye. The insert of fig. 10b, showing the overlapping of Visible and infrared luminescence (50% in transparency) images, clearly shows that the red hues were obtained by Dottori using at least two red pigments, one luminescent and the other non-emitting. The preliminary study, carried out on the red test panel, allows us to hypothesise that the emission behaviour is due to a cadmium based pigment whose presence has been in fact confirmed by non-invasive spectroscopic techniques [28].

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ACCEPTED MANUSCRIPT The second artwork examined is Untitled by Consagra. The blue induced luminescence image in the infrared range (fig.11b) shows a yellow-greenish luminescence that becomes more intense in correspondence of the yellow areas and weaker in the green and red painted

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zones suggesting once again the use of cadmium based pigments. XRF analysis, in fact, have confirmed the presence of cadmium in different amounts in all the luminescent areas

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(fig.11c). In particular the most intense signals are detected in paints corresponding to the use of yellow cadmium pigment, showing in the X-ray fluorescence spectra the signals of Ba, Zn, Cd, S and Sr. In the green areas the additional presence of chromium and cobalt suggests the

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use of a chromium oxide with cobalt blue and a yellow cadmium pigment. The presence of selenium in the red areas points out the employment of cadmium sulfoselenide. It has to be

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mentioned that this campaign survey was carried out during museum visiting hours without complete darkness. Despite the absence of optimal illumination conditions, the maximum power of the LED light sources and the use of 99% reflectance standard, allowed luminescence images to be acquired estimating and eliminating the contribution of the

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ambient stray light [29] .

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Finally this methodology has been successfully applied to the study of an illuminated manuscript. The purple areas excited with the blue LEDs, display a reddish luminescence (fig.12d) in the Visible range that has been also detected by spectrofluorimetric

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measurements. The UV-Visible reflectance (560 nm) and emission (600 nm) spectra (fig.12b and c respectively) suggest in fact the presence of a red vegetal lake [18]. Interestingly, MidFTIR spectroscopy reveals, in the same violet areas, the signals of anhydrite (CaSO4) that sometimes are found, as extender or substrate for organic colorants [30]. It has to be said that the size of the illuminated manuscript (the size of the miniatures are approximately 30X30 mm) made the detection of fluorescence in small painted areas more problematic. However, the narrow Visible band generated by LED light source system allows us to selectively excite the colorant, minimizing the interference with the parchment emission that is instead, observed under UV excitation [30, 31] and can generate important spectral distortion due to inner filter effects. With this regard it is important to note that the red painted details composed of cinnabar, revealed by XRF analysis through the presence of mercury, do not show any fluorescence. A completely false orange emission may instead be detected under UV excitation when this pigment is found together with a species exhibiting a yellow-green 17

ACCEPTED MANUSCRIPT fluorescence such as parchment and binders [12]. It is known that the extent of this distortion may be drastically reduced or removed by exciting the pigment with wavelengths very close

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to its absorption maximum.

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4 Conclusion

This work highlights that the developed system, consisting of interchangeable and tunable LED light sources associated with different detection system, a digital camera wide spectrum

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(WS) for the 380-1100 nm range and a Vidicon Camera for the 1100-2200 nm range, constitutes a suitable methodology for the non-invasive characterisation and distribution of

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luminescent artist’s materials. The analytical procedure, based on the study of opportunely prepared mock-ups containing more than 70 different kind of pigments and lakes, and the subsequent cross-validation through UV-Visible spectrofluorimetry and spectrophotometry allowed us to evaluate the actual sensitivity and reliability of the experimental set up adopted.

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Comparing the results obtained by photo-induced luminescence imaging, using the 99%

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reflectance standard as reference of the luminescence phenomena, with those obtained by UV-VIS-NIR spectroscopy, selecting a suitable excitation wavelengths, it has been demonstrated that the fluorescence images show only the intrinsic luminescence of the

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materials, completely avoiding any interference from spurious lights. In particular it has been shown that the excitation in the Visible range prevents the collection of eventual distortion or false emissions due to self-absorption and inner filter effect without resorting to particular image elaborations. The system has proven to be particularly sensitive and versatile in the detection of luminescent materials on real artworks of diverse nature where the narrow emission band and high intensity of the excitation source generated by LEDs allowed to detect the presence of cadmium based pigments, even in mixtures with other compounds, and a red vegetal lake applied as a thin layer on a fluorescent support such as parchment. Finally moreover, the interchangeability allows us to record Visible images and luminescence images under three excitation wavelengths by using the same irradiation source and without moving the art object or the experimental equipment. This advantage, together with the high intensity and selectivity, make the designed interchangeable and tunable LED 18

ACCEPTED MANUSCRIPT light source system an economical, user friendly and comfortable tool for the detection of luminescent images in museums or in restoration sites.

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Acknowledgments This research was carried out as part of the Regione Umbria project "Sviluppo delle attività di

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ricerca, valutazione e tutela conservativa", Progetto 1 del Primo atto integrativo all'APQ "Tutela e prevenzione dei beni culturali". We thank the FUTURAHMA project

(Dal

futurismo al ritorno al classicismo 1910-1922.Tecniche pittoriche, critica delle varianti e

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problemi conservativi) for the involvement in the analysis of several paintings of Dottori. A special acknowledgement to Palazzo della Penna museum in Perugia, Palazzo Collicola in

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Spoleto and Biblioteca Comunale G. Carducci in Spoleto for the access to the works of art. Captions:

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Fig.1: the emission spectra of blue (1), green (2), red (3) LEDs and the transmission spectrum of the B+W 486 UV IR CUT band-pass glass filter (a); the emission spectrum of blue LEDs and the transmission spectra of B+W486 UV IR CUT band-pass, B+W 040 cut-on and B+W 092 IR 695 cuton glass filters (b); emission spectrum of green LEDs and the transmission spectra of B+W486 UV IR CUT band-pass, B+W 090 cut-on and B+W 092 IR 695 cut-on glass filters (c); the emission spectrum of red LEDs and the transmission spectra of B+W486 UV IR CUT band-pass and B+W 092 IR 695 cut-on glass filters (d). Fig.2: Visible (a), blue-induced Visible luminescence in the 585-690 nm range (BIVL, b, irradiance= 650 lux; integration time= 30sec), green-induced Visible luminescence image in the 633-690 nm range (GIVL, c, irradiance= 900 lux; integration time= 30sec) and blue-induced infrared luminescence in 735-1100 nm range (BIL, d, irradiance= 1100 lux; integration time= 30sec) images respectively of the yellow panel. Fig.3: fluorescence spectra (a) of Y3, Y4, Y5, Y6 sectors (λexc= 445nm), and weld lake emission spectrum (λexc= 532 nm) in the insertion. On the right the emission spectra of cadmium yellow series (b) Y17, Y20, Y22, sectors (λexc= 445 nm), and (c) Y15, Y16, Y22 sectors (λexc= 532 nm). Fig.4: Visible image (a), green-induced Visible luminescence image in the 633-690 nm range (GIVL, b, irradiance= 900 lux; integration time= 30sec), blue-induced Visible luminescence image in the 585-690 nm range (BIVL, c, irradiance= 650 lux; integration time= 30sec) and green induced infrared luminescence in 735-1100 nm range (GIL, d, irradiance= 900 lux; integration time= 30sec) respectively of the red panel. 19

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Fig. 5: fluorescence spectra (a) (λexc= 445 nm and λexc= 532 nm in the insertion) and UV-Vis-NIR pseudoabsorbance spectra (b) of R3, R13, R14, R16 sectors. The blue and the green lines (b) indicate the emission wavelength output of the blue and green LEDs exciting sources respectively.

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Fig.6: XRF spectra of R19, R21, R24 and R27 (a). Relative amount of Se and Cd in the cadmium red pigments normalized to the maximum Kα line of the dataset.

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Fig.7: Visible image (a), and the red-induced infrared luminescence images in 735-1100 nm range (RIL695, b, irradiance= 430 lux; integration time= 30sec) and in 830-1100 nm range (RIL830, c, irradiance= 760 lux; integration time= 30sec) respectively of the blue panel.

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Fig.8: Visible image (a), and the Visible-induced infrared luminescence (white LEDs) images in 1100-2200 nm range (b) of the blue, yellow and red panels respectively. Fig.9: luminescence spectra of Y12 sector (λexc= 445 nm) collected in the 300-1150 nm range (grey line) and 950-1600 nm range (black line).

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Fig.10: Primavera, Gerardo Dottori. Visible (VIS, a), green-induced infrared luminescence (GIL, c, irradiance= 900 lux; integration time= 30 sec) images and a detail (b) of the superposition of Visible and luminescence (50% in transparency) images.

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Fig.11: Untitled, Pietro Consagra: Visible (VIS, a) and blue-induced infrared luminescence (BIL, b, irradiance= 1100 lux; integration time= 30 sec) images, XRF spectra (c) of the yellow, green and red areas (indicated by * in the Visible image). Fig.12: Illuminated Manuscript, (XVI century). Visible (VIS, a), blue-induced Visible luminescence (BIVL, b, irradiance= 1100 lux; integration time= 5 sec) images and UV-Vis-NIR reflectance (b) and emission (λexc= 445 nm) spectra (c).

References

[1] C.Fischer, I.Kakoulli, Multispectral and hyperspectral imaging technologies in conservation: current research and potential applications, Rev. in Conservation 7 (2006) 3-16; [2] A. Aldrovandi, E. Buzzegoli, A. Keller, and D. Kunzelman, Investigation of painted surfaces with a reflected UV false colour technique’, in Art’5 - 8th International Conference on "Non Destructive Investigations and Microanalysis for the Diagnostics and Conservation of the Cultural and Environmental Heritage." Lecce (Italy), 15-19 May 2005. [3] M.Bacci, A.Casini, C.Cucci, M.Picollo, B.Radicati, M.Vervat, Non-invasive spectroscopic measurements on the Il ritratto della figliastra by Giovanni Fattori: identification of pigments and colourimetric analysis, J. Cultural Heritage, 4 (2003) 329–336. 20

ACCEPTED MANUSCRIPT [4] G. Verri, The application of Visible-induced luminescence imaging to the examination of museum objects', in Proceedings of SPIE 7391, O3A: Optics for Arts, Architecture, and Archaeology II, eds.Pezzati L. and Salimbeni R. (2009) 739105.

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[5] G. Verri, The spatially resolved characterisation of Egyptian blue, Han blue and Han purple by photo-induced luminescence digital imaging, Analytical and Bioanalytical Chemistry 394(4) (2009), 1011-1021.

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[6] http://www.britishmuseum.org/pdf/charisma-multispectral-imaging-manual-2013.pdf

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[7] A. Romani, C.Clementi, C.Miliani, G.Favaro, Fluorescence Spectroscopy: A Powerful Technique for the Noninvasive Characterization of Artwork, Acc. Chem. Res. 43 (2010) 837-846

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[8] M. Thoury, J.K. Delaney, E.R Rie, M.Palmer, K. Morales and J. Krueger, Near-infrared luminescence of cadmium pigments: in situ identification and mapping in paintings, Appl. Spectrosc. 65 (2011) 939-951. [9] J.K. Delaney, J.G. Zeibel, M. Thoury, R. Littleton, M. Palmer, K.M Morales, E.R. de la Rie, A. Hoenigswald, Visible and infrared imaging spectroscopy of Picasso's Harlequin musician: mapping and identification of artist materials in situ, Appl Spectrosc. 2010 Jun;64(6):584-94. doi: 10.1366/000370210791414443

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[10] E.R. de la Rie, Fluorescence of paint and varnish layers (part 1), Studies in Conservation 27 (1982) 1–7. [11] G.Verri, D.Comelli, S.Cather, D.Saunders, F.Pique, Post-capture data analysis as an aid to the interpretation of ultraviolet-induced fluorescence images, Proceedings of SPIE 6810, Computer Image Analysis in the Study of Art, eds Stork, D.G. and Coddington, J. (2008) 681002-1 to 681002-12.

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[12] G.Verri, C.Clementi, D.Comelli, S.Cather, F.Piqué, Correction of Ultraviolet-Induced Fluorescence Spectra for the Examination of Polychromy, Applied Spectroscopy, 62 (2008) 12951302. [13] C.Clementi, C.Miliani, G.Verri, S.Sotiropoulou, A.Romani, B.G.Brunetti, A.Sgamellotti, Application of the Kubelka–Munk Correction for Self-Absorption of Fluorescence Emission in Carmine Lake Paint Layers, 63 (2009) 1323-1330. [14] G. Accorsi, G. Verri, A. Acocella, F. Zerbetto, G. Lerario, G. Gigli, D. Saunders and R.Billinge, Imaging, photophysical properties and DFT calculations of manganese blue (barium manganate(VI) sulphate) – a modern pigment, Chem. Commun., 2014, 50, 15297. [15] L. Simonot, M. Thoury, J. K. Delaney, Extension of the Kubelka-Munk theory for fluorescent turbid media to a nonopaque layer on a background. J. Opt. Soc. Am. A, 28(7):1349-1357, 2011. [16] http://www.doernerinstitut.de/downloads/Back_to_the_Roots/Back_to_the_roots_Kirby_I.pdf [17] A. Romani, C. Grazia, C. Anselmi, C. Miliani, B.G.Brunetti, Proc. SPIE 8084, O3A: Optics for Arts, Architecture, and Archaeology III, 808403 (June 06, 2011); doi:10.1117/12.889529; 21

ACCEPTED MANUSCRIPT [18] C.Clementi, B.Doherty, P.L.Gentili, C.Miliani, A.Romani, B.G.Brunetti, A.Sgamellotti, Appl. Phys. A 92 (2008) 25-33.

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[19] Z. G. Cerovic, A. Ounis, A. Cartelat, G. Latouche, Y. Goulas, S. Meyer and I. Moya, The use of chlorophyll fluorescence excitation spectra for the non-destructive in situ assessment of UV-absorbing compounds in leaves, Plant, Cell and Environment 25 (2002) 1663–1676.

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[20] A.Cesaratto, C. D'Andrea, A.Nevin, G.Valentini, F.Tassone, R. Alberti, T.Frizzi, D. Comelli, Analysis of cadmium-based pigments with time-resolved photoluminescence, Anal. Methods 6 (2014) 130-138.

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[21] C.F. Bridgman and H.L. Gibson, Infrared Luminescence in the Photographic Examination of Paintings and Other Art Objects, Studies in Conservation 8, 77-83 (1963).

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[22] D.Anglos, M.Solomidou, I.Zergiotti, V.Zafiropulos, T. Papazoglou, C. Fotakis, Laser-induced fluorescence in artwork diagnostic: an application in pigment analysis, Appl.Spectrosc. 50 (1996) 1331-1334. [23] F. Rosi, C. Grazia, F. Gabrieli, A. Romani, M. Paolantoni, R. Vivani, B.G Brunetti, P. Colomban, C. Miliani, UV-Vis-NIR and micro Raman spectroscopies for the non destructive identification of Cd1-xZnxS solid solutions in cadmium yellow pigments accepted in Microchemical Journal.

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[24] C.Clementi, G.Basconi, R.Pellegrino, A.Romani, Carthamus tinctorius L.: A photophysical study of the main coloured species for artwork diagnostic purposes, Dyes and Pigments 103 (2014) 127137. [25] H.Berke, H.G. Wiedemann The chemistry and fabrication of the anthropogenic pigments chinese blue and purple in ancient China, EASTM 17 (2000), 94-120.

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[26] G. Accorsi, G. Verri, M. Bolognesi, N. Armaroli, C. Clementi, C. Miliani, and A.Romani,, ‘The exceptional near-infrared luminescence properties of cuprorivaite (Egyptian blue)’, Chemical Communications (2009) 3392–3394. [27] N.Eastaugh, V. Walsh, T. Chaplin, R. Siddall Pigment Compendium, A dictionary and optical microscopy of historical pigments. [28] F. Rosi, M.Patti, L. Cartechini, F.Gabrieli, L.Monico, A.Romani, C. Anselmi, C. Grazia, A. Daveri, A.Sgamellotti, C. Miliani , Dottori Gerardo the futurist view, Fabrizio Fabbri Editore, Milano (2014), 33-45. [29] G.Verri, D.Saunders Xenon flash for reflectance and luminescence (multispectral) imaging in cultural heritage applications, Technical Research Bulletin, British Museum 8 (2014). [30] B. Doherty, A. Daveri, C. Clementi, A. Romani, S. Bioletti, B. Brunetti, A. Sgamellotti, C. Miliani, The Book of Kells: A non-invasive MOLAB investigation by complementary spectroscopic techniques, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 330– 336.

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[31] M.Aceto, A.Arrais, F. Marsano, A. Agostino, G. Fenoglio, A.Idone, M.Gulmini, A diagnostic study on folium and orchil dyes with non-invasive and micro-destructive methods, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 142 (2015) 159–168.

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Highlights We propose a user-friendly tool for luminescence acquisition images of works of art Custom made interchangeable and tunable LEDs for the visible-induced luminescence Crossvalidation of luminescence images with UV-VIS-NIR spectrofluorimetry NIR luminescence of a modern yellow pigment, nickel titanate, has been documented

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