Laser-induced fluorescence study of medieval frescoes by Giusto de’ Menabuoi

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Laser-induced fluorescence study of medieval frescoes by

Giusto de’ Menabuoi

Roberta Fantoni, Luisa Caneve, Francesco Colao, Luca Fiorani, Antonio

Palucci, Ramiro Dell’Erba, Vasco Fassina

To cite this version:

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Italian National Agency for New Technologies, Energy and

Sustainable Economic Development

http://www.enea.it/en

http://robotica.casaccia.enea.it/index.php?lang=en

This paper is a pre-print. The final paper is available on:

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Author's personal copy

JournalofCulturalHeritage14S(2013)S59–S65

Available

online

at

www.sciencedirect.com

Laser-induced

ﬂuorescence

study

of

medieval

frescoes

by

Giusto

de’

Menabuoi

Roberta

Fantoni

a,∗

_,

_Luisa

_Caneve

a

_,

_Francesco

_Colao

a

_,

_Luca

_Fiorani

a

_,

_Antonio

_Palucci

a

_,

Ramiro

Dell’Erba

b

_,

_Vasco

_Fassina

c

a_ENEA_Technical_Unit_for_the_development_of_applications_of_radiations,_CR_Frascati,_V._E._Fermi_45,₀₀₀₄₄_Frascati,_Italy

b_ENEA_Technical_Unit_technologies_for_energy_and_industry_–_Robotics_Laboratory,_CR_Casaccia,_Via_Anguillarese_301,₀₀₁₂₃_Roma,_Italy c_{Soprintendenza}_ai_Beni_Storici_Artistici_ed_{Etnoantropologici}_per_le_province_di_Venezia,_Belluno,_Padova_e_Treviso,_Rio_Marin,₃₀₁₂₄_Venezia,_Italy

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received24October2012 Accepted31October2012 Availableonline20February2013 Keywords:

Laser-inducedﬂuorescence Frescoes

PCanalysis

Spectralanglemapper Giustode’Menabuoi

a

b

s

t

r

a

c

t

Laser-inducedfluorescence(LIF)isapowerfulremoteandnon-invasiveanalysistechniquethathasbeen successfullyappliedtothereal-timediagnosisofhistoricalartworks.Hyperspectralimagescollectionon fresco’sandtheirfalsecoloursprocessingallowedtorevealfeaturesinvisibletothenakedeyeandto obtainspecificinformationonpigmentscompositionandconsolidantsutilization,thelatteralsorelated toformerrestorations.ThisreportpresentstheresultsobtainedbyENEALIFscanningsystemduringa fieldcampaignconductedinJune2010onfresco’sbyGiustode’MenabuoiinthePaduaBaptistery.The datacollectedbyLidArtallowedthedetectionofParaloidB72andMovilith/PrimalAC33,guidingthe restorersintheirconservationactions.

1. Introduction

Laser-inducedﬂuorescence(LIF)isapowerfulremoteanalysis toolthathasbeensuccessfullyappliedtothereal-timediagnosisof historicalartworks,allowingtheobservationoffeaturesinvisible tothenakedeye,aspigmentcomposition[1],biologicalattack[2]

andtraceofformerrestorations.

InatypicalLIFinstrument,anultraviolet(UV)laserbeam irra-diatesasampleandanopticalsystemmeasurestheﬂuorescence spectrum that contains informationonthe target composition. Nevertheless,alternative spectroscopic techniques,suchas XRF andmicro-Ramanaresuitabletoobtainanalogousinsitu informa-tion[3].LIFisuptonowtheonlyoneforwhichremoteapplication hasbeendemonstrated up toten of meters.LIF is indeed fast, remote[4],non-invasive,sensitiveandselective.Theseadvantages have encouragedits application in the real-time monitoringof historicalfrescoes,mosaics,paintingsandstones.Formerstudies showeditshighpotentialasadiagnostictoolforculturalheritage eitheroperatinginspectral[1,2]ortime[5]resolvedmode.

TheTechnicalUnitforthedevelopmentofapplicationsof radi-ations,partoftheItalianNationalAgencyforNewTechnologies, EnergyandSustainableEconomicDevelopment(ENEA)hasworked inthisfieldsince2003andhasdevelopedvariousprototypes.The systemheredescribed(LidArt)isalidarfluorosensor(opticalradar basedonLIF)and itsfirstversion waspatented in2007[6]. At

∗ Correspondingauthor.

E-mailaddress:fantoni@frascati.enea.it(R.Fantoni).

present,itisevenmorecompact(containedinacylinderofradius 29cmandheight18cm),hyperspectralandtimeresolved(the ﬂu-orescencespectrumismeasuredwithawavelengthresolutionof few nm and a timeresolution of few ns).Moreover,it is light, robust,transportable,user-friendlyandcost-effective.Eventually, itsscanning(theartworksurfaceisprobedremotelyaimingthe laserbeam)isbasedonaninnovativeapproach[7]thatreduces theacquisitiontimefromhourstominutes.

Thanks tosophisticated dataprocessing techniquesas false-colorimaging,principalcomponentanalysis(PCA)onspectraand spectralanglemapping(SAM)onimages,LidArthasdetected char-acteristicsinvisibletothenakedeye,aspigmentcompositions(e.g. titaniumwhitevs.zincwhite[8]),pigmentdiffusions(limeand casein)[9],biologicalattacks(algaeandfungi)[10],consolidants (usuallyresins)[11],deteriorations,depigmentations, retouches andvarnishes.

In particular,restorers needa precise knowledge of surface materialsonceengagedinremovingtracesofformerrestorations. LIFimages,collectedinthePaduaBaptisteryandprocessedinorder toretrieveinformationonconstituentmaterials,areherepresented anddiscussed.TheﬁeldcampaignwascarriedoninJune2010prior totheplanningofsuccessiverestorations.Tothisaim,the investiga-tionwasfocusedontotherecognitionofconsolidantsandbinders, withthesupportofanavailabledatabaseandthedevelopmentof statisticaltoolsformainspectralbandsidentiﬁcation.

TheconstructionofPaduaBaptistery,locatedontherightside ofthecathedral,startedin12thcenturyandwasaccomplishedin 1281.Itsfresco’sareconsideredGiustode’Menabuoi’smasterpiece. Inthepresentstudy,carriedonwithinacontractsignedbyENEA

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S60 R.Fantonietal./JournalofCulturalHeritage14S(2013)S59–S65

andSoprintendenzaaiBeniStoriciArtisticiedEtnoantropologiciper leprovincediVenezia,Belluno,PadovaeTreviso,selectedareasof theParadisefrescoonthecentraldomeandoftheGenesisonthe tambourhavebeeninvestigatedbymeansoftheENEALIFscanning system.

Aftera shortdescriptionofthelastLidArt prototype,results obtainedduringtheﬁeldcampaignconductedfromJune7to11 arepresentedanddiscussed.Someconclusionsonthe appropri-atenessofthedevelopedtechnologicaltooltotherequiredremote investigationonapaintedCHsurface,aregivenattheend.

2. Experimental

2.1. Theset-up

The LIF scanningsystem developed and patented [6] atthe ENEAlaboratoryinFrascatiiscapabletocollecthyperspectral ﬂu-orescenceimagesscanninglargeareasforapplicationstocultural heritagesurfaces(e.g.frescoes,decoratedfacadesetc.).Anew com-pactset-uphasbeenbuilt,andsuccessivelypatented[7],aimed toincrease theperformancesin termsofspaceresolution, time resolvedcapabilitiesand dataacquisitionspeed.Major achieve-mentshavebeenreachedbyacriticalreviewoftheopticaldesign andconsequentlyofthedetectorutilized:thepointfocalization modehasbeenchangedintoalinefocalizationbyusingaquartz cylindricallensandaimagingspectrograph(Jobin-YvonCP240); thelineararraydetector,responsibleforthemultichannelspectral resolution,hasbeenreplacedwithasquareICCDsensor(ANDOR iStarDH734,pixelsize13␮m),mountedbehindaslitparallelto thelaserlinefootprintduringthescanning.

This arrangementis characterizedby havingthespatial and spectral information on two mutually orthogonal directions imagedonthedetector,withsubmillimetricspatialresolutionand aspectralresolutionbetterthan2nm.Additionally,itispossibleto implementtimeresolvedmeasurementsonthenanosecondscale bygatingthespectraldetector.

Theoverallcurrentsystemperformancesarehorizontal resolu-tion640pixel,0.1mradangularresolution,minimumacquisition timeperline200ms,ﬁeldofview(FOV)aperture5.7◦ (correspond-ingtoascannedlineof2.5mat25mdistance).Withthelatter optics,animageof1.5×5m2_is_scanned_in_less_than₂_minutes_at

25m.

The compact arrangement of the experimental apparatus is showninFig.1,togetherwiththeadoptedoptical scheme.The Nd:YAGcompactlasersourcecouldbeoperatedattwodifferent

wavelengthsintheUV:355nmor266nm,1mJofoutputenergy wereproducedinshortpulses(10ns)at20Hz.Specialcarehasbeen paidtocheckonreferencesamplesinlaboratorythattheutilized laserﬂuencewaslowenoughtoavoidanyphoto-damageinduced bythelasertothepaintedsurface(energydensity<7␮J/cm2₎_as

testedonfrescospecimenstothepurposerealized.Inparticular measurementswerecarrieddefocussingthelaserbeam.The com-binationof(low)laserrepetitionratewith(high)scanningspeed allowedtoavoidtheoccurrenceofanyphotochemicalprocessat thesurface(e.g.pigmentoxidationorconsolidantdecomposition) relatedwithcontinuousirradiationofthesamepoints.

2.2. Operatingmodes

Thelinescanningsystemcanoperateintwooperatingmodes byslightlychangingtheactivecomponentsintheentiredevice: i.e.reﬂectanceandﬂuorescencemeasurements.Thesemodesshare indeedmostoftheoptoelectronicequipment,namelythe collec-tionopticsandthespectraldetector,whilediffering,ononehand, inthewaythesampleisexcitedand,ontheotherhand,intheway thespectraldetectorisoperated.

Reflectancemeasurementsareperformedinascaninwhichthe laserisswitchedoffwhilethesampleisexposedtothelightemitted byaNISTtraceablelamp.Thecollectionopticsarefocussedontothe scannedsurfaceandtheacquisitioncameraisrunningbylocking itsinternalsynchronismtothepowerline(toavoidimage flicker-ing).Theresultgivesthereflectancespectrumforeachpixelofthe scannedarea,fromwhichtheCIE/labcoordinatescanbecomputed oncethesystemiscalibratedagainstareferencesurface.

Theacquisitionofﬂuorescencespectrainducedbylaseris car-riedonduringascanwiththelaserswitchedon.Thesystemuses aproceduretorecordspectraalsoinfulldaylight:inthiscase, a proprietarymethodis usedtodiscriminatebetweenthelight inducedfromthelaser(LIFsignal)withrespecttothelight diff-usedbytheareaunderstudyandbytheenvironment.Theresult givestheﬂuorescencespectrumforeachpixelofthescannedarea; thenasuccessivedataanalysisoftheacquiredspectrumprovides detailedinformationontheareaunderstudy.

LIFﬂuorescencemeasurementsrequiretheadoptionofa pecu-liarmeasurement protocolinordertoachievea gooddetecting efﬁciencyandhighspatialresolutionevenintheverybroadspectral rangeofradiationemitteduponlaserexcitationat266nm.Infact, becauseoftheuseofrefractivecollectingoptics,thefocalplanesfor ultraviolet(UV)andvisible(VIS)radiationoccurindifferent posi-tion,causingdefocuseffectsanddegradingthespatialresolution

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R.Fantonietal./JournalofCulturalHeritage14S(2013)S59–S65 S61

ineitheroftheregions.Toovercometheproblem,successive sep-arateacquisitionrunsarescheduledforeveryscan,eachofthem optimizedforaspeciﬁcwavelengthrange.

Actually,threeconsecutiveacquisitionrunsarescheduled:in thefirstthecollectingopticsisfocusedontheUVregionfrom250 to450nm;thesuccessivetwoscansarefocusedonthespectral regionfrom400to750nmtoacquirerespectivelythefluorescence inducedinthevisibleandthereflectanceimage.

2.3. Analysisofspectralmeasurements

Tospeeduptheprocessingofacquiredimages,themost rele-vantspectralfeaturesareidentifiedbyPrincipalComponent(PC) analysis.Althoughitis commonlyadmittedthatthePCsdonot possessanydirectphysicalmeaning,theycanneverthelessbe rep-resentedasspectrasuitabletobedescribedintermsofbands.In somecases,agivenPCcorrespondstowelldefinedspectroscopic bandswithassociatedpeaks,whileinothercasesmore compli-catedtrendsandshapesareobserved:mostfrequentisthecaseof abandsetagainstanother.FewofthePCsareusuallyretainedfor subsequentanalysis:typicallyfivetoeightcomponentsareenough todescribetheentirespectraldataset.Itisalsoworthnoticingthe possibilitytobuildsuitablelinearcombinationsofthecomputed PCstohaveafaithfulrepresentationofeachpixelspectra, even-tuallyusedforthecomputationofstandardCIE/labcolorimetric measurements.

Inthepresentpaper,thePCanalysisisdevoted tothe iden-tiﬁcationofprominentspectralfeatures,thusrelievingfromthe lengthytimeconsumingexaminationofeachacquiredspectrum

[12].Thisprocedureisadvantageousbecauseitisfastandcanrunin asemi-automaticmode.However,ithastheinconveniencetogivea globalanalysis,possiblyignoringthoselocalpeculiaritieswhichdo notpossessenoughstatisticalsigniﬁcance.Toovercomethis draw-backlocalPCanalysisarealsoperformedondifferentportionof thescannedareas,thentheresultsareanalyzedseparately.Once identiﬁed,spectralbandsaresoughtforintheacquiredLIFspectra, completingthedataanalysis.

Adifferentmethod,usedintheanalysisofspectralimages, con-cernstheidentificationofregionshavingaspecificspectralcontent. Typicalisthecaseofidentificationofagivenpigmentinanimage: suchtaskisaccomplishedeitherbyabandanalysis,orbyusing projectionalgorithmslikeSpectralAngleMapper(SAM)or Spec-tralCorrelation Mapper(SCM)incombinationwithanavailable database[13].Althoughthemapperalgorithmsperformwellwith alowcomputationalcost,theirperformancesaregenerallylower withrespecttothebandanalysisprocedures.

3. Results:LIFreferencespectrafromfrescomaterials

Beforepresentingimagesremotelycollectedonfrescoesatthe Baptistery,inordertoproceedwithaneffectivedataanalysis,it isworthwhiletoreportthereferencespectrawhichcanbe uti-lizedforSAMprojectionsinordertoidentifyspeciﬁccontribution fordifferentchemicalsatthefrescosurface.Tothisaimlaboratory sampleswerecharacterizedaspreparedaccordingtotraditional receiptsconcerningbothmaterialsandmethods.Aﬁnallayerof consolidanthasbeenaddedontopofeachsamplesurface.Based onformerinformationofrestorationscarriedoninPadua Baptis-teryinthefollowing,wedescribeonlyapartoftheavailabledata base,includingacrylicresins,bindersandpigments,whicharemost likelyrelevanttothefrescoesuponinvestigation.Notethat nor-malizedspectra,gatherafterdifferentsamplepreparationresulted alwaysverywellcomparable,thussupportingthepeculiar spec-tralfeatureassociatedtoeachcomponentuponmonochromatic UVexcitation. 300 400 500 600 700 800 900 0.0 0.2 0.4 0.6 0.8 1.0 Calaton Mowilith 50 Primal AC33 Paraloid B72 Vinavil background

Fluorescence Intensity (a.u.)

wavelength(nm)

Fig.2.Fluorescencespectraofacrylicresinsonlaboratoryfrescossamplesexcited at266nm.

3.1. LIFreferencespectraofacrylicresins

Sinceabout40yearsacrylicresinswerecommonlyutilizedas consolidantinrestorationofpaintedsurfaces,includingfrescoes. LIFspectroscopyhasbeenutilizedforthecharacterizationofﬁve differentresins:paraloidB72,calaton,primalAC33,mowilith50 andvinavil.Alaboratorysamplesetwaspreparedduringa coop-erationLjubljanaRestorationCentre.Eachsamplewasformedbya plastersubstrate,apigmentlayerandcoveredbyanacrylicresinat differentthickness.Referencespectrawerecollectedupon excita-tionat355nmand266nm,theformershowedabroadblue-green bandcharacteristicfortheentirefamily,whilethespectracollected fromthelatterareshowninFig.2,theirmainfeaturesare summa-rizedinTable1.

Thespectracollectedafterexcitationat266nmaresuitableto resinsclassification,sincetheyshowspecificemissionbandsfor eachdifferentsample[11];namelyonlycalatonpresentsa char-acteristicblue-greenbroadvisibleemission,whileallothersare strongUVemitters.Amongthemvinavilcanberecognizedbyits slightlybroaderUVbandaccompaniedbyaweakerwelldefined secondpeakat510nm.Finally,thesubstrateemissionaloneis com-pletelydifferent,notshowinganysignatureoverlappingwiththe examinedconsolidants.

3.2. LIFreferencespectraofbinders

Since historical times, organic binders are utilized to keep togetherpigmentspowders.Eightdifferentcompounds,listedin

Table2,wereinterrogatedbyLIFattwodifferentexcitation wave-lengths. Samples wererealized at theChemistry Dep.of Rome UniversitySapienza,byplacingdirectlythebinderontopofanon ﬂuorescentsubstrate.

Referencespectra,collectedat2mdistanceuponlaser excita-tionat355nm,areshowninFig.3,respectivebandcentrepositions

Table1

Bandcentresofﬂuorescencespectraofacrylicresinsonlaboratoryfrescossamples excitedat266nm.

Resin Bandcentre(nm)

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Table2

Pigmentspeakemissionexcitedat266nm.

Pigment Bandcentres(nm)

(exc)=255nm Lineoil 460 Leadwhite 460 Calcite 450,580 Leadsulfate 450 Nopigment(plaster) 460 400 450 500 550 600 650 700 750 800 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 x 104 Wavelength [nm]

LIF intensity [A.U.]

Bgnd_C3 RabbitGlue_C2 EggWhite_C1 EggYolk B3 LineseedOil B1 Casein3 A1

Fig.3.Fluorescencespectraofbinderonaninertsubstrateuponexcitationat 355nm.

aregiveninTable3.Dataobtaineduponexcitationat266nmare

notshownsince,apartfromrabbitgluepeakedat310nm,all spec-traappearverysimilarwithamaximumaround340nmandatail alongthevisible.Wemaythusconcludethat,onceinformationon bindersaresearchedfor,itisworthwhiletoutilizethelongerUV wavelength(355nm)todetectspectralfeatureswhichmightallow fortheiridentiﬁcation.

4. Results:thecampaignonPaduaBaptisteryfrescoes

4.1. Thescannedareas

IntheBaptistery,differentportionsofthefrescoeshavebeen scannedwithinthedomeandonthetambour.Siximageswere collectedonthedomevaultand eight imagesonthetambour. Datawereacquiredat15mdistancefromthedomeataslightly smallerdistancefromthetambour.Imagescollectedareshaped asrectangularstripesabout1.5mbroadandtypicallyfrom8mto 12mlong.

Table3

Binderspeakemissionexcitedat355nm.

Binder Bandcentres(nm)

(exc)=355nm Casein 450 Lineoil 520 Eggwhite 520 Rabbitglue 520 Redwhite 540 Background(silica) 520

Fig.4.SectionsofthedomevaultscannedbyLIF,greylevelscorrespondto differ-entemissionintensity(blackrepresentthehighestintensity);(A)and(B)bandat 293nm;(C)and(D)bandat370nm.

Data wereacquiredoperating theset-upboth in reﬂectance

andﬂuorescenceacquisitionmode,thelatteruponexcitationat

266nm.Datawereprocessedbymeansofanautomaticstatistical

analysis(PCA)aimedtorecognizethepresenceofprominentbands.

Oncemajorﬂuorescencebandshavebeenidentiﬁed,their

assign-menthasbeencheckedagainsttheavailableconsolidantdatabase

bymeansofSAMprojections,andtherelevantdistributiononthe

imagehasbeenobtained.

The band analysis, upon which the monochromatic image

reconstructionreportedinFigs.4and5isbased,derivesdirectly

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Fig.5.(A)FalsecolourRGBreconstructionofasectionofthedomevaultbasedonbandsat293nm,370nmand550nm;(B)averagespectralcontentinselectedspectral regions,whicharemarkedontheleftimage.Notetheintensebandappearsat375nmtentativelyassignedtovinylconsolidants.

Afurtherqualitative resultsofLIF scanningcan beobtained combiningthreesignificantbandsinasinglefalsecolourimages, an example of this elaboration is presented in Fig. 5 (for the monochromaticimageshowninFig.4D).Theinset(A)showthe RGBreconstructionbasedonbandpeakedat550nm,370nmand 293nm;theinset(B)showstheaveragespectruminthe respec-tivecircledareaontheleft,significantemissionsat375nmand at293nm(confirmingPCAresults) areobserved, theirpossible assignmenttodifferentconsolidantsbeingabovediscussed.

TheSAM application gives a preciselocalization of different materials, however this is not true when overlapping contrib-utionsarefoundwithcomparableintensities.Anexampleofthis unfavourablecaseisshowninFig.6:theleftinset(A)showsthe similaritymapreferredtoparaloidB72ﬂuorescencespectrum, cov-eringabout15%oftheexaminedsurface,howeverananalogous percentageisobtainedwhenconsideringthesubstratumspectrum in theright inset(B). The remarkablespacecorrelation among theretrievedmapisanadditionalindicationofthestrong over-lapbetweentheconsideredspectralcontributionwhichcannotbe effectivelydisentangledbytheusedSAMalgorithm.

As a last example of consolidant identification, a vinyl compound,whosefluorescencespectrumisavailableaspure com-pound(Fig.2),hasbeenconsidered.Sincethecompound could notbedepositedonaplastersimilartotheoneutilizedinPadua Baptisteryanditwasnecessarytohaveagoodspectralreference toretrieveareliablespacedistribution,atwo-stepanalysiswas carriedon.First,thesimilarity mapwasproduced withrespect to the purecompound, small areas (about 1%) unambiguously characterizedby itspresence were identified (Fig. 7A).Second, the spectral signature of these areas was assumed as internal

referenceofthecompoundontheusedplasterandtheﬁnal distri-butionwasobtained(Fig.7B),coveringabout11%oftheexamined surface.Althoughthisindirectproceduremightgiveriseto arte-facts,incaseofinitialincorrectassumptionornoisyspectra,and theconﬁrmationoftheassignmentshouldbeachievedbymeans of complementary techniqueswhenever possible,theproposed methodisa validtoolforanimmediatenon-invasive screening andalsotothemosteffectivelocalizationofsuccessivesampling areas.

4.2. Reﬂectancemeasurementsonpigments

AnRGBimageobtainedfromreflectancemeasurementsonthe vaultisshown inFig.8A.Spectral featureshavebeenidentified byPCAThereconstructedreflectanceimagerevealsaninteresting feature under Christ’srighthand: a redarea withhighly satu-rated color, whose spectral analysisshows an alterationin the ratiobetweentheredandbluemaxima(at622nmand477nm, respectively).

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Fig.6. SAMmapsassociatedwith:paraloidB72(A)andplaster(B).

Fig.7. SAMmapsassociatedto:vinylfromourdatabase(A)andavinylwithan internalreference(B).

not clear, since differentcauses might contribute to its occur-rence:retouches withadifferentredpigment,humidityonthe wall, differentbinders or added varnishes withdiffusive inho-mogeneous reﬂectance at the surface. Once again, additional complementarymeasurementswouldbenecessaryforadeﬁnite answer, however, the present measurements indicate where samplesshouldbecollectedandhowmuchtheanomalyextends onthepaintedsurface.

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5. Conclusions

Laboratorymeasurements,aimedtotherealizationofaLIFdata baseforremoteinvestigationoffrescoes,incombinationwiththe developedimageanalysismethodology,havebeenutilizedto dis-cussresultscollectedduringthecampaignofcharacterizationof PaduaBaptisteryfrescoes,asanexampleofmonumentalpainted surface.Presentresultsallowtosummarizemajorachievementsof theproposedinnovativetechnique,asfollows:

• theLIF instrument developed is suitable tonon-invasive and remotesurfacediagnostics;namelyitpermitstoscanlargeareas withmedium/highresolutionandtoreleasedigitalimageswhose false colors canbe associatedwith detectedspectral features observeduponUVexcitationat266nm(e.g.presenceofspeciﬁc consolidant/binder);

• theidentiﬁcationoftheconsolidant/binder,ispossibleprovided that itsLIF spectrum is present in the availabledata baseas detectedona similarcomplexmatrix;alternatively the pres-enceofa classof consolidants(e.g.acrylic,vinyl, etc.)can be assumedanditsdistributioncanbemeasuredontheentireimage byusingasreferenceandinternalstandardwhosespectrumhas beenobtainedinanareaoftheimagewhereitdominates; • theadditionalavailabilityofreﬂectanceimages,measuredupon

externalilluminationorutilizinganadditionalvisiblelampwhile keepingthelaserswitchedoff,bymeansofthesamescanning system,givesinformationonpigmentsdistributionand degra-dation.

Thelimitationsofthediagnostictechniquerelymostlyonthe natureoftheCHtargetconsidered,duetoitscomplexitywitha multilayeredstructureoftencontainingmaterialswithsmall chem-icaldifferences.Majordrawbacksforfrescoesarereportedinthe followinglist:

• difﬁcultiestobuilta reliabledatabase,containingalloriginal materialspreparedaccordingtospeciﬁchistoricalreceiptsand ondocumentedrestorations(thelatterforconsolidantsaddedin moderntimes);

• modest differences in spectral signatures, either related to chemicalsimilarity,ortobroadbandemissioncharacteristicof ﬂuorescenceforsolidsurfaces,stronglyalteredbythe ﬂuores-centsubstrate(e.g.plaster),whichwouldpreventtogetimproved resultsbyhardwareimplementationofhighspectralresolution acquisition.

Duetothementionedlimitations,aquantitativeLIFscanning remoteanalysisseemsnottobefeasiblealsoinperspective. Con-versely,theadvantagesofferedbyfastandsemi-automaticanalysis

capabletorecognizeanomaliesinthedistributionofsurface com-ponents(eitherconsolidants,binders,orevensomepigments)are of interest to theconservator who is aware where to proceed with accurate point analyses utilizing either different spectro-scopic(Raman,LIBS),Spectrometric(SIMS),orimaging(IR,XRF) techniquestohavedeﬁnitematerialidentiﬁcationandrespective quantitativeanalysis,thelattersuitableforrenormalizationofLIF scanningintensitydistributiononremotelycollectedimages.

References

[1]D.Anglos,M.Solomidou,I.Zergioti,V.Zafﬁropulos,T.G.Papazoglou,C.Fotakis, Laser-inducedﬂuorescenceinartworkdiagnostics:anapplicationinpigment analysis,Appl.Spectrosc.50(1996)1331–1334.

[2]D.Lognoli,G.Lamenti,L.Pantani,D.Tirelli,P.Tiano,L.Tomaselli,Detectionand characterizationofbiodeteriogensonstoneculturalheritagesbyﬂuorescence lidar,Appl.Opt.41(2002)1780–1787.

[3]J.Striová,G.Coccolini,S.Micheli,C.Lofrumento,M.Galeotti,A.Cagnini,E. Castellucci,Non-destructiveandnon-invasiveanalysesshedlightonthe real-izationtechniqueofancientpolychromeprints,Spectrochim.ActaA73(2009) 539–545.

[4]G.Cecchi,L.Pantani,V.Raimondi,L.Tomaselli,G.Lamenti,P.Tiano,R.Chiari, Fluorescencelidartechniquefortheremotesensingofstonemonuments,J. Cult.Herit.1(2000)29–36.

[5]D.Comelli,C.D’Andrea,G.Valentini,R.Cubeddu,C.Colombo,L.Toniolo, Fluo-rescencelifetimeimagingandspectroscopyastoolsfornondestructiveanalysis ofworksofart,Appl.Opt.43(2004)2175–2183.

[6]F.Colao,R.Fantoni,L.Fiorani,A.Palucci,Fluorescence-basedportabledevice basedforthespatialscanofsurfaces,inparticularintheareaofCultural Her-itage,ItalianpatentRM2007A000278depositedon21.05.2007.

[7]L.Caneve,F.Colao,L.Fiorani,A.Palucci,Portablelaserradarsystemforremote surfacediagnostics,ItalianpatentRM2010A000606depositedon17.11.2010. [8]I.Borgia,R.Fantoni,C.Flamini,T.DiPalma,A.GiardiniGuidoni,A.Mele,

Lumi-nescencefrompigmentsandresinsforoilpaintingsinducedbylaserexcitation, Appl.Surf.Sc.127(1998)95–100.

[9]F.Colao,R.Fantoni,L.Fiorani,A.Palucci,Applicationofascanninghyperspectral lidarﬂuorosensortofrescodiagnosticsduringtheCULTURE2000campaignin Bucovina,RevistaMonumentelorIstorice/ReviewofHistoricalMonumentsn. LXXV/1-2(Bucharest,2006)53–61.

[10]F.Colao,L.Caneve,A.Palucci,R.Fantoni,L.Fiorani,Scanninghyperspectrallidar ﬂuorosensorforfrescodiagnosticsinlaboratoryandﬁeldcampaigns,in:J.Ruiz, R.Radvan,M.Oujja,M.Castillejo,P.Moreno(Eds.),LasersintheConservation ofArtworks(LACONAVII),CRCPress,2008,pp.149–155.

[11]L.Caneve,F.Colao,R.Fantoni,L.Fornarini,Laserinducedﬂuorescence anal-ysisofacrylicresinsusedinconservationofculturalheritage.Proceedings ofOSAV’2008,The2ndInt.TopicalMeetingonOpticalSensingandArtiﬁcial Vision,St.Petersburg,Russia,2008,pp.57–63.

[12]A.C.Rencher,MethodsofMultivariateAnalysis,WileyInterscience,NewYork, 2002.

[13]G.Girouard,A.Bannari,A.ElHartiandA.Desrochers,ValidatedSpectralAngle MapperAlgorithmforGeologicalMapping:ComparativeStudybetween Quick-birdandLandsat-TM,PresentedattheXXthISPRSCongress,Geo-Imagery BridgingContinents,Istanbul,Turkey,12–23July2004.

[14]C.Miliani,M.Ombelli,A.Morresi,A.Romani,Spectroscopicstudyofacrylic resinsinsolidmatrices,Surf.CoatingsTechnol.151–152(2002)276–280. [15]S.Valadas,A.Candeias,J.Mirao,D.Tavares,J.Coroado,R.Simon,A.S.Silva,M.Gil,