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Table2isshowninFig.1.Withoutsuchcontrol,comparisonsofefectsareunreliable.FromTable2,it semsthattherubylaser(694nm-red)isbestfortheremovalofgrentatos,whereasthefrequency doubledNd:YAG(532nm-gren)isbestforredink.However,thisexperiencecontradictstheobserva- tionsofthefirststudy'7ofTable2.Inthisstudy,itwasfoundthatgrentatoswerethemostdificultof alcolourstoremovewiththerubylaser.Someredtatoswereremovedaseficientlyase.g.blue,while otherspresentedificulties.Thislatersetofobservationscontradictsthesimplestexpectationsbasedon considerationsoflowlightintensityabsorptionspectroscopywhicharethatagrenpigmentshould absorbredlightandaredpigmentshouldabsorbgrenlight.Itmustberemembered,ofcourse,thatfor pigmentmixturesmetamericolourmatchingisposiblewhentwocoloursapearthesametotheye, butdifermarkedlyinspectralcontent(e.g.yelow+bluecanapearthesameasanapropriatemixture ofgren+red).Unfortunately,therecanbenoguarantesthattatoistswilusesingle,purepigmentsto obtainagiven"red"or"gren".Clearly,thereisnedforsomesimplespectroscopytorevealthepres- enceofsuchmixturesandtogiveanindicationofthespectroscopicabsorbanceofagivenpigment.
Thepurposeofthepresentpaperistodemonstratesuchasimple,reliablespectroscopyandtouseit toexaminethemechanismoflaserinducedtatoremovalbasedontheaboveclinicalresults.
3.MATERIALSANDMETHODS
3.1. Spectroscopy
particlessonprecipitate,althoughthefinerparticlesresistcentrifuging,takingdaystofinalysetleout. Thepigmentsthemselvesareusualyextremelybrightandareoftendilutedwithbariumsulphate,calcium
Forthepurposeofaplyingatato,pigmentparticlesofaveragediameter6tmaretakenupin liquids,typicalyproprietarymouthwash.Wehavefoundthatthisisnotasolution,butisbeterdescribed asaslury.Indedthesepigmentsaresentialyinsolubleinanyliquid,poiarornon-polar.Thelarger
sulphateortitaniumdioxidebeforeuseintatos.Inbothcases,itisclearthataplyingstandard
spectroscopictechniquestotheseinhomogeneous,time-varyingsamplespresentsgreatdificultiesof interpretationandsimpleargumentsbasedontheresultsofsuchstudieswilbeunreliable.
Theliquidphase"absorption"spectrawereobtainedonaPhilipsPU8720UV/visiblespectrometer usinga1cmpathlengthcelandfreshlycentrifugedpreparationsofpigmentinmouthwash.Thesolid statereflectancespectrawereobtainedwithaSpectratechdifusereflectanceatachmentinBomemDA3 Fouriertransformspectrometerandreferencedtofilterpaper(WhatmanN°1)whichwasfoundtohavea flatresponseoverthespectroscopicrangeofinterest.Thesolidfilmwassuportedbythesamefilter paperandidentical,highlyreproducible,spectrawereobtainedirespectiveofthethicknesoftheaplied film.Solidstatetransmisionspectrawereobtainedfrompigmentfilmssandwichedbetwentwoglas microscopeslidesinanIRPlanmicroscopeatachedtotheBomemDA3instrument.Siteswereselected wherethesolidfilmwasesentialycontinuousandthefilmthickneswasmeasuredtobeabout10pm. Again,identicalspectrawereobtainedirespectiveofthesitechoseninagivenfilm.Therewasinsufi- cientsignalstrengthforthisarangementtobeusedtobtainthereflectancespectra.However,theper- centageoflightreflectedinbothsystemsislikelytobeverysimilar,giventheindependenceofthe
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