Page 53 - Mike's published articles

hasbenfoundtobelargelyextra-celularlocatedin"pols"inthemidtodepdermis.Profesional tatowork,however,lodgespigmentbothextra-andintra-celularly,thelaterbeingcontainedinthe cytoplasmofmacrophages.Thisisprobablywhyprofesionaltatosaremoredificulttoremove2,23• Aditionaly,thegrenpigmentisnotapurecompound,but,asdeducedabove,probablycontainsa mixtureofbrominatedcoperphthalocyanineandotherorganicpigments.Unlikethepure,bluecoper phthalocyanine,thismixturewilnotgrindtosuchafinepowder,butwiltendtoforma"paste"andgive thelargeragregatesasobserved.
Likewise,fromfig.7itisclearthatbothalexandriteandrubylaserlightarereflectedsimilarlyby thesepigmentsandthussimilarbehaviourwouldbexpectedfromlowlightintensityarguments.The alexandritelaserisfoundtobelesefective24thantheruby25,however,andthismustthereforebedueto thediferenceinflux(Wcm2).Notonlymustthenergy(J)beconsideredinsuchacomparison,butthe pulsewidthandiradiatedarearealsovitalyimportant,althesebeinginvolvedinthecalculationofthe flux(seabove).Multiphotonprocesesarefluxdrivenotfluencedriven,andsosuchamechanismis mostprobablyoperatinginthetatoremovalprocesaspredictedfromthecalculationsshownabove.
5.DISCUSION
"Absorbance"spectraofpigmentssluriedinliquidhavebenshownottobetrueabsorbance spectrafortypicalcompoundsandfromtheirinsolubilityinesentialyanysolvent.Ithasalsoben shownabovethattheabsorbancesofalpigmentsthroughoutthevisiblespectrumaybeconsideredto bextremelystrongevenatquitelargewavelengthseparationsfromthecentreofthetransitions.These spectrarethereforedisregardedasusefuldiagnosticseitherfortheidentificationofpigmentsassimilar orforpredictingtheirspectroscopicbehaviourinlaserasistedtatoremoval.
Solidstatespectrahavebenobtainedbyasimpledifusereflectancetechniqueandintransmision modeunderamicroscope.Inthelaterexperiments,the"transmited"lightcloselycoresponds, spectroscopicaly,toreflectedlight.Aditionaly,therewerenovisibledirectpathsthroughthesample.
Itisthereforededucedthatthis"transmited"lightpermeatesthesamplebymultiplereflectioninvoids withinthesamplethicknesandisnotlightremainingaftertrueabsorption.Thisisexpectedfromthe estimationoflikelyextinctioncoeficientsforsuchmaterials.
Ourinterpretationofthereflectancexperimentsisthatthesegiveareasonablyacuratemeasureof thelosoflightenergyatthesurfaceofthesolidpigment.Thismeasurewilbevalidbothacrosthe wavelengthrangeandbetwendiferentpigments,althoughinthelatercase,samplepreparationcanlead tominor(<0.1reflectanceunits)ireproducibities.Giventhenormousabsorptioncoeficientsinthe visiblespectrumexpectedforalthesepigmentsfromredthroughtoblueandthattheircolourasper- ceivedbytheyeisdirectlyrelatedtotheirreflectancespectrasweobtainthem,considerationsoftheir colourintermsoftheirrelativeabsorbancearecertainlyirelevant.Ofcourse,reflectanceandabsorbance arerelated,butthextremelyhighabsorbanceofalthesesolidsatparticlesizesrepresentativeoftato practiceimpliesthattheywildiferinthisrespectlessharply,bothasafunctionofwavelengthandin theirdepthofsurfacepenetrationofthelight,thanwouldbexpected.Inalcasesthiswilbextremely smal,i.e.,allightabsorptionofalcoloursofpigmentwilocuresentialywithin<1tmofthesurface. Thisproceswil,atsuficientlyhighflux,leadtomultiphotonabsorption,ionisation,plasmaformation, folowedbyshockwaveproductionandmechanical,presure-wave-inducedbreakdownofthepigment
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