Page 52 - Mike's published articles
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Inthiscasethetransmitedlightissimplytheforwardreflectedlight.Thereislitlediferenceinthe spectrumwhetherthetransmitedcomponentisincludedornot,althoughthereisaslightredshiftinthe absorbancecalculatedbytheformermethodandsuchashiftcanonlybeduetoasignificantabsorbance componentafectingthetransmitedlight.Thefiguresforabsorbancesestimatedthiswaymustbein arbitraryunits,sincethespectrometerwilfailtocolectalthescateredlightbysomedegrewhich varieswitheachsample.
IfweconsiderthereflectancespectraoftheblueandgrenpigmentsinFig.7,rememberingthat theserepresentthelightlostbyreflectionandbackscatering,itisclearthatthelosoflightislowand similarforbothpigmentsattherubylaserwavelength.Theunitsshownarexactlyasproducedbythe spectrometer.However,diferencesinsurfacetexturemaychangethesebyaround0.1reflectanceunits. ThusthebehaviourofthesepigmentstothislasershouldbesimilatIfthesepigmentsbehavediferently, itisnot,therefore,duetothespectroscopicharacteristicsofthecompounds,buttosomeotherfactor. Giventhattheionisationpotentialofthesepigmentswilalsobecloselysimilar,theshockwavegenera- tionfolowingmultiphotonionisationisexpectedtobesimilar.Closerinvestigation21ofthetwopigments revealsthattheparticlesizedistributionisdiferent,however.Thegrenpigmentcontainsparticlesupto 20pmdiameterwhiletheblueparticleshaveamaximumdiameterof10lIm.Sincetheblueandgren pigmentspectraresimilarintherubylaserregime,however,thefectoftheskinandthetisue/pigment interfaceasagainsttheair/pigmentinterfacewilbecomontoboth,weatributethediferentbehaviour ofthesetwopigmentstotherubylasertothisdiferenceinparticlesize.Inamateurtatosthepigment
A
Fig.6.Grenpigment,solidline=(1-solid Fig.7.Reflectancespectraoftheblue(solid
phasereflectance)spectrum,dotedline=liq- line)andgren(dotedline)pigmentsrefer- uidphase"absorbance"spectrum,bothinar- encedtoanaluminumiror.Theverticallines bitraryunits. indicatelaserwavelengths,
(a) =Nd:YAG, (b)=Alexandrite, (c)=ruby, (d)=frequencydoubledNd:YAG.
SPIEVol.2623/47
