Page 47 - Mike's published articles
P. 47
tiesof2overapathlengthof1cmquitereadilyatthewavelengthofmaximumabsorption(i.e.sucha sampleistransmiting1%oftheincidentlight).Asolidofdensity2gmcm3andmolecularweightof30
(typicalofapigment)hasanefectiveconcentrationof2x10130=6.7molar.Thisshouldhavean opticaldensityof2x6.7x10x6x10=80overthe6pmofapigmentparticlediameter.Nospec-
trometercouldetectsuchaweaksignal.Evenanopticaldensityof8isbeyondcomercialyavailable machines.Thus,suchparticlesaresentialy10%absorbingofthelightfailingonthemoveravery widerangeofwavelengthsprovidingthatitisnotreflectedorscatered.Moreover,thelightwilbetotaly absorbedveryneartothesurface.Thishasimportantimplicationsforthemechanismtobediscused later.Acordingly,wehaveresortedtodifusereflectancespectroscopywhichisasimpletechnique measuringacombinationofscateredandreflectedlighti.e.measuringthelosoflightavailableforthe absorptionproces.Thistechniquealowsreadycomparisonofthespectroscopicbehaviourofdiferent laserfrequenciesacrosthespectrum.Forthepurposesofcomparisonwithliquidphaseabsorptionspec- traonlywepresentsomeofthesereflectancespectraintheform(1-reflectance).
ThegeneralcharacteristicsofpulsedlasersusedintatoremovalareshowninTable1.Hereitis clearthat,whilethemaximumenergydensities(fluences)fortheselasersdonotvarygreatly,thefluxcan varybyfiveordersofmagnitude.Phenomenarelatedtothefluxwilthusvarygreatlywithlasertype.
TABLE1.GeneralCharacteristicsofLasersUsedforTatoRemoval Nd:YAG Alexandrite Ruby Dye*
Wavelength
1064nm 75nm 694nm 585nm 8 J cm2 8 J cm2 12 J cm2 8 J cm2
Energydensity(max)
Typicalbeamdiam. 3m 3m 4m 5m Beamcrossection/cm2 0.07 0.07 0.126 0.196 Pulsewidth 10ns 10ns 25ns 450ps Flux/photonscm2s4 6x1028 4x1027 1.3x1028 2.7x1023
*insertedhereforcomparison.Thesehavebensugestedfortatoremovalinthepast,butarenot usedmuchforthispurposecurently.
Fig.1.TheintensityprofileoftheDerma-Laserrubylaser. ThiscloselyaproximatestotheTEM0profile.Thiswas obtainedwithaBigSkybeamViewAnalyser,modelBVA1O1 withaCohusolidstatecameraNoER502.Gausianfit coeficients0.9horizontaland0.93vertical,diameters2.86 mhorizontaland2.79verticalat1/epoints
421SPIEVol.2623
