Laser Safety: Safe clinical application of laser radiation

Transcription

1 Lser Sfety: Sfe clinicl ppliction of lser rdition Frits F.M. de Mul MEPHYS Softwre & Services emil: info(t)medphys.nl

2 NOH: Nominl Oculr Hzrd istnce NOH: sfe distnce to void eye dmge Stndrd distnce for sfety: 0.25 m for 10 s exposure time. If lser intensity [W/m 2 ] t the stndrd distnce is too high: protection glsses compulsory. 2

3 mge of excessive lser exposure to eye nd skin eye skin Wvelength Photochemicl Retin Therml Erythem formtion Therml < 400 nm >

4 NOH: Nominl Oculr Hzrd istnce lser trget focus Sfety condition t trget: Intensity I Mximum Permissible Exposure MPE I nd MPE in W/m 2 NOH is minimum distnce where I MPE 4

5 MPE: mximum permissible exposure MPE is function of: Wvelength Pulse durtion (or continuous) Pulse frequency Angulr subtense (viewing ngle of light source from trget) MPE-formule re tbulted in irective 2006/25/EU of Europen Prliment nd Commission IEC NEN

6 MPE: mximum permissible exposure : eye α = ngulr subtense 6

7 MPE: mximum permissible exposure : eye α = ngulr subtense 7

8 NOH: Nominl Oculr Hzrd istnce Sfety condition: I MPE in W/m 2 With I = P.PC / S I = intensity [W/m 2 ] P = lser power [W] PC = lser Profile Correltion Fctor [-] S = trget re [m 2 ] 8

9 PC: Profile Correltion Fctor PC depends on lser mode structure Cylindricl trnsverse modes Rectngulr trnsverse modes Locl intensity in modes is lrger thn verged over the mode envelope. Profile Correltion Fctor PC. 9

10 NOH: Nominl Oculr Hzrd istnce lser R focus, width w θ/2 trget Effective trget re: S 2, 4 w = effective dimeter 4S 2 w 2 Sfety condition: I P PC. S MPE S P. PC MPE 4S w 4P. PC. MPE w 10

11 NOH: Nominl Oculr Hzrd istnce lser R θ/2 Effective trget re: 2 S, w 4 focus, width w trget Sfety condition: 4S w 4P. PC. MPE w NOH R 2.tn( / 2) 1 2.tn( / 2) 4P. PC. MPE - w 11

12 NOH: Nominl Oculr Hzrd istnce lser R focus, width w θ/2 trget NOH depends on: 1/θ P PC 1/MPE -w NOH R 2.tn( / 2) 1 2.tn( / 2) 4P. PC. MPE - w 12

13 NOH: Nominl Oculr Hzrd istnce Focus s seen from the trget lser NOH is lso dependent on w /2 R focus, width w α : ngulr subtense ngulr subtense = ngle to view the source (= focus) from the trget position 1 2 tn 1 w 2 R 2( w NOH ) A smller wist will led to shrper (intenser) spot on the trget For certin situtions (e.g. visible light): MPE depends on α (tbulted in EU-irective), nd thus on NOH 13

14 NOH: Nominl Oculr Hzrd istnce lser lser Two reltions between NOH nd MPE : w /2 R focus, width w R focus, width w θ/2 trget 1 tn 2 w 2R 2( So, NOH =f (α) But lso: 1 4P. PC NOH R - 2.tn( / 2). MPE w NOH w ) NOH = f (MPE) =f (α)! From Sfety Condition: I MPE Thus: itertive pproch necessry!! 14

15 NOH: Nominl Oculr Hzrd istnce Two reltions between NOH nd MPE : lser w /2 R focus, width w (1): NOH =f (α) lser R θ/2 focus, width w trget (2): NOH = f (MPE) = f (α) Itertive pproch: Strt α -vlue Clc NOH 1 from (1) nd NOH 2 from (2) NOH 1 NOH 2 < ε? yes redy no α α ±Δα 15

16 Trnsmission of sfety goggles lser R θ/2 NOH = R + focus, width w trget In cse NOH > sfety distnce b (from focus): sfety goggles necessry! Trnsmission T of sfety goggles nd Opticl density O: T b NOH 2 ; O 10 logt trget re ~ R 2 Stndrd sfety distnce = 0.25 m for 10 sec exposure. 16

17 Lser tretment In generl: pulsed lser tretment will consist of seprte trins of seprte pulses I I I I I I I I I I I I I I I I I I I I I Trin 1 Trin 2 Trin 3 Lser pulses Totl tretment Time Continuous lser: 1 long trin of 1 long pulse. 17

18 Exmple LASER INPUT ATA VALUE Wvelength 532 nm (full) ivergence ngle rd Profile correltion fctor 2 - Wist(spot)dimeter 400e-6 m istnce lser to focus(wist) 0.3 m Bsic pulse: pulse power 1.000E+06 W...: pulse energy 5.000E-03 J...: durtion 5.000E-09 s...: rep.freq Hz Trin of pulses: power 5.000E+00 W...: energy 5.000E+01 J...: durtion 1.000E+01 s...: rep.freq Hz...: pulse t end of trin (yes/no=1/0) : nr. of pulses per trin Totl tretment: power 5.000E+00 W...: energy 5.000E+01 J...: durtion 1.000E+01 s...: nr. of trins in totl 1 - Sfe distnce from focus (for O-glsses) m 18

19 Exmple OUTPUT of clcultions: For this wvelength the dmge is: EYE: photochemicl nd retin (therml) nd therml SKIN: therml RESULTS: (H in J/m^2 - E=MPE in W/m^2) CALCULATION formul MPE [W/m^2] lph [rd] NOH [m] T-glsses O-glss eye: pulse H=5E-3.CE 1.000E E E id. with CF1.. N= 10000, CF= 1.000E E E E id. with CF2.. N= 10000, CF= 1.000E E E E eye: trin H=18.CE.t^ E E E eye: totl H=18.CE.t^ E E E skin: pulse H=200.CA 4.000E E E id. with CF1.. N= 10000, CF= 1.000E E E E id. with CF2.. N= 10000, CF= 1.000E E E E skin: trin H=1.1E4.CA.t^ E E E skin: totl H=1.1E4.CA.t^ E E E CF = correction fctor, needed for therml dmge when >1 pulse in Tmin. (Tmin is 1.8E-5 s for 532 nm). 19

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