Laser Safety. Safe clinical application of laser radiation. Frits F.M. de Mul ffmdemul(at)gmail.

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1 Lser Sfety Sfe clinicl ppliction of lser rdition Frits F.M. de Mul emil: ffmdemul(t)gmil.com

2 Lser Sfety Sfe clinicl ppliction of lser rdition Contents: 1. Physicl / mthemticl explntion of the clcultion 2. The computer progrm: exmple of clcultions. Progrm options (similr clcultions): 1. Subprogrm of the Hygird -pckge (stnd-lone). 2. Web-ppliction. More informtion nd downlods: ; Avilble softwre....

3 Lser Sfety uses : NOHD: Nominl Oculr Hzrd Distnce NOHD: sfe distnce to void eye (or skin) dmge Stndrd distnce for eye 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. 3

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

5 NOHD: Nominl Oculr Hzrd Distnce lser trget focus Sfety condition t trget: Intensity I MUST be Mximum Permissible Exposure MPE I nd MPE in W/m 2 MPE is tbulted by EU NOHD is minimum distnce where I MPE 5

6 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 Directive 2006/25/EU of Europen Prliment nd Commission IEC NEN

7 MPE: mximum permissible exposure : eye α = ngulr subtense (ngle of lser wist, s seen from trget) α = 1, 10, 100 mrd (0.06, 0.6, 6 0 ) NB. Logrithmic scles 7

8 MPE: mximum permissible exposure : eye α = ngulr subtense (ngle of wist, s seen from trget) α = 1, 10, 100 mrd (0.06, 0.6, 6 0 ) If α smller: brighter lser spot, MPE smller. 8

9 NOHD: Nominl Oculr Hzrd Distnce: Clcultion Sfety condition: I MPE in W/m 2 I = intensity [W/m 2 ] I ~ lser power P [W] I ~ 1/ trget re S [m 2 ] I dependent on type of lser bem: PC = lser Profile Correltion Fctor [-]. I = (P/S). PC = P.PC / S 9

10 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 > 1. 10

11 NOHD: Nominl Oculr Hzrd Distnce Sfety condition: I = intensity t trget P = power PC= prof.corr.fctor S = trget re. lser R focus, width w P PC I. MPE S Effective trget re: θ/2 D trget P. PC S MPE 2 S D, D D w 4 D = effective dimeter S 4S ( D w) 4 D w 2 2 Sfety condition for trget dimeter : D 4S w 4P. PC. MPE w 11

12 NOHD: Nominl Oculr Hzrd Distnce lser R focus, width w θ/2 trget D Effective trget re: 2 S D, D D w 4 Sfety condition for trget dimeter : D 4S w 4P. PC. MPE w NOHD R D 2.tn( / 2) NOHD 1 4P. PC - 2. tn( / 2). MPE w 12

13 NOHD: Nominl Oculr Hzrd Distnce lser R focus, width w θ/2 trget D NOHD depends on: D, or θ P PC 1/MPE w NOHD 1 4P. PC - 2. tn( / 2). MPE w 13

14 NOHD: Nominl Oculr Hzrd Distnce NOHD is lso dependent on α : ngulr subtense ngulr subtense α = ngle to view the source (= lser focus) from the trget position lser w /2 R focus, width w D Focus s seen from the trget 1 2 tn 1 w 2 R 2( w NOHD ) A smller wist will led to shrper (intenser) spot on the trget, thus MPE lower. For certin situtions (e.g. visible light): MPE depends on α (tbulted in EU-Directive), nd thus on NOHD 14

15 NOHD: Nominl Oculr Hzrd Distnce Two reltions between NOHD nd MPE : lser w /2 R focus, width w D 1 tn 2 w 2R 2( So, NOHD =f (α) w NOHD ) lser R focus, width w θ/2 trget D But lso: NOHD = f (MPE) =f (α)! from Sfety Condition: I MPE 1 4P. PC NOHD R - 2.tn( / 2). MPE w Thus: itertive pproch necessry!! 15

16 NOHD: Nominl Oculr Hzrd Distnce Two reltions between NOHD nd MPE : lser θ/2 D focus, width w trget R lser w /2 R focus, width w D (1): NOHD = f (MPE) = f (α) (2): NOHD =f (α) Itertive pproch: ε = tolernce Strt α -vlue Clc NOHD 1 from (1) nd NOHD 2 from (2) NOHD 1 NOHD 2 < ε? yes redy no α α ±Δα 16

17 Trnsmission of sfety goggles lser R focus, width w θ/2 trget D NOHD = R + In cse NOHD > sfety distnce b (from focus): sfety goggles necessry! Trnsmission T of sfety goggles nd Opticl density OD: T b NOHD 2 ; OD 10 log T trget re ~R 2 Stndrd sfety distnce = 0.25 m for 10 sec exposure. 17

18 Lser tretment In generl: pulsed lser tretment will consist of 1 seprte trins of 1 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. 18

19 Hygird-progrm: Exmple of clcultions LASER INPUT DATA VALUE Wvelength 532 nm (full) Divergence ngle rd Profile correltion fctor 2 - Wist(spot)dimeter 400e-6 m Distnce 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 OD-glsses) m 19

20 Hygird-progrm: Exmple of clcultions 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] NOHD [m] T-glsses OD-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). 20

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