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Laser Safety Calculations Tom Lister

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Laser Safety Measurements 28 th Nov 2014 Tom Lister NOTES The following three calculations were given as homework for delegates of the IPEM LPA Update in Bath, 28 th Nov 2014 and worked through as an exercise during the afternoon. The answers given are not necessarily correct. There was a consensus agreement that the consideration of pulsed sources and < min are appropriate conservative approaches for these particular calculations

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Laser Safety Measurements 28 th Nov 2014 Tom Lister Calculation 1 Alexandrite epilatory laser = 755 nm Aperture = 15 mm (focal spot 12 mm at 40 mm) Beam half angle = 2.7 Max Energy Density = 35 Jcm -2 Pulse Duration = 20 ms Operating frequency = 1.5 Hz

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Laser Safety Measurements 28 th Nov 2014 Tom Lister MPE AOR Directive In carrying out the assessment, measurement or calculation, the employer must follow the [...] standards of the IEC 60825-1:2014 The MPE levels represent the maximum level to which the eye or skin can be exposed without consequential injury immediately or after a long time and are related to the wavelength of the laser radiation, the pulse duration or exposure duration, the tissue at risk and, for visible and near infra-red laser radiation in the range 400 nm to 1 400 nm, the size of the retinal image.

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Laser Safety Measurements 28 th Nov 2014 Tom Lister Maximum Permissible Exposure 755 nm, 20 ms pulse Thermal effects on the retina 2.7 x? 12 mm 40 mm

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Laser Safety Measurements 28 th Nov 2014 Tom Lister C 6 =1? 60825-1 page 27 Most laser sources have an angular subtense less than min, and appear as an apparent point source (small source) when viewed from within the beam (intra-beam viewing). If we assume < min, C 6 =1 Table A.1 in 60825-1 MPE = 18t 0.75 C 4 Exposure time, t = 20 ms t 0.75 = 5.310 -2 C 4 = 10 0.002(755-700) = 1.3 MPE = 1.24 Jm -2

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Laser Safety Measurements 28 th Nov 2014 Tom Lister NOHD 60825-1:2014 distance from the output aperture beyond which the beam irradiance or radiant exposure remains below the appropriate corneal maximum permissible exposure (MPE)

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Laser Safety Measurements 28 th Nov 2014 Tom Lister NOHD C 6 =1 Maximum energy output 35 Jcm -2 over a 12 mm diameter spot 35(0.6 2 ) = 39.6 J Spot size required 39.6 J / 1.24 Jm -2 = 32 m 2 Radius = (32/) = 3.2 m

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Laser Safety Measurements 28 th Nov 2014 Tom Lister NOHD C 6 =1 Distance required 3.2 m /4710 -3 = 68 m Manufacturer: NOHD = 100 m 3.2 m ? 47 mrad 68 m

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Laser Safety Measurements 28 th Nov 2014 Tom Lister C 6 1? 60825-1 p13: The angular subtense of the laser source should not be confused with the divergence of the beam. The angular subtense of the laser source cannot be larger than the divergence of the beam but it is usually smaller than the divergence of the beam. 100 mm 12 mm 40 mm 100 mm smallest spot size 17 mm 2.7

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Laser Safety Measurements 28 th Nov 2014 Tom Lister C 6 1? 60825-1 p13: The angular subtense of the laser source should not be confused with the divergence of the beam. The angular subtense of the laser source cannot be larger than the divergence of the beam but it is usually smaller than the divergence of the beam. If we assume = beam divergence, C 6 1 C 6 = / min = 47/1.5 = 31 MPE = 18t 0.75 C 4 C 6 (Table A.2) MPE = 1.24 x 31 MPE = 39.0 Jm -2

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Laser Safety Measurements 28 th Nov 2014 Tom Lister NOHD C 6 1 Spot size required 39.6 / 39.0 = 1.0 m 2 Radius = (1.0/) = 0.57 m Distance required 0.57 m /4710 -3 = 12 m 0.57 m 47 mrad 12 m ? ?

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Laser Safety Measurements 28 th Nov 2014 Tom Lister Calculation 2 EVLT laser = 1470 nm Aperture = 400 m Numerical Aperture = 0.37 Max Power = 12 W (15%) Pulse Duration = 100-9900 ms Pulse Interval = 100-1000 ms Max pulse rate = 5 Hz

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Laser Safety Measurements 28 th Nov 2014 Tom Lister Maximum Permissible Exposure 1470 nm, >100 ms pulse length Thermal effects on the cornea If we assume < min, C 6 =1 MPE = 5600t 0.25 Exposure time, t = 100 ms t 0.25 = 0.56 MPE = 3150 Jm -2 Exposure time, t = 9900 ms t 0.25 = 1.8 MPE = 9933 Jm -2

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Laser Safety Measurements 28 th Nov 2014 Tom Lister NOHD 9900 ms Maximum energy output 12 W over 10 s; 9900 ms on, 100 ms off Approx. 108 J Spot size required 108 / 9933 = 1.0910 -2 m 2 Radius = (1.0910 -2 /) = 5.9 cm

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Laser Safety Measurements 28 th Nov 2014 Tom Lister Beam Divergence NA = 0.37 NA = nsin Refractive index of fibre = 1.62 (Wikipedia) = sin -1 (NA/n) = 230 mrad

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Laser Safety Measurements 28 th Nov 2014 Tom Lister NOHD 9900 ms Distance required 0.059 m /0.23 rad = 26 cm 5.9 cm ? 230 mrad 26 cm

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Laser Safety Measurements 28 th Nov 2014 Tom Lister NOHD 100 ms Maximum energy output 12 W over 10 s; 100 ms on, 100 ms off Approx 6 J Spot size required 6 / 3150 = 1.9010 -3 m 2 Radius = (1.9010 -3 /) = 2.5 cm Distance required 0.025 m /0.23 = 10.8 cm

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Laser Safety Measurements 28 th Nov 2014 Tom Lister Calculation 3 Ophthalmic YAG Laser = 1064 nm Aperture = 50 mm (8 m spot size) Focal length 107 mm Cone Angle = 16 Max Energy = 10 mJ Pulse Duration = 4 ns Single pulse

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Laser Safety Measurements 28 th Nov 2014 Tom Lister Maximum Permissible Exposure 1064 nm, 4 ns pulse length Thermal effects on the retina (any pulse length) Assume < min, C 6 =1 MPE = 210 -2 C 7 C 7 = 1 MPE = 20 mJm -2

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Laser Safety Measurements 28 th Nov 2014 Tom Lister Divergence Cone angle (16)? Divergence 8 (140 mrad)?

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Laser Safety Measurements 28 th Nov 2014 Tom Lister NOHD Spot size required 20 mJm -2 / 10 mJ = 2 m 2 Radius = (2 /) = 80 cm Distance required 0.80 /0.14 = 5.7 m Effect of ionisation? 80 cm 140 mrad ? 5.7 m ?

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Laser Safety Measurements 28 th Nov 2014 Tom Lister Thank you

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Laser Safety Measurements 28 th Nov 2014 Tom Lister Beam diameter d 63 definition? If d 86 is 12 mm, then d 63 = 8.5 mm

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Laser Safety Measurements 28 th Nov 2014 Tom Lister LaserBee NOHD Assume full power of 12 W for different times. Set power profile to continuous and enter 12 W. This gives the default 100 s NOHD of 25.8 cm. If you change the exposure time to 100 ms then the NOHD is 47.0 cm

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Laser Safety Measurements 28 th Nov 2014 Tom Lister LaserBee NOHD Assume that the cone angle is the required full angle d63 divergence source diameter of 8 m (we are measuring the NOHD from the focal spot). Enter a pulse energy of 10 mJ and lookup the NOHD from the tab to get 2.84 m. Increasing the energy by 20% to allow for fluctuations makes the NOHD 3.1 m

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Laser Safety Measurements 28 th Nov 2014 Tom Lister BS EN 50825-1:2014 Page 69: x = 100 mm

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Laser Safety Measurements 28 th Nov 2014 Tom Lister Maximum Exposure Assume 7 mm pupil 50825-1 p90: The pupil is a variable aperture but the diameter may be as large as 7 mm when maximally dilated in the young eye. 18.8 Jcm -2 x (0.35 2 ) = 7.25 J