Laser Safety Calculations Tom Lister. Laser Safety Measurements– 28 th Nov 2014 Tom Lister NOTES...
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Laser Safety Calculations Tom Lister. Laser Safety Measurements– 28 th Nov 2014 Tom Lister NOTES The following three calculations were given as ‘homework’
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
Slide 3
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
Slide 4
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.
Slide 5
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
Slide 6
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
Slide 7
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)
Slide 8
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
Slide 9
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
Slide 10
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
Slide 11
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
Slide 12
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 ?
?
Slide 13
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
Slide 14
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
Slide 15
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
Slide 16
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
Slide 17
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
Slide 18
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
Slide 19
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
Slide 20
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
Slide 21
Laser Safety Measurements 28 th Nov 2014 Tom Lister Divergence
Cone angle (16)? Divergence 8 (140 mrad)?
Slide 22
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 ?
Slide 23
Laser Safety Measurements 28 th Nov 2014 Tom Lister Thank
you
Slide 24
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
Slide 25
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
Slide 26
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
Slide 27
Laser Safety Measurements 28 th Nov 2014 Tom Lister BS EN
50825-1:2014 Page 69: x = 100 mm
Slide 28
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