1. Laser safety Outline : Laser effects on tissues (skin and eyes)
The laser safety standards
Precautionary measures
François BALEMBOIS

2. Laser effects on biological tissues
Photochemical effects
Photoablative effects
(breaking of molecular bounds)
Ionization
Absorption
Burns T°
Tearing of tissues
Shock wave

3. Effects of temperature increase
Incident photons T°
Absorbing zone
Cw laser or long pulse laser
(ms-s)
Short pulse lasers (fs-ns)
Tearing
Gas ejection
Heat diffusion
Burn extension
«Recoil» of tissues
(reactionnary force)
Mechanical shock

4. Laser effects on the skin
Photo-sensibilization
Erythema
Cancer risks
Photo-ablation
UV ( nm)
Visible and IR ( nm)
Burns
Coagulation
Carbonization
But the skin can heal up !

5. Cross section of the human eye
Sclerotic
Ciliary muscles
Aqueous humor
Choroid    
Cornea
Retina
Macula or Fovea
Foveola
Pupil
Optic disk
Crystalline lens
Vitreous humor
Iris
Optic nerve
Ciliary body
Pigment layer

6. Vitreous and aqueous absorption
Eye transmission up to the retina
Crystalline lens absorption
Vitreous humor absorption
100 80
Vitreous and aqueous absorption 60
Eye transmission (%) 40 20
400
600
800
1000
1200
1400
Wavelength (nm)

7. Spectral absorption of the different eye media
Cornea
Crystalline lens
Aqueous humor
Vitreous humor
Wavelength (nm)

8. Retina burns and injuries
Laser effects on the eye
Photokeratisis
«Sunburn» of cornea
Burn of cornea
Cornea
Cataract
Cataract +
Cornea burn
Retina burns and injuries
Crystalline lens
Wavelength (nm)

9. Focusing effects on the retina :
Spectral transmission characteristics
Near UV
Far UV and far IR
Visible and near IR
Focusing effects on the retina :
Eretina= 105 Eincident
In W/cm2
Or in J/cm2

10. Retina absorption (visible and near IR)
(400nm<l<1400 nm)
Sclerotic
Light
Cones and rods
Pigmentary epithelium
Absorption by cones and rods : 5%
Absorption by epithelium : 95%
Cones and rods are indirectly destroyed
by heat diffusion or by tearing effects

11. Danger of visible and near IR lasers
Retina image diameter
Irradiance (W/cm2)
HeNe 1mW at 633 nm
20 µm
> 102
Sun
100 µm 10

12. Ti:saphirre laser (790 nm)
Bottom of the eye after a laser accident
Ti:saphirre laser (790 nm)
Foveola completely burned (200 µm diameter, at the center of the fovea)
Half visual acuity lost
Permanent damages

13. Laser safety Outline : Laser effects on tissues (eyes and skin)
The laser safety standards
Precautionary measures
François BALEMBOIS

14. The laser safety standards
(EN or CEI 60825)
The goals :
Set the damage threshold for users
Sort the different lasers versus the risks

15. Permissible maximum exposure (PME)
Corresponding to the damage threshold
- PME generally in W/m2 or in J/m2.
- PME are different following the tissues involved :
Skin PME > Eye PME
- PME : Density averaged over a reference surface.
Skin PME : Reference surface : 1mm diameter
(Energy or power measured through an 1mm diameter aperture)
Eye PME : Reference surface : the cornea
Between 400 et 1400 nm : 7mm diameter aperture
(pupil opened at maximum)
Else : 1mm diameter aperture.

16. (thermal effects in steady state)
Skin PME
Depends on pulse duration and on wavelength
Extract from EN :
Duration (s)
10-9 à
10-7
10-7 à 10
10 à
10 3
103 à
3 104
<10-9
l(nm)
180 à 302,5
30 J.m-2
C2 J.m-2
302,5 à 315
C2 J.m-2
W.m-2
C1 J.m-2
t>T1
t<T1
C1 J.m-2
315 à 400
104 J.m-2
10 W.m-2
400 à 1400
W.m-2
200 J.m-2
1,1 104 t0,25 J.m-2
2000 W.m-2
1400 à 106
1011 W.m-2
200 J.m-2
5600 t0,25 J.m-2
1000 W.m-2
Influence of peak power
Influence of energy
Influence of power
(thermal effects in steady state)

17. Skin PME Influence of the laser wavelength Energy density (J/m2).10320 15
Energy density (J/m2).103 10 5
2000
1500
1000
500
Wavelength (nm)
Curve given for an exposure of 10 s

18. Eye PME Between 400 and 1400 nm : depends on the exposure
Direct exposure or «mirror like» reflection
Indirect exposure :
Diffused reflection
Eye
Secondary source
Glass plate
Eye
Piece of paper
The eye images the secondary source
Laser focused to a tiny spot on the retina
Retina image larger
PME is an energy density
or a power density
PME is a luminance (W/sr/m2)

19. Focusing effect on the retina
Eye PME
Influence of the laser wavelength
Energy density (J/m2)
Wavelength (nm)
Focusing effect on the retina
Curve given for an exposure of 10 s

20. Part of the beam coming in the eye without optics
Laser classes (1/2)
Class 1 : Class 1 lasers are safe E<PME
Class 1 M : Class 1M lasers are safe if no optical instruments are used
Part of the beam coming in the eye without optics
Laser beam
Class 2 : Visible laser ( nm) : safe providing the blink eye response operates (0,25s max exposure).
Order of magnitude : cw output power < 1mW.
Class 2M : Visible laser ( nm) : safe if no optical instruments and if blink response operates

21. Laser classes (2/2) Class 3R : « relatively dangerous lasers »
Visible : 5 times more powerful than Class-2 lasers
Other wavelength : 5 times more powerful than Class-1 lasers
Class 3B : Dangerous lasers by direct exposure or by «mirror like» reflections
Order of magnitude : cw output power < 500 mW.
Class 4 : Dangerous lasers even by diffused reflections
Risk of skin injuries, risk of fire.
cw output power > 500 mW.

22. Laser safety standard problems
Complex biological effects :
- Cumulative and cooperative effects of different wavelengths
- Effect of pulse repetition rate
- Effects of femtosecond pulses
- Long term effects of UV laser (carcinogenic?)
Classe 2 : blink reflex? Not so sure!
(only a few cones involved: nervous signal not high enough to trigger the blink reflex)
Classe 3R : «relatively» dangerous?
OLE Septembre 2001 p13
Flash blindness = Danger for pilots (more restrictive than standard)

23. Eye safe lasers Eye safe zone around 1500 nm
Cornea
Vitreous humor
Aqueous humor
Crystalline lens
Wavelength (nm)
Absorption spread in the cornea
« Skin » regeneration of cornea after exposure

24. Laser safety Outline : Laser effects on tissues (eyes and skin)
The laser safety standards
Precautionary measures
François BALEMBOIS

25. Practical recommandations (1/2)
«Control of the laser beam from the source to the detector»
- All the mirrors must be solidly fixed
- Localization of the laser beam
Be careful with vertical beams
- Locate parasitic reflections and avoid it
(thanks to black plates)
- Avoid diffused reflections
Diffusion of a class 4 laser by a simple metal plate is dangerous
Use of a « laser beam absorber »
Laser
Diffusing object
Tube
Light trap
Laser

26. Practical recommandations (2/2)
« The laser user must be careful »
- Beam alignement always at low power level
- Never put the eyes in the plane of the laser
- Do not wear «reflective objects» (watch, bracelet…)
- Work in a room correctly ligthed
(In order to reduce the eye pupil size)
- Wear safety goggles (Class 3B and 4 - lasers)
Be careful with «team working» on the same laser
(more than one person!)

27. Safety goggles Generally doped glasses or plastics Examples :
Transmission (%)
Wavelength (nm)
Side protections

28. How to choose safety goggles?
- Measure of E (Energy density or power density) at the laser output
- Goggles Transmission * E < PME
- Choice of an optical density < -10 log (PME/E)
Be careful with pulsed lasers!
(Saturation of absorption problems)

29. Information on safety goggles Typical pulse duration
Wavelength, optical density, laser type
Symbol
Laser type
Typical pulse duration D
CW lasers
>0,2 s I
Pulsed lasers
>10-6 to 0,25 s R
Q-switched lasers
>10-9 to 10-6 s M
Mode-locked lasers
<10-9 s