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

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

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

Laser effects on the skin Photo-sensibilization Erythema Cancer risks Photo-ablation UV (100-400 nm) Visible and IR (400-106 nm) Burns Coagulation Carbonization But the skin can heal up !

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

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)

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

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)

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

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

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

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

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

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

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.

(thermal effects in steady state) Skin PME Depends on pulse duration and on wavelength Extract from EN 60825 : 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 3 1016 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 2 1011 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)

Skin PME Influence of the laser wavelength Energy density (J/m2).103 20 15 Energy density (J/m2).103 10 5 2000 1500 1000 500 Wavelength (nm) Curve given for an exposure of 10 s

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)

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

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 (400-700 nm) : safe providing the blink eye response operates (0,25s max exposure). Order of magnitude : cw output power < 1mW. Class 2M : Visible laser (400-700 nm) : safe if no optical instruments and if blink response operates

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.

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)

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

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

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

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!)

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

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)

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