prof. dr. frank mücklich dr. andrés lasagni lehrstuhl … · laser processing of materials prof....
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Functional MaterialsFunctional Materials Saarland UniversitySaarland University
Laser processing of materials
Prof. Dr. Frank MücklichDr. Andrés Lasagni
Lehrstuhl für Funktionswerkstoffe Sommersemester 2007
Laser safety
Functional MaterialsFunctional Materials Saarland UniversitySaarland University
LASER Safety
Contents:• Laser-tissue interaction• Type of interaction
– Thermal interaction– Thermo-acoustic Interaction– Photochemical Interaction
• Absorption of radiation by the organism– Absorption of laser light by the skin and eye
• Effect of ultraviolet radiation• Effect of Infra-Red radiation• Wavelength bands as relevant for photobiology• Laser Exposure Limits – Terms
– Maximum permissible exposure (MPE)– Nominal hazard zone (NHZ)
• Laser Safety Classes• Subdivision of Potential Hazards
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Laser radiation affects that kind of tissue, which absorbs the radiation. The absorption of laser radiation in tissue, especially in ocular tissue, is strongly wavelength dependent. The type of interaction depends on the wavelength and on the interaction duration.
Laser-tissue interaction
Lambert-Beer Law: I(z) = I0 . e-γ.z
γ [cm-1] = Absorption coefficient
Reflectiondiffus direct
Dispersion Absorption
Transmission
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Type of interaction
Depending on the interaction duration and peak irradiance values, each interaction type can be assigned a general domain:
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High power densities in small volumes
strong local heating
The most frequent damages are:
•Skin turning red to burns
•cooking and evaporation
Thermal interaction
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Thermo-acoustic Interaction
Explosion-like evaporation mechanism(Popcorn-Effect) in i.e. veins and arteries
Formation of Pressure-waves
veins and arteries are broken into pieces,particles are ejected
painful, partially to strongly bleeding injuries
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Photochemical Interaction
• Chemical properties are changed
A B
• Biological functions are destroyed
A + C Biological Function 1B + C ≠ Biological Function 1
Example: UV-Radiation: Skin Cancer
h ν
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Laser-tissue interaction (examples)
Examples:
• When the temperature of the tissue is increased above a critical temperature, proteins are denaturised and thermal damage occurs.
• If temperatures above 100 °C are induced, water in the tissue begins to boil and further temperature increases lead to a carbonisation of the tissue.
• In the ultraviolet and blue end of the visible spectrum, photochemical damage can occur, as photon energies are sufficiently high to cause direct damage to macromolecules of cells such as to the DNA.
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Abs
orpt
ion
coef
ficie
nt α
[µm
-1]
Penetration depth d [µm
]
Water
Water
Tm:Y
AG
Ho:Y
AG
Er:Y
SSG
Er:Y
AG
Nd:Y
AG
Diod
en
HeNe
Ar-Io
nen
ArF-
Excim
erXe
Cl-E
xcim
er
CO2
Wavelength λ [µm]
Absorption of radiation by the organism
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Absorption of laser light by the skin and eye
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300 700500 1000 1400 1800 2200Wavelength λ [nm]
(From Seiler, Lasertechnik in der Medizin)
100
50
0
Ref
lect
ance
[%]
bright
dark
Absorption of radiation by the skin
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Absorption of radiation by the skin
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Absorption of radiation by the eye
RPE (retinal pigment epithelium ): in VIS range absorbs practically all of the incident optical radiation =>
very little power is needed to produce large temperature rises
Cross section through the retina
Laser Light
5 µm For near IR wavelengths => radiation is partially transmitted through the RPE and is absorbed in the choroid(absorption volume is much larger and also for long term exposure, the blood support reduces the temperature rise)
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Absorption of radiation by the eye
In contrast to light from conventional sources to which the retina is regularly exposed, if laser radiation is imaged onto the retina, the diameter of the irradiated spot on the retina is as small as 10 - 20 µm!
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Absorption of radiation by the eye
6P
2
6
3
PN 103I104107II ⋅⋅=
⋅⋅
⋅= −
−
Example: λ = 550 nm; f = 17,05 mm; D = 7 mm
db = 4 µm
It means, that if a Energy density lp penetrates the pupil, the Energy density IN at the retina is given by:
Df44,2db
⋅⋅=λ
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=> Laser Pointers can damage eyes!
Absorption of radiation by the eye
Green laser pointers commonly soldin stores and on the Internet nowconclusively have been shown to cause eye damage, Mayo Clinicresearchers announced in May 2005.
(about 240.000 situations in Internet!)
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Absorption of radiation by the eye
With larger energies, holes in the retina are produced which result either in bleeding injuries
Injuries induced with a Nd:YAG laser on a monkey retina.
white spots: thermal burns => coagulation of retinal layers.
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Effect of ultraviolet radiation
In the entire UV spectral region (100 - 380 nm) the biological effect of the radiation is cumulative.
For the evaluation of the exposure one must calculate therefore the TIME-INTEGRAL (30,000 s = 1 working day) of the irradiancy.
UV-A (315 - 380 nm):The Penetration depth into the skin is some millimeters.Biological effects:
Pigmentation of the skin (max. at 380 nm, Threshold value: 10 J/cm²)Formation of cataract
UV-A (200 - 315 nm): Photokeratitis: A burn of the cornea (the clear front surface of the eye)
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Effect of Infra-Red radiation
The damaging effect of the infrared radiation is practically thermal.
Near IR (IR-A, 780 - 1400 nm):
• penetrates up to the retina•Biological effects: Formation of cataract
Middle IR (IR-B, 1400 - 3000 nm) & Large IR (IR-C, 3 µm - 1 mm):
• high water absorption, retina cannot be achieved
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radiation absorbed in uppermost cell layers of eye and skin3000 nm - 1 mmIR-C
radiation absorbed in volume of the eye1400 nm - 3000 nmIR-B
radiation focussed onto the retina, but not visible; deep penetration into the skin
700 nm - 1400 nmIR-A*
penetrates deep into eye and skin; possible damage to the lens
315 nm - 400 nmUV-A*
intermediate absorption depth; highly effective in producing photokeratoconjunktivitis and sunburn
280 nm - 315 nmUV-B
absorbed in uppermost cell layers of eye and skin; highly effective in producing photokeratoconjunktivitis ; germicidal. Radiation with wavelengths smaller than about 180 nm - 200 nm are heavily absorbed by the oxygen of the air and is also termed "vacuum ultraviolet". Vacuum UV usually need not be considered for hazard evaluation.
100 nm - 280 nmUV-C
Tissue InteractionWavelengthRange
CIE Shorthand
Wavelength bands as relevant for photobiology
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Wavelength bands as relevant for photobiology
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Laser Exposure Limits - Terms
Nominal hazard zone (NHZ)Area within the MPE is equalled or exceeded
Nominal Ocular Hazard distance (NOHD)Distance along the axis of the direct laser beam to the human eye beyond which the MPE is not equalled or exceeded
Maximum permissible exposure (MPE)The highest laser energy to which the eye or skin can be exposed for a given laser
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Maximum permissible exposure (MPE)
1) Optical and thermal properties of the skin and the eye are different => MPE for the eye and the skin differ => MPEskin , MPEeye!(especially in the retinal hazard wavelength region)
2) The MPE values are specified in units of J m-2 and W m-2
3) MPE values depend on the exposure duration (for longer exposure durations, the maximum safe exposure level generally is smaller than for shorter exposure durations)
4) MPE values depend on the laser wavelength
5) The ocular MPE is defined at the position of the cornea, i.e. the focussing properties of the eye and the pupil size are accounted for in the derivation of the MPEs in the retinal hazard region.
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Maximum permissible exposure (MPE)
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Maximum permissible exposure (MPE)
How are the MPE values calculated?
Exposure dose at which 50 % of the exposures lead to a lesion is called "Effective Dose 50%" or ED-50 (for a given laser wavelength, pulse duration and spot size)
A typical dose-response curve as obtained in threshold experiments (here for a 850 nm laser, 180 ns pulse duration, minimal retinal spot size, beam diameter = 8mm)
MPE
MPE < 10%
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(a) Specular Reflection (b) Diffuse Reflection
Nominal hazard zone (NHZ)
NHZ: The space within which the level of direct, scattered or reflected laserradiation exceeds the MPE
The NHZ must be calculated in each case differently
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Nominal hazard zone (NHZ)
NHZ – Calculations:
= 0.1 W/cm²
From the tableNHZ
The location where the irradiance or the exposure per pulse equals the maximum permissible exposure (MPE) defines the border of the Nominal Hazard Zone
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Nominal hazard zone (NHZ)
MPE
Case 2. NHZ calculation for a collimated beam which is focussed by a lens of focal length f.
Nd:Yag Laser
λ = 1064 nm
MPE = 17.10-6 W/cm²
d = 10 mm
P = 0.2 W
f = 500 mmr = 61 mts
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Nominal hazard zone (NHZ)
MPE
Case 1. NHZ calculation for a divergent beam, under the assumption of a linear divergence (far field approximation; Θ is the full angle divergence).
Nd:Yag Laser
λ = 1064 nm
MPE = 17.10-6 W/cm²
d = 10 mm
P = 2 W
Θ = 0.5 mrad
r = 24270 mts = 24 Km!!!
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Nominal hazard zone (NHZ)
MPE
Case 3. NHZ calculation for diffuse reflection from a rough surface
Nd:Yag Laser
λ = 1064 nm
MPE = 17.10-6 W/cm²
ρ= 80 % (Pt at λ=1064nm )
P = 0.2 W
ε = 30° r = 0.5 mts
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Nominal hazard zone (NHZ)
MPE
Case 4. NHZ for a fibre with half divergence angle β
Nd:Yag Laser
λ = 1064 nm
MPE = 17.10-6 W/cm²
P = 0.2 W
β = 10°
r = 2.9 mts
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Nominal hazard zone (NHZ)
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Nominal hazard zone (NHZ)
Nominal Hazard Zone and Entryway Controls
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Laser Safety Classes
As MPE evaluations and the determination of hazard areas are quite complicated and involved, a laser safety classification scheme has been developed by international standardisation committees according to which laser products are grouped into classes with similar hazard potentials
Laser Safety ClassesLegislation:IEC 60825-1 (International ElectrotechnicalCommission)EN 60825-1 (European standardisation organisation) BS EN 60825-1 (British Standard)DIN EN 60825-1 (Deutsches Institut für Normung)
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Laser Safety Classes
No hazard area for the naked eye, but hazard area for the use of optical instruments (extended NHZ)
MPEs are not exceeded for the naked eye, even for long exposure durations, but maybe exceeded with the use of optical instruments
Safe for the naked eye, potentially hazardous when optical instruments are used
Very low power lasers; either collimated with large beam diameter or highly divergent
Class 1M
No hazard area (NHZ)
MPEs are not exceeded, even for long exposure duration (either 100 s or 30000 s), even with the use of optical instruments
Safe
Very low power lasers or encapsulated lasers
Class 1
(CD-ROM players)
Hazard AreaRelationship to MPEMeaningType of lasersClass
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Laser Safety Classes
Hazard AreaRelationship to MPEMeaningType of lasersClass
No hazard area for the naked eye when based on accidental exposure (0.25 s exposure duration), but hazard area for the use of optical instruments (extended NHZ)
MPE for 0.25 s not exceeded for the naked eye, but maybe exceeded with the use of optical instruments
Same as Class 2, but potentially hazardous when optical instruments are used
Visible low power lasers; either collimated with large beam diameter or highly divergent
Class 2M
No hazard area when based on unintended exposure (0.25 s exposure duration)
Blink reflex limits exposure duration to nominally 0.25 s. MPE for 0.25 s not exceeded, even with the use of optical instruments.
Safe for unintended exposure, prolonged staring should be avoided
Visible lowpower lasers
Class 2(Super-market scanners)
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Laser Safety Classes
Hazard AreaRelationship to MPEMeaningType of lasersClass
Hazard area for the eye (NOHA), no hazard area for the skin
Ocular MPE with naked eye and optical instruments may be exceeded more than 5 times. Skin MPE usually not exceeded.
Hazardous when eye is exposed. Usually no hazard to the skin. Diffuse reflectionsusually safe
Medium powerlasers
Class 3B(research)
5 times the limit of Class 1 in UV and IR, and 5 times the limit for Class 2 in visible, i.e. 5 mW
MPE with naked eye and optical instruments may be exceeded up to 5 times
Safe when handled carefully. Only small hazard potential for accidental exposure
Lowpowerlasers
Class 3R(Laser pointers)
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Laser Safety Classes
Hazard AreaRelationship to MPEMeaningType of lasersClass
Hazard area for the eye and skin, hazard area for diffuse reflections
Ocular and skin MPE exceeded, diffuse reflections exceed ocular MPE
Hazardous to eye and skin, also diffuse reflection may be hazardousFire hazard
High powerlasers
Class 4(research)
Accessible Emission Limit (AEL): maximum value of accessible laser radiation that an individual may be exposed to during the operation of a laser.
No limit4
500 mW3B
5 times the limit of Class 1 in UV and IR, and 5 times the limit for Class 2 in visible, i.e. 5 mW3R
Same as Class 2, distinction with measurement requirements2M
1 mW2
Same as Class 1, distinction with measurement requirements1M
40 µW for blue1
Typical AEL for cw lasersLaser Class
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Subdivision of Potential Hazards
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Priority 1:• Eliminating or minimizing dangers through constructive measures• Example: covering dangerous areas (danger of being crushed, struck,
etc.)
Priority 2:• Implementing necessary safety measures for dangers which cannot
be eliminated• Example: optical sensors for securing moving machine parts
Priority 3:• Informing users about remaining dangers which cannot be avoided by
constructional or safety measures• Example: a note in the operating instructions about wearing gloves as
a means of protection against sharp edges or hot workpieces
Safety goals
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Commercial Examples
Closed safety cabin
Optical sensors