Prof. Dr. Frank Mücklich Dr. Andrés Lasagni Lehrstuhl … · Laser processing of materials Prof. Dr. Frank Mücklich Dr. Andrés Lasagni Lehrstuhl für Funktionswerkstoffe Sommersemester

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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


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


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


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


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


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


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)



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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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


=> Laser Pointers can damage eyes!


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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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.


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


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


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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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%


(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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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



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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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!!!



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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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 and Entryway Controls


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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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




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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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)




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


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

Documents

Prof. Dr. Frank Mücklich Dr. Andrés Lasagni Lehrstuhl … · Laser processing of materials Prof. Dr. Frank Mücklich Dr. Andrés Lasagni Lehrstuhl für Funktionswerkstoffe Sommersemester