Laser processing of materials. Laser safety

Transcription

1 Laser processing of materials Laser safety Prof. Dr. Frank Mücklich Dr. Andrés Lasagni Lehrstuhl für Funktionswerkstoffe Sommersemester 2007 Contents: LASER Safety 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

2 Laser-tissue interaction 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. Reflection diffus direct Dispersion Absorption Lambert-Beer Law: I(z) = I 0. e -γ.z Transmission γ [cm -1 ] = Absorption coefficient Type of interaction Depending on the interaction duration and peak irradiance values, each interaction type can be assigned a general domain:

3 Thermal interaction High power densities in small volumes strong local heating The most frequent damages are: Skin turning red to burns cooking and evaporation 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

4 Photochemical Interaction Chemical properties are changed h ν A B Biological functions are destroyed A + C Biological Function 1 B + C Biological Function 1 Example: UV-Radiation: Skin Cancer Examples: Laser-tissue interaction (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.

5 Absorption of radiation by the organism ArF-Excimer XeCl-Excimer Ar-Ionen HeNe Dioden Nd:YAG Ho:YAG Tm:YAG Er:YSSG Er:YAG CO 2 Absorption coefficient α [µm -1 ] Water Penetration depth d [µm] Water Wavelength λ [µm] Absorption of laser light by the skin and eye

6 Absorption of radiation by the skin 100 bright Reflectance [%] 50 dark Wavelength λ [nm] (From Seiler, Lasertechnik in der Medizin) Absorption of radiation by the skin

7 Absorption of radiation by the eye Laser Light 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 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) Cross section through the retina 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 µm!

8 Absorption of radiation by the eye Example: λ = 550 nm; f = 17,05 mm; D = 7 mm d b = λ f 2,44 D d b = 4 µm It means, that if a Energy density l p penetrates the pupil, the Energy density I N at the retina is given by: N = IP = I 6 P 3 10 I 4 10 Absorption of radiation by the eye => Laser Pointers can damage eyes! Green laser pointers commonly sold in stores and on the Internet now conclusively have been shown to cause eye damage, Mayo Clinic researchers announced in May (about situations in Internet!)

9 Absorption of radiation by the eye white spots: thermal burns => coagulation of retinal layers. 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. Effect of ultraviolet radiation In the entire UV spectral region ( 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 ( 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 ( nm): Photokeratitis: A burn of the cornea (the clear front surface of the eye)

10 Effect of Infra-Red radiation The damaging effect of the infrared radiation is practically thermal. Near IR (IR-A, nm): penetrates up to the retina Biological effects: Formation of cataract Middle IR (IR-B, nm) & Large IR (IR-C, 3 µm - 1 mm): high water absorption, retina cannot be achieved Wavelength bands as relevant for photobiology CIE Shorthand UV-C UV-B UV-A* IR-A* IR-B IR-C Wavelength Range 100 nm nm 280 nm nm 315 nm nm 700 nm nm 1400 nm nm 3000 nm - 1 mm Tissue Interaction absorbed in uppermost cell layers of eye and skin; highly effective in producing photokeratoconjunktivitis ; germicidal. Radiation with wavelengths smaller than about 180 nm 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. intermediate absorption depth; highly effective in producing photokeratoconjunktivitis and sunburn penetrates deep into eye and skin; possible damage to the lens radiation focussed onto the retina, but not visible; deep penetration into the skin radiation absorbed in volume of the eye radiation absorbed in uppermost cell layers of eye and skin

11 Wavelength bands as relevant for photobiology Laser Exposure Limits - Terms Maximum permissible exposure (MPE) The highest laser energy to which the eye or skin can be exposed for a given laser 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

12 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 => MPE skin, MPE eye! (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. Maximum permissible exposure (MPE)

13 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) MPE < 10% 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 Nominal hazard zone (NHZ) NHZ: The space within which the level of direct, scattered or reflected laser radiation exceeds the MPE (a) Specular Reflection (b) Diffuse Reflection The NHZ must be calculated in each case differently

14 NHZ Calculations: Nominal hazard zone (NHZ) NHZ From the table = 0.1 W/cm² The location where the irradiance or the exposure per pulse equals the maximum permissible exposure (MPE) defines the border of the Nominal Hazard Zone 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 = W/cm² d = 10 mm P = 0.2 W f = 500 mm r = 61 mts

15 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 = W/cm² d = 10 mm P = 2 W Θ = 0.5 mrad r = mts = 24 Km!!! Nominal hazard zone (NHZ) MPE Case 3. NHZ calculation for diffuse reflection from a rough surface Nd:Yag Laser λ = 1064 nm MPE = W/cm² ρ= 80 % (Pt at λ=1064nm ) P = 0.2 W ε = 30 r = 0.5 mts

16 Nominal hazard zone (NHZ) MPE Case 4. NHZ for a fibre with half divergence angle β Nd:Yag Laser λ = 1064 nm MPE = W/cm² P = 0.2 W β = 10 r = 2.9 mts Nominal hazard zone (NHZ)

17 Nominal hazard zone (NHZ) 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 Classes Legislation: IEC (International Electrotechnical Commission) EN (European standardisation organisation) BS EN (British Standard) DIN EN (Deutsches Institut für Normung)

18 Laser Safety Classes Class Type of lasers Meaning Relationship to MPE Hazard Area Class 1 (CD-ROM players) Very low power lasers or encapsulated lasers Safe MPEs are not exceeded, even for long exposure duration (either 100 s or s), even with the use of optical instruments No hazard area (NHZ) Class 1M Very low power lasers; either collimated with large beam diameter or highly divergent Safe for the naked eye, potentially hazardous when optical instruments are used MPEs are not exceeded for the naked eye, even for long exposure durations, but maybe exceeded with the use of optical instruments No hazard area for the naked eye, but hazard area for the use of optical instruments (extended NHZ) Laser Safety Classes Class Type of lasers Meaning Relationship to MPE Hazard Area Class 2 (Supermarket scanners) Visible low power lasers Safe for unintended exposure, prolonged staring should be avoided Blink reflex limits exposure duration to nominally 0.25 s. MPE for 0.25 s not exceeded, even with the use of optical instruments. No hazard area when based on unintended exposure (0.25 s exposure duration) Class 2M Visible low power lasers; either collimated with large beam diameter or highly divergent Same as Class 2, but potentially hazardous when optical instruments are used MPE for 0.25 s not exceeded for the naked eye, but maybe exceeded with the use of optical instruments 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)

19 Laser Safety Classes Class Type of lasers Meaning Relationship to MPE Hazard Area Class 3R (Laser pointers) Low power lasers Safe when handled carefully. Only small hazard potential for accidental exposure MPE with naked eye and optical instruments may be exceeded up to 5 times 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 Class 3B (research) Medium power lasers Hazardous when eye is exposed. Usually no hazard to the skin. Diffuse reflections usually safe Ocular MPE with naked eye and optical instruments may be exceeded more than 5 times. Skin MPE usually not exceeded. Hazard area for the eye (NOHA), no hazard area for the skin Laser Safety Classes Class Class 4 (research) Type of lasers High power lasers Meaning Hazardous to eye and skin, also diffuse reflection may be hazardous Fire hazard Relationship to MPE Ocular and skin MPE exceeded, diffuse reflections exceed ocular MPE Hazard Area Hazard area for the eye and skin, hazard area for diffuse reflections Accessible Emission Limit (AEL): maximum value of accessible laser radiation that an individual may be exposed to during the operation of a laser. Laser Class 1 1M 2 2M 3R 3B 4 Typical AEL for cw lasers 40 µw for blue Same as Class 1, distinction with measurement requirements 1 mw Same as Class 2, distinction with measurement requirements 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 500 mw No limit

20 Subdivision of Potential Hazards Safety goals 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

21 Commercial Examples Closed safety cabin Optical sensors