Laser safety

1 Lasers, light and safety 1 1.1 Lasers: stimulating light 1 1.1.1 Creating light 1 1.1.2 Quantifying light 10 1.2 The properties of laser radiation 13 1.3 The safety of laser technology 15 1.4 Safety standards 17 References 19 2 Quantifying levels of laser radiation 21 2.1 Power and energy 21 2.2 Irradiance and radiant exposure 27 2.2.1 Terminology 29 2.2.2 Averaging over area—limiting aperture 30 2.3 Angle and intensity 32 2.3.1 Plane angle 33 2.3.2 Solid angle 34 2.3.3 Radiant intensity 35 2.4 Field-of-view—angle of acceptance 36 2.4.1 Terminology and optical set-up 37 2.5 Radiance 40 2.5.1 Averaging over the FOV 43 2.5.2 Transforming radiance to irradiance 44 2.5.3 Actual measurement FOV—simplification for small sources 45 2.6 Wavelength issues 46 2.6.1 Wavelength bands 46 2.6.2 Visible radiation 46 2.6.3 Spectral quantities 48 2.6.4 Action spectra 49 2.6.5 Photometric quantities and units 51 2.7 Absorption, reflection and scattering 522.7.1 Absorption law 54 2.7.2 Volume scattering 54 2.7.3 Diffuse reflection—surface scattering 56 2.8 Measurement instruments and detectors 56 2.8.1 Parameters and uncertainty 57 2.8.2 Types of radiometers 62 References 65 3 Laser radiation hazards 66 3.1 Introduction 66 3.2 The human skin 67 3.3 The human eye 68 3.4 The concept of exposure limits (MPE) 74 3.4.1 Exposures above the MPE 78 3.5 Laser–tissue interaction 79 3.5.1 General optical absorption characteristics 80 3.5.2 Types of interaction 80 3.6 MPE evaluation and measurement concept 87 3.6.1 Limiting aperture and angle of acceptance 87 3.6.2 Exposure location and exposure duration 90 3.6.3 Representation of MPE values 93 3.6.4 Summary and overview of dependencies 96 3.6.5 Evaluation and measurement position 99 3.6.6 Background to the concept of dosimetry 102 3.7 Injury to the skin 103 3.7.1 Aversion response, typical exposure durations 106 3.8 Skin MPE values 107 3.9 Injury to the eye 113 3.9.1 Ultraviolet radiation 115 3.9.2 Retinal damage 116 3.9.3 Corneal damage from infrared radiation 119 3.9.4 Aversion response and typical exposure durations 120 3.10 MPE values for the eye—also relevant to AEL values 122 3.11 MPE values in the ultraviolet 123 3.11.1 Multiple pulses 126 3.11.2 Ultrashort pulses 131 3.12 Retinal MPE values 132 3.12.1 Apparent source 135 3.12.2 General evaluation approach 149 3.12.3 Retinal thermal—wavelength dependence 151 3.12.4 Retinal thermal—time dependence 156 3.12.5 Retinal thermal—dependence on ل 161 3.12.6 Retinal photochemical 186 3.12.7 Comparison of thermal and photochemical retinal limits 197 Contents ix 3.12.8 Multiple pulses in the retinal hazard region 200 3.13 MPE values in the far-infrared 214 3.13.1 Multiple pulse exposures 217 3.14 Multiple wavelength exposures 218 References 220 4 Laser product classification 222 4.1 Overview 225 4.1.1 Diffuse versus intrabeam (direct) viewing 227 4.1.2 Viewing duration 228 4.1.3 Naked (unaided) eye versus exposure with optical viewing instruments 229 4.1.4 Tabular overview 229 4.1.5 Manufacturing requirements 230 4.2 Classification scheme 231 4.2.1 Derivation of the AEL values 234 4.2.2 Time base 236 4.2.3 Measurement requirements 239 4.2.4 Classification scheme summary 247 4.2.5 Embedded laser products 247 4.2.6 Old Class 3A and USA Class IIIa 249 4.2.7 Overview table 250 4.3 Manufacturer’s classification procedure 254 4.3.1 Introduction 254 4.3.2 General issues 254 4.3.3 Single fault condition 259 4.3.4 Measurement requirements 261 4.3.5 Measurement requirements for extended sources 267 4.3.6 Equivalence to MPE evaluation 271 4.4 Requirements for the manufacturer 272 4.4.1 General hardware 272 4.4.2 Labels 279 4.4.3 Informational requirements 283 4.5 US requirements 283 4.5.1 Registering laser products in the US 283 4.5.2 Changes to CDRH requirements 286 4.6 Enclosure and classification 287 4.6.1 Embedded Class 1 laser products—nice but not necessary! 287 4.6.2 Requirements for laser guards IEC 60825-4 289 4.7 Application specific requirements 291 4.7.1 Laser processing machines (ISO 11553 and EN 12626) 292 4.7.2 Medical laser products, IEC 60601-2-22 294 4.7.3 Optical telecommunications 296 4.7.4 Laser light shows 298 x Contents 4.8 Case studies 300 4.8.1 HeNe alignment laser 300 4.8.2 Low-level therapy laser 300 4.8.3 Line laser 302 4.8.4 Scanner 307 4.8.5 Near-IR and visible beam 312 References 313 5 Beam propagation and exposure assessment 314 5.1 Measurement versus calculation 314 5.2 Classification apertures 318 5.3 Beam profiles 321 5.3.1 Gaussian beams 321 5.3.2 Beam divergence 323 5.3.3 Fractional power through apertures 327 5.3.4 Emission from optical fibres 329 5.3.5 Non-Gaussian beams 331 5.4 Hazard distance 334 5.5 Beam reflections 339 5.6 Optical viewing instruments 342 5.6.1 Aided viewing 342 5.6.2 Binocular viewing 343 5.6.3 Close-up viewing 347 5.6.4 Magnified viewing of extended sources 351 5.7 Assessment accuracy 351 References 352 6 Additional laser hazards 353 6.1 Other hazards of laser operation 353 6.2 Additional beam hazards 354 6.2.1 Dazzle 354 6.2.2 Beam-initiated fire and explosion 355 6.2.3 Other thermal hazards 355 6.2.4 Fume 356 6.2.5 Additional laser emission 358 6.3 Non-beam hazards 358 6.3.1 Electricity 358 6.3.2 Non-beam fire and explosion hazards 359 6.3.3 Collateral radiation 359 6.3.4 Hazardous substances 359 6.3.5 Laser-generated noise 360 6.3.6 Mechanical hazards 361 6.3.7 Temperature and humidity 361 6.3.8 External shock and vibration 362 6.3.9 Computer malfunction 362 Contents xi 6.3.10 Ambient noise 362 6.3.11 Compressed gases 362 References 363 7 Assessment of laser risk 364 7.1 Workplace evaluation 364 7.1.1 The laser class 364 7.1.2 Does ‘safe’ mean Class 1? Does Class 1 mean ‘safe’? 370 7.1.3 Supplier and purchaser responsibilities 371 7.2 Risk assessment 374 7.2.1 Hazards and risks 374 7.2.2 The risk assessment process 375 7.2.3 Risk factors 377 7.2.4 Determining the level of risk 379 References 381 8 Protective measures and safety controls 382 8.1 Introduction to protective control measures 382 8.1.1 The use of safety control measures 382 8.1.2 Control measures as a function of the laser class 383 8.2 Laser controlled areas 386 8.2.1 Types of laser controlled areas 386 8.2.2 Controlling access 389 8.2.3 Use of warning signs for laser controlled areas 389 8.3 Engineering control measures 390 8.3.1 Class-dependent safety features 390 8.3.2 Additional engineering control measures 392 8.4 Administrative control measures 398 8.4.1 The use of product safety features 399 8.4.2 Other procedural control measures 400 8.5 Personal protection 401 8.5.1 Personal protective equipment 401 8.5.2 Types of protection 402 8.6 Eye protection 403 8.6.1 The use of protective eyewear 403 8.6.2 Specifying eye protection 406 8.6.3 European standards for laser protective eyewear 415 8.7 Working in laser controlled areas 420 8.8 Laser servicing 421 References 423 xii Contents 9 The management of laser safety 424 9.1 Health and safety responsibilities 424 9.2 The framework policy 425 9.3 The role of the laser safety officer 427 9.4 Safety training 430 9.5 Human factors 433 Appendix A Glossary 435 Appendix B Special parameters 444 Appendix C Common misunderstandings 447 Appendix D Some MPE and AEL values 452