Lasers and Laser Safety

Lasers and Laser Safety A short course taught as part of module PY3108 Department of Physics 1

Contents Basics of Lasers and Laser Light Laser Beam Injuries Laser Hazard Classes Laser Safety Standards Laser Control Measures and Procedures Calculations and Estimates Quantifying Laser Safety Department of Physics Laser Safety 2

Basics of Lasers and Laser Light L ight A mplification by S timulated E mission of R adiation Department of Physics Laser Safety 3

Wave nature of light l Wavelength Blue: l = 400 nm Red: l = 700 nm Light is an electromagnetic wave. Different wavelengths in the visible spectrum are seen by the eye as different colors. Department of Physics Laser Safety 4

Electromagnetic Spectrum Visible Gamma Ray X-ray Ultraviolet Infrared Microwaves Radio Radio Short Wavelength Long Wavelength Lasers operate in the ultraviolet, visible, and infrared. Department of Physics Laser Safety 5

Stimulated Emission Incident Photon Excited Atom Incident Photon Stimulated Photon same wavelength same direction in phase Laser-Professionals.com Department of Physics Laser Safety 6

Laser Light - Characteristics MONOCHROMATIC DIRECTIONAL COHERENT The combination of these three properties makes laser light focus 100 times better than ordinary light Department of Physics Laser Safety 7

Monochromatic one colour Department of Physics Laser Safety 8

Directional Department of Physics Laser Safety 9

Coherent Department of Physics Laser Safety 10

Laser Components Optical Resonator Active Medium Output Beam High Reflectance Mirror (HR) Excitation Mechanism Output Coupler Mirror (OC) Department of Physics Laser Safety 11

Helium-Neon Gas Laser Department of Physics Laser Safety 12

Neodymium YAG Laser Rear Mirror Adjustment Knobs Safety Shutter Polarizer Assembly (optional) Coolant Beam Tube Adjustment Knob Output Mirror Q-switch (optional) Nd:YAG Laser Rod Flashlamps Pump Cavity Laser Cavity Harmonic Generator (optional) Beam Beam Tube Courtesy of Los Alamos National Laboratory Laser-Professionals.com Department of Physics Laser Safety 13

Feedback amplifier speaker microphone Physics, Photonics & Ethics November 11, 2009 Laser Safety

Introduction to Laser Physics Mirror with Reflectivity = r Start with one unit of optical power 1 (1-r)rg 2 rg 2 r 2 g 2 Gain or Amplification Region g rg (1-r)g Mirror with Reflectivity = r End with r 2 g 2 units of optical power after one round trip. 3 possibilities : r 2 g 2 < 1 optical field decays r 2 g 2 = 1 stable optical field (so called lasing condition) r 2 g 2 > 1 optical field increases Physics, Photonics & Ethics November 11, 2009 Laser Safety

Laser Spectrum Gamma Rays X-Rays Ultra- Visible Infrared Micro- Radar TV Radio violet waves waves waves waves 10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 1 10 10 2 Wavelength (m) LASERS Retinal Hazard Region Ultraviolet Visible Near Infrared Far Infrared ArF 193 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 10600 KrF 248 XeCl 308 Ar 488/515 HeNe Ruby 633 694 2w Alexandrite Nd:YAG 755 532 GaAs 905 Nd:YAG 1064 Wavelength (nm) Communication Diode 1550 CO 2 10600 Laser-Professionals.com Department of Physics Laser Safety 16

Laser Beam Injuries High power lasers can cause skin burns. Lasers can cause severe eye injuries resulting in permanent vision loss. Department of Physics Laser Safety 17

Skin Burn from CO 2 Laser Accidental exposure to partial reflection of 2000 W CO 2 laser beam from metal surface during cutting Department of Physics Laser Safety 18

Types of Laser Eye Exposure LASER INTRABEAM VIEWING EYE SPECULAR REFLECTION REFLECTED BEAM LASER MIRROR DIFFUSE REFLECTION LASER SCATTERED LIGHT ROUGH SURFACE Department of Physics Laser Safety 19

Human Eye Choroid Sclera Retina Aqueous Lens Macula Cornea Optic Nerve Vitreous Department of Physics Laser Safety 20

The Retinal Hazard Region 400-1400 nm Affects the retina Department of Physics Laser Safety 21

Ultraviolet Light UV-C (100-280nm) Cornea surface UV-A (315-400nm) Affects the lens UV-B (280-315nm) Absorbed by the cornea Department of Physics Laser Safety 22

Infra-red Region Near IR (< 1400nm) Affects the retina Far IR Affects cornea and aqueous humor Department of Physics Laser Safety 23

25 m THERMAL BURNS ON PRIMATE RETINA Department of Physics Laser Safety 24 Laser-Professionals.com Photo courtesy of U S Air Force

Multiple Pulse Retinal Injury Department of Physics Laser Safety 25

Q-Switched Laser Injury Retinal Injury produced by four pulses from a Nd:YAG laser range finder. Department of Physics Laser Safety 26

Causes of Laser Accidents Studies of laser accidents have shown that there are usually several contributing factors. The following are common causes of laser injuries: Inadequate training of laser personnel Alignment performed without adequate procedures Failure to block beams or stray reflections Failure to wear eye protection in hazardous situations Failure to follow approved standard operating procedures or safe work practices Department of Physics Laser Safety 27

Laser Hazard Classes Lasers are classified according to the level of laser radiation that is accessible during normal operation. Department of Physics Laser Safety 28

Important terminology Maximum Permissible Exposure (MPE) Accessible Emission Limits (AEL) Nominal Ocular Hazard Distance (NOHD) Department of Physics Laser Safety 29

Class 1 MPE less than 7 microwatts of visible radiation. A Class 1 laser is considered safe under reasonably foreseeable conditions of operation and they present no hazard to the eye or skin. This Class also includes laser systems which cannot emit hazardous levels of laser radiation because of the inherent design of the product. Note that there may be a more hazardous laser system embedded in the product and additional precautions would be necessary if the system is opened up. Department of Physics Laser Safety 30

Class 1M Exceed accessible emission limits for Class 1 but, because of the geometrical spread of the emitted radiation, the laser does not cause harmful levels of exposure to the unaided eye. Safe limit can be exceeded, and injury can occur, if magnifying viewing instruments are used, eg binoculars, telescopes, magnifying glasses, microscopes. Department of Physics Laser Safety 31

Class 2 MPE less than 1 mw of visible radiation. A Class 2 laser or laser system must only emit a visible laser beam. Momentary viewing of a Class 2 laser beam is not considered hazardous since the upper limit for this type of device is less than the maximum permissible exposure (MPE) for momentary exposures of 0.25 second or less - the so-called blink reflex. Intentional extended viewing is considered hazardous. Department of Physics Laser Safety 32

Class 2M Laser products which exceed the permitted accessible limits for Class 2 but, because of the geometric spread of the emitted radiation, protection of the unaided eye is normally afforded by natural aversion responses to bright light. The use of magnifying viewing instruments can give rise to higher levels of exposure and injury can occur. These instruments include: binoculars; telescopes; magnifying lenses; and microscopes. Department of Physics Laser Safety 33

Class 3R Laser products having a level of accessible emission up to five times the limits for Class 1 (if invisible) or Class 2 (if visible). The maximum permissible exposure may be exceeded but the risk of injury is low. Department of Physics Laser Safety 34

Class 3B Less than 500 mw of visible radiation note that Class 3B also covers pulsed lasers and invisible radiation. Reference should be made to the MPE limits contained in the British Standard in order to verify classification. Direct intra-beam viewing is always hazardous. Viewing diffuse reflections from a distance is normally safe provided the exposure duration is less than 10 seconds. Department of Physics Laser Safety 35

Class 4 Over 500 mw visible radiation note that Class 4 also covers pulsed lasers and invisible radiation. Reference should be made to the MPE limits contained in the British Standard in order to verify classification. These lasers are very powerful and may also be a fire hazard. Exposure of the skin to the beam may cause injury. Even diffuse reflections are hazardous. Very stringent control measures are required. Laser Safety

Some Examples & Numbers Department of Physics Laser Safety 37

Engineering controls Engineering controls remove the dependence to follow rigorous procedures and the possibility of failure of personal protective equipment. Engineering controls all the safety features built into the design of the laser and its associated equipment. Examples of Engineering Controls include: key control; remote interlock; beam enclosures; protective filter installations. Department of Physics Laser Safety 38

Administrative controls Administrative and Procedural Controls are intended to supplement Engineering Controls to ensure that laser personnel are fully protected from laser hazards. Administrative/Procedural Controls include: information, instruction and training; signage; protocols arrangements for maintenance; arrangements for servicing. Department of Physics Laser Safety 39

Signage Class 1 Class 2 Class 2M Class 3B Class 4 CLASS 1 LASER PRODUCT LASER RADIATION DO NOT STARE INTO BEAM CLASS 2 LASER PRODUCT LASER RADIATION DO NOT STARE INTO BEAM OR VIEW DIRECTLY WITH OPTICAL INSTRUMENTS CLASS 2M LASER PRODUCT LASER RADIATION AVOID EXPOSURE TO BEAM CLASS 3B LASER PRODUCT LASER RADIATION AVOID EYE OR SKIN EXPOSURE TO DIRECT OR SCATTERED RADIATION CLASS 4 LASER PRODUCT Department of Physics Laser Safety 40

International Laser Warning Labels INVISIBLE LASER RADIATION AVOID EYE OR SKIN EXPOSURE TO DIRECT OR SCATTERED RADIATION CLASS 4 LASER PRODUCT WAVELENGTH MAX LASER POWER EN60825-1 1998 10,600 nm 200 W Symbol and Border: Black Background: Yellow Legend and Border: Black Background: Yellow Department of Physics Laser Safety 41

Personal Protective Equipment Personal protective equipment should only be used when the above measures do not provide sufficient control. Personal protective equipment includes: protective eyewear; and protective clothing. Department of Physics Laser Safety 42

Laser eye protection Selection of eyewear should be based on: wavelength(s) being used; radiant exposure; maximum permissible exposure (MPE); optical density of eyewear; visible light transmission requirements; adequate peripheral vision; prescription lenses; and, comfort. Department of Physics Laser Safety 43

Optical Density A log l 10 Example 1: I I A l out in I I out in Example 2: A 3 l I I 10 out in 1 1,000,000 log 10 I out 1 1,000,000 I in 6 A l I 10 3 in 1000 Department of Physics Laser Safety 44

Eyewear Labels All eyewear must be labeled with wavelength and optical density. Guidelines in EN207/8 (Handout) Department of Physics Laser Safety 45

Optical Density Example: Plastic Eyewear Characteristics 10 9 8 Orange 2w Nd:YAG filter 7 6 5 Green Nd:YAG filter 4 3 2 1 0 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 Wavelength (mm) Department of Physics Laser Safety 46

Who has the primary responsibility for laser safety any time a CLASS 3 or CLASS 4 laser is operated? The person operating the laser always has the primary responsibility for all hazards associated with laser use. Department of Physics Laser Safety 47

Suggested Format for Standard Operating Procedures (SOP) 1. Introduction Description of laser Type and wavelength; Intended application & Location Average power or energy per pulse Pulse duration and repetition rate for pulsed lasers 2. Hazards List all hazards associated with laser Eye and skin hazards from direct and diffuse exposures Electrical hazards Laser generated air contaminants Other recognized hazards 3. Control Measures List control measures for each hazard Include the following: Eyewear requirement, include wavelength and OD Description of controlled area and entry controls Reference to equipment manual Alignment procedures (or guidelines) 4. Authorized Personnel 5. Emergency Procedures Department of Physics Laser Safety 48

Safe Beam Alignment Most beam injuries occur during alignment. Only trained personnel may align class 3 or class 4 lasers (NO EXCEPTIONS!) Laser safety eyewear is required for class 3 and class 4 beam alignment. ANSI REQUIRES approved, written alignment procedures for ALL class 4 laser alignment activities and recommends them for class 3. Department of Physics Laser Safety 49

1. Exclude unnecessary personnel from the laser area during alignment. 2. Where possible, use low-power visible lasers for path simulation of high power visible or invisible lasers. 3. Wear protective eyewear during alignment. Use special alignment eyewear when circumstances permit their use. 4. When aligning invisible beams, use beam display devices such as image converter viewers or phosphor cards to locate beams. Alignment Guidelines for Class 3 and 4 lasers 5. Perform alignment tasks using high-power lasers at the lowest possible power level. 6. Use a shutter or beam block to block high-power beams at their source except when actually needed during the alignment process. 7. Use a laser rated beam block to terminate high-power beams downstream of the optics being aligned. 8. Use beam blocks and/or laser protective barriers in conditions where alignment beams could stray into areas with uninvolved personnel. 9. Place beam blocks behind optics to terminate beams that might miss mirrors during alignment. 10. Locate and block all stray reflections before proceeding to the next optical component or section. 11. Be sure all beams and reflections are properly terminated before high-power operation. 12. Alignments should be done only by those who have received laser safety training. Department of Physics Laser Safety 50

Safe Work Practices Never intentionally look directly into a laser. Do not stare at the light from any laser. Allow yourself to blink if the light is too bright. Never view a Class 3 (or any higher power) laser with optical instruments. Never direct the beam toward people. Operate lasers only in the area designed for their use and be certain that the beam is terminated at the end of its use path. Never allow a laser beam to escape its designated area of use. Position the laser so that it is well above or below eye level. Only block the beam with appropriate (enclosing or diffuse reflecting) beam blocks/shutters. Always enclose laser beams as much as possible with appropriate materials. Remove all unnecessary reflective objects from the area near the beam s path. This may include items of jewelery and tools. Do not enter a designated Class 3b or Class 4 laser area (posted with a DANGER sign) without approval from a qualified laser operator. Eye protection is required in these areas. Always wear laser safety eyewear in designated areas. Department of Physics Laser Safety 51

Keys to a successful laser safety program Active involvement and support of laser users Effective oversight by local (Departmental ) Laser Safety Officers Support from management at all levels Department of Physics Laser Safety 52

Safety Calculations Maximum Permissible Exposure (MPE) Wavelength Exposure CW or Pulsed And ANSI Standards Accessible Emission Limits (AEL) Area of limiting aperture MPE Nominal Ocular Hazard Distance (NOHD) [also referred to a Nominal Hazard Zone (NHZ)] Department of Physics Laser Safety 53

What is the duration time - t? What is the duration time? a. Is it intentional viewing? Duration time = exposure time. b. Is it accidental exposure? i. Visible (400-700 nm), ocular Duration time = 0.25 s ii. <400 nm, ocular Duration time = 30000 s iii. >700 nm, ocular Duration time = 100 s iv. Visible or invisible, skin Duration time = 10 to 100 s MPE: l=10.6 mm? CO 2 laser (IR) Department of Physics Laser Safety 54

MPE: l=10.6mm 100 sec 10.6mm 10,600nm 1000Wm 2 l=10.6mm? Laser Safety 55

What is the duration time - t? What is the duration time? a. Is it intentional viewing? Duration time = exposure time. b. Is it accidental exposure? i. Visible (400-700 nm), ocular Duration time = 0.25 s ii. <400 nm, ocular Duration time = 30000 s iii. >700 nm, ocular Duration time = 100 s iv. Visible or invisible, skin Duration time = 10 to 100 s MPE: l=980 nm? Yb:crystal laser (NIR) Department of Physics Laser Safety 56

MPE: l=980nm CW 100 sec l=980nm What is C 4? Department of Physics Laser Safety 57

MPE: l=980nm page 36 Laser guidance notes C 4 10 10 980700/500 280 0.56 3.63 10 /500 t 100s MPE 0.75 2 18C t 2066Jm 21Wm 4 2 Department of Physics Laser Safety 58

How about laser pointers? What is the duration time? a. Is it intentional viewing? NO Duration time = exposure time. b. Is it accidental exposure? YES i. Visible (400-700 nm), ocular YES Duration time = 0.25 s t=0.25s ii. <400 nm, ocular Duration time = 30000 s iii. >700 nm, ocular Duration time = 100 s iv. Visible or invisible, skin Duration time = 10 to 100 s Department of Physics Laser Safety 59

How about laser pointers? MPE 6.36Jm 2 25.5Wm 2 Department of Physics Laser Safety 60

Accessible Emission Limits (AEL) Area of limiting aperture MPE AEL MPE A LA Index LA = limiting aperture Department of Physics Laser Safety 61

10.6mm Laser MPE 1000Wm 2 AEL 2 1000Wm r 2 0.001m 1000 0.78mW 2 2 Department of Physics Laser Safety 62

980nm Laser MPE 21Wm 2 AEL 2 21Wm r 2 21 0. 007m 2 2 0. 8mW Department of Physics Laser Safety 63

Laser Pointers MPE 25.5Wm 2 AEL 2 25. 5Wm r 2 25. 5 0. 007m 2 2 1mW Department of Physics Laser Safety 64

Beam Type Department of Physics Laser Safety 65

Beam Type Department of Physics Laser Safety 66

Beam Type Department of Physics Laser Safety 67

Angular Measures Hint: look up steradian Department of Physics Laser Safety 68

The Steradian A graphical representation of 1 steradian. The sphere has radius r, and in this case the area of the patch on the surface is A = r 2. The solid angle is W = A/r 2 so in this case W = 1. The entire sphere has a solid angle of 4π sr 12.56637 sr. http://en.wikipedia.org/wiki/steradian Department of Physics Laser Safety 69

Analog to Radians L r A r 2 W r L r A http://en.wikipedia.org/wiki/steradian W steradians Department of Physics Laser Safety 70

Steradian What is the area? r A For small angles: A 2 2 0 4 0 r r 2 2 sin 2 2 sin Department of Physics Laser Safety 71 d 4 d 2 2r cos r 1 cos 2 0 2 2 2 4r 2 4 r 2 Large angles 2 2

Surface Irradiance 1 metre away: r = 1m I P A Emits into all angles: 40W light bulb: 5W light 5W 5 Wm 2 0. 5Wm 2 2 4 r 4 A 2r 2 1 cos 4r 2 Department of Physics Laser Safety 72

Surface Irradiance 1 mw, Collimated 2mm diameter beam A r 1 metre away: directed beam, diameter at 1m still 2mm 2 I P A 1mW 0.001 Wm 2 300Wm 2 2 2 r (0.001) Department of Physics Laser Safety 73

Surface Irradiance Department of Physics Laser Safety 74

Spatial Coherence Department of Physics Laser Safety 75

Retinal Irradiance (Example) Image factor 100 times smaller Image factor ~3330 times smaller Department of Physics Laser Safety 76

Optical fibre & diverging beams Department of Physics Laser Safety 77

Gaussian beam profiles Department of Physics Laser Safety 78

Gaussian Cross-section Department of Physics Laser Safety 79

Example Optical Fibre d 63 z 2zNA 1. 7 d 63 z 2 2z w 0 l Department of Physics Laser Safety 80

Power Through Apertures Department of Physics Laser Safety 81

Large Beam Department of Physics Laser Safety 82

Small Beam Department of Physics Laser Safety 83

Intermediate Beam Department of Physics Laser Safety 84

Power through Aperture Department of Physics Laser Safety 85

Nominal Ocular Hazard Distance (NOHD) Department of Physics Laser Safety 86

NOHD Department of Physics Laser Safety 87

Collimated Department of Physics Laser Safety 88

Focussed Department of Physics Laser Safety 89

Calculating NOHD Emitted Power = P Beam Divergence = Beam Area: Irradiance: I A r P A 2 2 4P z 2 2 For small angles! r becomes distance z Department of Physics Laser Safety 90

Calculating NOHD Insert: MPE for I Insert NOHD for z I 4P z 2 MPE 4P NOHD 2 NOHD 2 P MPE Department of Physics Laser Safety 91

Example Bare Diode Laser Two divergence angles! Department of Physics Laser Safety 92

Calculating NOHD (diode laser) Emitted Power = P Divergence Angles = Beam Area: Irradiance: A r P I A, 2 z 1 2 2 2 4P 2 1 2 For small angles! z 2 1 2 2 2 Department of Physics Laser Safety 93

Calculating NOHD (diode laser) Insert: MPE for I Insert NOHD for z I z 4P 2 1 2 MPE 4P 2 1 2 NOHD NOHD 2 P MPE Product of divergences,,replace θ 2 1 2 1 2 Department of Physics Laser Safety 94

Example 12mW at 980nm 100 mm 300 mm MPE 21Wm 2 Department of Physics Laser Safety 95

12mW at 980nm d 63 z Therefore: 2 2z w 0 l 2 2 w l 0 2 2 0. 98 10 m m m m 0. 088 rad A r 2 2 2 Department of Physics Laser Safety 96

Calculating NOHD NOHD P 2 MPE 0.3m 0. 012 2 2 21 0 088. 2 Department of Physics Laser Safety 97

12mW at 980nm 100 mm 300 mm Can we verify the AEL found earlier? AEL 0. 8mW Department of Physics Laser Safety 98

12mW at 980nm d 63 z Therefore: 2 2z w 0 l z 100mm d 63 8.82mm z 300mm d 63 26.5mm Department of Physics Laser Safety 99

12mW at 980nm 1 e d d 2 63 d 7mm aperture to be used (pupil diameter) Therefore: z 100mm d 63 8.82mm 0.467 z 300mm d 63 26.5mm 0.068 Department of Physics Laser Safety 100

12mW at 980nm AEL 0. 8mW 100 mm 300 mm 0.467 P 5. 6mW 0.068 P 0. 8mW Department of Physics Laser Safety 101

12mW at 980nm 300 mm What about safety parameters for Class 1M? AEL and MPE? At what distance does the laser become essentially class 1M? Department of Physics Laser Safety 102

What is the duration time - t? What is the duration time? a. Is it intentional viewing? Duration time = exposure time. b. Is it accidental exposure? i. Visible (400-700 nm), ocular Duration time = 0.25 s ii. <400 nm, ocular Duration time = 30000 s iii. >700 nm, ocular Duration time = 100 s iv. Visible or invisible, skin Duration time = 10 to 100 s MPE: l=980nm? Department of Physics Laser Safety 103

4 AEL 1. 210 C4W Department of Physics Laser Safety 104

Class 1 @ l=980nm 4 AEL 1. 210 C4W C 4 10 980700/ 500 280 10 / 500 10 0. 56 3. 63 AEL 0. 436 mw 0. 436mW P 0. 436mW 0. 0363 12mW Department of Physics Laser Safety 105

Class 1 @ l=980nm 1 e d d d 2 d 63 63 e 1 2 d d ln 63 1 d 63 2 ln ln d 1 7mm 1 0. 0363 36. 4mm Department of Physics Laser Safety 106

Class 1 @ l=980nm d 63 2 2 w zl 0 z d 63 2 w 2 0 l z 36. 4mm 10mm 2 20. 98mm 412mm Department of Physics Laser Safety 107

12mW at 980nm AEL 0. 436mW z 412mm Class 1 @ l=980nm Department of Physics Laser Safety 108

An Easier Way!! Calculate using MPE! AEL MPE A LA MPE AEL A LA 0 43610 3 W 11 22Wm 2 2 MPE.. 0. 007 2 Department of Physics Laser Safety 109

An Easier Way l=980nm Or, use the MPE table with the long viewing time Department of Physics Laser Safety 110

Class 1 @ l=980nm MPE 3.2C 4 Wm 2 C 4 10 980700/ 500 280 10 / 500 10 0. 56 3. 63 MPE 11.6Wm 2 Previously calculated 11.22 Wm -2 Values self-consistent but not exact! Department of Physics Laser Safety 111

An Easier Way!! NOHD 2 P MPE 2 Already know: 2 11. 22 0. 012 0. 088 2 0. 42m 420mm Department of Physics Laser Safety 112

How about laser pointers? What is the duration time? a. Is it intentional viewing? NO Duration time = exposure time. b. Is it accidental exposure? YES i. Visible (400-700 nm), ocular YES Duration time = 0.25 s ii. <400 nm, ocular Duration time = 30000 s iii. >700 nm, ocular Duration time = 100 s iv. Visible or invisible, skin Duration time = 10 to 100 s Department of Physics Laser Safety 113

7 AEL 3. 910 C3W Department of Physics Laser Safety 114

Class 1 Laser C 3 10 0. 015 17. 58 @ l550 l 633nm AEL 6.86mW 532nm @ l AEL 0.39mW Department of Physics Laser Safety 115

Class 1 And, this is consistent with the numbers provided in the laser safety manual Department of Physics Laser Safety 116

Summary 1) Choose safety classification 2) Use tables for appropriate MPE or AEL: AEL MPE A LA MPE AEL A LA 3) Use laser power, diffraction angle and MPE to NOHD NOHD 2 P MPE 1 2 Department of Physics Laser Safety 117