Massachusetts Institute of Technology Laser Safety Training William B. McCarthy, Ph.D. Radiation Protection Office

Laser Safety at MIT Massachusetts State Regulations ANSI Standard for Laser Use MIT Radiation Protection Office MIT Laser Safety Policies Laser Safety Officer Laser Safety Liaison for each department

MIT Radiation Protection Office and the Laser Safety Officer Register laser systems with the State Register laser workers Instruct as to laser hazards and safe use of lasers Perform hazard analysis for all Class 3b and 4 lasers Provide safety related information to laser users such as proper eyewear Coordinate eye exams for laser users

Laser Basics Light Amplification by Stimulated Emission of Radiation

Laser Light vs White Light Laser light is: Monochromatic Directional Coherent These three properties of laser light make it different than normal light.

Electromagnetic Spectrum X-Rays Cosmic RF : AM, FM, TV MW Power IR UV - Rays Visible Light

Laser Light Spectrum Wavelength (microns) 0.2 0.4 0.6 0.8 1.0 2.0 3.0 10.0 Wavelength (microns) Ultraviolet Visible Near Infrared Mid Infrared Far Infrared 0.7 Photochemical Retinal Damage Not “Eye Safe” Heating Surface Effects of the Eye Eximer Argon HeNe Ruby GaAs Nd:YAG Ho:YAG Er:YAG CO2

How A Laser Works ACTIVE MEDIUM EXCITATION MECHANISM FEEDBACK MECHANISM ACTIVE MEDIUM High Reflectance mirror Output coupler mirror EXCITATION MECHANISM

Types of Lasers Active Medium - Gas - Solid State - Diode - Dye Time Duration - Continuous Wave (> 0.25 sec) - Pulsed (< 0.25 sec) - Single Pulse (< 1Hz) - Pulsed (Q-switched)

Laser Characteristics Power Output (watts) 10 W to MW Wavelength (nanometers) 200 nm to 10.6 m Pulse Duration and Delay (nanoseconds - seconds) 1 psec to continuous A 100 watt light bulb has 100,000 times the power of a 1 mW laser, the same laser is 10 million times brighter

Examples of Lasers

Laser Safety Terminology MPE - Maximum Permissible Exposure - “The level of laser radiation to which a person may be exposed without hazardous effect or adverse biological changes in the eye or skin.” NHZ - Nominal Hazard Zone - “The space within which the irradiance or radiant exposure exceeds the appropriate MPE.”

More Laser Terminology O.D. - Optical Density - Approximately the order of magnitude of transmittance (). More accurately the OD is equal to log10 (1/). Eyewear is chosen with an OD that will reduce the eye exposure to the MPE or below. Irradiance - Watt/cm2 Radiant exposure - Joule/cm2

Laser Hazards laser safety is no joke

Laser Hazards Primary Beam Hazards Scattered Beam Hazards Non-Beam Hazards Fire Chemical Air Contaminants Electrical Primary Beam Scattered Beam

Specular vs Diffuse Reflections Specular - reflection from a “shiny” object. This can be as hazardous as the primary beam. Diffuse - reflection from a rough object. The “roughness” depends on the wavelength of the light.

Laser Classifications Class 1 - No Biological effect Class 2 - Eye hazard for chronic viewing only Class 3a - Eye hazard for chronic viewing or use of collecting optics Class 3b - Eye and Skin hazard for direct beam exposure Class 4 - Eye and Skin hazard for direct beam and scattered radiation, also a fire hazard More detail on laser classification later

Skin - Biological Effects Thermal (all wavelengths) Skin burns surface burn from CO2 (typical thermal burn) deep burn from YAG (very painful) Photochemical (< 550 nm) “tanning” skin cancer photosensitive reactions (some are medication related) Shockwave (Acoustic)

Degree of Bio-effect depends on: Wavelength of the beam Irradiance or radiant exposure on the tissue Duration of exposure and pulse repetition characteristics Extent of vascular flow to the area Size of the area irradiated

Tissue absorption - wavelength dependence Skin is fairly transparent to red light and near infrared. Here one can see why the CO2 laser causes a surface burn and a YAG causes a deeper burn.

Eye - Biological Effects Corneal Damage photokeratitis - welder’s flash corneal burn Retinal damage blindness - fovea/macula retinal detachment retinal burn Lens cataracts light rays focus on the retinal back surface of the eye fovea optic nerve blood supply macula lens cornea retina

A detailed picture of the human eye

Ocular transmission and retinal absorption The highest retinal absorption is in the blue region. Blue light poses a relatively greater hazard to the retina than other visible wavelengths. Ultraviolet and mid to far infrared light is absorbed by the cornea. Near infrared is absorbed by the lens and can produce cataracts.

Absorption of laser light by the eye Visible and near infrared (0.4 - 1.4 um) wavelengths are focused by the cornea and lens and absorbed by the retina. Mid and far infrared (1.4 - 1000 um) wavelengths and UV-B and C (0.18 - 0.315 um) are absorbed by water on the surface of the eye. UV-A (0.315 - 0.390 um) wavelengths are absorbed in the cornea and lens structure.

Laser Eye Exam Baseline Incident Periodic Termination Eye exams are required of all personnel directly involved with the use of Class 3b and 4 lasers. The eye exam is done by the MIT Medical Department. Baseline Incident Periodic Termination

How Do You Avoid Laser Eye Injuries? A Multiple Choice Question How Do You Avoid Laser Eye Injuries? a) Always keep your eyes closed when working with lasers or b) Wear the proper eyewear for your laser system

Laser Eyewear Laser eyewear should be readily available and worn whenever a hazardous condition exists. RPO recommends and approves the appropriate protective eyewear. Eyewear should let as much visible light through while still blocking the laser light. Special Alignment eyewear should be used during alignments. Also, eyewear should fit comfortably.

Laser Classification Class 1 Class 2 no biological effect less than 0.4 uW visible light if higher class laser is fully enclosed and interlocked it can be classified as a Class 1 laser. Class 2 visible light only less than 1mW hazard for chronic viewing (>0.25 sec) aversion response adequate protection Class 2a supermarket scanners

Laser classification cont. Class 3a visible light only (ANSI makes some exception for invisible) 1 to 5 mW same as Class 2 except it is also a hazard with collecting optics Class 3b visible and invisible light 5 to 500 mW of continuous wave power eye and skin hazard with direct beam Class 4 greater than 500 mW eye and skin hazard for exposure to both direct beam and scattered radiation fire hazard

Laser Warning Signs CAUTION DANGER Class 2 and 3a lasers Class 3b and 4 lasers Laser Radiation - Do Not Stare into Beam 2.5 mW - HeNe - 543 nm Class 3a laser Class 4 laser DANGER 750 mW - Nd:YAG - 1064 nm Invisible laser beam - Avoid Eye or Skin Exposure to Direct or Scattered Radiation MIT Radiation Protection Office 3-2180

Laser Safety Controls Engineering Controls Administrative Controls Procedural Controls Use appropriate controls Danger Class 4 Laser

Engineering Controls Operation Laser beam Entry/access to laser remote firing key switch Laser beam beam stop or attenuation beam shutters controlled beam path protective housing Entry/access to laser door interlocks filtered windows laser Controlled Area warning lights DANGER Class 4 laser

Administrative Controls Standard operating procedures startup - shutdown - specific operations - emergencies Administrative procedures limitations on use of laser according to classification training: Class 3b and 4 lasers must be operated by experienced and trained operators. Spectators are subject to entry requirements operating manuals eyewear labeling Laser warning signs

Using Appropriate Controls Class 1 - No warning signs are required Class 2 - Caution signs/labels are required Class 3a - Warning (Caution or Danger) signs/labels are required, eyewear required if laser beam can not be controlled

Appropriate controls cont. Class 3b - Danger signs/labels, beam path control, and protective eyewear required, plus the following are recommended: protective housing key switch area interlocks beam stops/attenuators activation warning system Class 3b laser controlled area establish Nominal Hazard Zone (NHZ)

Appropriate controls cont. Class 4 - Those items recommended for Class 3b are required for Class 4 lasers. The following are recommended: supervised by an individual knowledgeable in laser safety require approved entry for all non-involved personnel use diffusely reflecting materials near the beam light tight room remote firing or remote viewing beam stops made of fire resistant materials CPR training (strongly recommended)

Safe Work Practices Never intentionally look directly into a laser. Do not stare at the light from a laser. Allow yourself to blink if the light is too bright. Never direct the beam toward other people. Remove all unnecessary reflective objects from the area near the beam path. This may include items of jewelry or tools. Never allow a laser beam to escape from its designated area of use. Do not enter a designated Class 3b or 4 (posted with a Danger sign) laser area without the proper eyewear. Position the laser so that it is well above or below eye level (both standing and sitting). When not in use a laser should be stored to prevent unauthorized use by untrained individuals.

Laser Accidents Electrocution at MIT “Accident Victim's View” page 19 of the MIT Laser Safety Program manual. Over 1/2 of the reported incidents have been with graduate students. Visiting Professor removes his eyewear so he can see better. Retinal burn, permanent damage. Bystander not wearing eyewear during an alignment procedure. Immediate retinal burn, lesion at edge of macula Professor measuring output, reflected beam into his eye. Permanent damage to macula (blind spot). No eyewear.

Laser Warning Sign DANGER Class 4 laser MIT Radiation Protection Office 3-2180 Laser Radiation - Avoid Eye or Skin Exposure to Direct or Scattered Radiation Type:________________________ Power:_______________________ Wavelength:___________________ Pulse:________________________