Massachusetts Institute of Technology

Laser Safety TrainingLaser Safety Training

William B. McCarthy, Ph.D.Radiation Protection Office

Laser Safety at MITLaser 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 OfficeMIT Radiation Protection Officeand 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 BasicsLaser Basics

• Light

• Amplification by

• Stimulated

• Emission of

• Radiation

Laser Light vs White LightLaser Light vs White Light

Laser light is:• Monochromatic• Directional• Coherent

These three properties of laser light make it different than normal light.

Electromagnetic SpectrumElectromagnetic Spectrum

Visible Light

RF :AM, FM, TVMW

IR UV

X-Rays

- RaysPower

Cosmic

Laser Light SpectrumLaser Light Spectrum

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 SurfaceEffects of the Eye

Exi

mer

Arg

on

HeN

e

Rub

y

GaA

s

Nd:

YA

G

Ho:

YA

G

Er:

YA

G

CO

2

How A Laser WorksHow A Laser Works

ACTIVE MEDIUM

EXCITATION MECHANISM

FEEDBACK MECHANISM

High Reflectance mirror

Output coupler mirror

Types of LasersTypes of LasersActive Medium

- Gas

- Solid State

- Diode

- Dye

Time Duration

- Continuous Wave (> 0.25 sec)

- Pulsed (< 0.25 sec)

- Single Pulse (< 1Hz)

- Pulsed (Q-switched)

Laser CharacteristicsLaser 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 LasersExamples of LasersLaser Media Type Wavelength (nm) Application

Ruby Solid Visible (694.3) Ranging, hole drilling, surgery

Neodymium - YAG Solid Near IR, Visible, andUV- A

Metal processing, welding, eyesurgery, military weapons and ranging

Carbon Dioxide Gas Far IR (10600) High power metal treatments, surgery

Argon Gas Visible (488 – 514) Eye surgery, cell counters, diagnostics,photoetching, light shows

Helium Neon Gas Visible and UV-A(351, 363, 632.8)

UPC checkout systems, alignment,construction, video discs

Gallium Arsenide Diode Near IR(820, 1310, 1550)

Communications, infrared beacons inarray format, laser printers, CD players

Krypton Gas Visible(676,647,568,528,476)

Light shows holography, diagnostics

Rhodamine Dye Visible(Tunable 570 – 650)

Spectroscopy, IC circuit etching,photochemical reactions

Laser Safety TerminologyLaser 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 TerminologyMore 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 HazardsLaser Hazards

laser safety is no joke

Laser HazardsLaser Hazards

• Primary Beam Hazards

• Scattered Beam Hazards

• Non-Beam Hazards

Fire

Chemical

Electrical

Air Contaminants

Primary Beam

Scattered Beam

Specular vs Diffuse ReflectionsSpecular 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 ClassificationsLaser ClassificationsClass 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 EffectsSkin - 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: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 dependenceTissue 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.

light rays focus on the retinal back

surface of the eye fovea

optic nerve

blood supply

macula

lens

cornea

retina

Eye - Biological EffectsEye - Biological Effects• Corneal Damage

– photokeratitis - welder’s flash

– corneal burn

• Retinal damage– blindness -

fovea/macula– retinal detachment

– retinal burn

• Lens– cataracts

A detailed picture of the human eyeA detailed picture of the human eye

Ocular transmission and retinal absorptionOcular 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 eyeAbsorption 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 ExamLaser Eye Exam• 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) Always keep your eyes closed when working with lasers

or

b) Wear the proper eyewear for your laser system

A Multiple Choice QuestionA Multiple Choice Question

Laser EyewearLaser 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 ClassificationLaser Classification• Class 1

– 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.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– visible and invisible light– greater than 500 mW– eye and skin hazard for exposure to both direct beam and scattered

radiation– fire hazard

Laser Warning SignsLaser Warning Signs

• Class 2 and 3a lasers

• Class 3b and 4 lasers

Class 3a laser

Laser Radiation - Do Not Stare into Beam

Class 4 laser750 mW - Nd:YAG - 1064 nm

Invisible laser beam - Avoid Eye or Skin Exposure to Direct or Scattered Radiation

MIT Radiation Protection Office 3-2180

2.5 mW - HeNe - 543 nm

Laser Safety ControlsLaser Safety Controls

• Engineering Controls

• Administrative Controls

• Procedural Controls

• Use appropriate controls

Engineering ControlsEngineering Controls• Operation

– 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

Administrative ControlsAdministrative 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 ControlsUsing 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.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.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 PracticesSafe 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 AccidentsLaser 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 SignLaser Warning Sign

Class 4 laserMIT Radiation Protection Office 3-2180

Laser Radiation - Avoid Eye or Skin Exposure to Direct or Scattered Radiation

Type:________________________

Power:_______________________

Wavelength:___________________

Pulse:________________________