Lasers&

Health Physics

April 6, 2017

David R. Bisson, CHPJHU/APL Laser Safety Officer

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Health Physics at the Applied Physics Lab

Health Physicists are dedicated to maximizing the beneficial use of radiation while minimizing the risk to people and the environment.

APL routinely uses Lasers to do great things…including carving our pumpkins and even making pumpkin pie!

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Outline

Laser usesLaser characteristicsParameters affecting safetyDamage mechanismsStandards and regulatory bodiesLaser classificationLaser safety program elements

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Laser Use by Consumers

• Medical Treatment– Vision correction (e.g., LASIK)– Repair of retinal bleeds and detachments– Dental treatments– Removal of hair, birthmarks, and tattoos

• Commercial Products– Laser pointers (Class 3R)– Laser levels– Laser printers/CD burners (Class 4 laser packaged

in Class 1 product)*– Laser Cutters/Etchers/Engravers* – Barcode scanners (Class 3B laser operated as

Class 2 system)– Telecommunication devices

• Entertainment– Laser tag (Class 2 laser)– Laser ranging devices (e.g., golf, hunting)– Laser light shows (e.g., Disney, concerts, trade

shows)

www.madisonavesmiles.com/laser-dentistry.html

commons.wikimedia.org/wiki/File:Classical_spectacular_laser_effects.jpg

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Laser Use by Government & Industry

Law Enforcement Speed monitoring

Disorient perpetrators (e.g., Dazzler)

Perimeter security systems

Environmental Aerial mapping with LIDAR

Pollution monitoring

Engineering Use Structural analysis with laser vibrometer

Surveying (includes use of high-powered lasers operated within a controlled area)

www.gilroydispatch.com/news/contentview.asp?c=207294

minnesota.publicradio.org/display/web/2008/07/09/digitalmap/

www.laserdazzler.net/

http://www.saminc.biz/land_survey.html

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Standards and Regulatory BodiesOrganization Role Purpose

American National Standards Institute (ANSI)

Standards U.S. standards organization that recommends safety guidelines and details methods to calculate relevant safety parameter values

International Electrotechnical Commission (IEC)

Standards International standards organization analogous to ANSI. The IEC and ANSI standards for safe laser use were harmonized in 2007.

Food and Drug Administration (FDA)

Regulatory (Products) U.S. federal agency that specifies requirements for safety features (e.g., interlock, shutter) and imposes these requirements on manufacturers

Federal Aviation Administration (FAA)

Regulatory(Usage)

U.S. federal agency overseeing outdoor laser safety and specifically how laser use must comply with restrictions to accommodate air travel

Occupational Safety and Health Organization (OSHA)

Regulatory(Usage)

U.S. federal organization that sets occupational safety rules. indirectly regulates laser use in the workplace through the “General Duty Clause” which mandates a workplace free of recognized hazards.

State & Local Governments Regulatory(Usage)

Enforcement of laser safety by the user is left to state and local governments in the U.S. These codes vary across state and municipality.*

Military Services Regulatory (Exemption for DoD Products &

Usage)

Military organizations can impose more stringent constraints on use. Contractors supporting military can petition the FDA for exemptions for DoD use.

*Note: Alaska, Arizona, Florida, Georgia, Illinois, Massachusetts, New York, and Texas are some states with laser regulatory programs1

1 Barat, Kenneth. Laser Safety Management, p. 162-163. CRC Press, Taylor & Francis Group. 2006.

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Laser Characteristics

Lasers are designed to emit light that is: Coherent (i.e., photons are in phase) Same wavelength or color (i.e., monochromatic) Highly directional (i.e., collimated with limited beam

divergence)

Laser light scattering Specular (energy remains directionally concentrated) Diffuse (energy spread out in multiple directions)

http://twistedphysics.typepad.com/cocktail_party_physics/optics/

Adapted from Trager, Frank , Editor (2007). Springer Handbook of Lasers and Optics. Chapter 21, Fig 21.3, p. 1255.

Frequency

Wave Number 10,000 cm-1 1,000 cm-1

3 x 1014 Hz 3 x 1013 Hz1.5 x 1015 Hz

50,000 cm-1

– Ultraviolet (100 – 380 nm)– Visible (380 – 780 nm)– Infrared (780 – 1,000,000 nm)

• Provides a “bright” light with minimal spreading (divergence)

• Laser wavelength or color occupies a very narrow region of the electromagnetic spectrum within one of three bands

LASER

Coherent, monochromatic, & directional

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Parameters Affecting Safety

Type of Laser

Continuous wave

Pulsed

Wavelength

“Color”

Divergence

Beam spread

Waist

Min. beam dia.

Reflection

“Mirror”

Distance traveled

Focus (Convergence)

“Lens”

Transmission

AttenuationAbsorption

“Scattering”

Irradiance

Power per beam area or rate of exposure (W/cm2)

Effective Dose

Radiant exposure equals Irradiance * Time (J/cm2)

• Direct (or intrabeam) exposure• Hazard distance• Control of hazard domain• Intended vs. unintended exposure• Aversion response times• Personal protective equipment• Optics in the system (e.g., magnifying optics

such as binoculars)• Optically induced hazards

Specular Reflections

“Mirror”

Opt

ical

Ele

men

tsOutput Power

Pulse width

Rep rate

Diffuse Reflections

“Diffuser”

Safety concerns arise if laser energy is concentrated in a small area

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Laser Damage Mechanisms

Amount of radiant exposure translates directly into damage caused Damage can result from direct, specular, or diffuse exposure Some damage can heal (e.g., corneal damage) Other injuries, such as retinal damage, are permanent, but can be

treated to minimize the extent of the effects Different mechanisms dominate in different regions of the spectrum

given composition of tissue affected and characteristics of the optical radiation

For example, retinal burns do not occur in UV because UV photons are quickly absorbed in the cornea and lens

Damage mechanisms fairly well understood and characterized; however, specification of thresholds evolves as new data acquired

Sources: 1 ANSI Z136.1-2007, Safe Use of Lasers, 2007.2 Trager, Frank , Editor (2007). Springer Handbook of Lasers and Optics. Chapter 21: pp. 1251-1276.

Thermal (e.g., 2nd

Degree Burn)

Photochemical (e.g., Sunburn on Eye)

Acoustic (e.g., Retinal Detachment)

Mechanism Description

Photochemical Incident light initiates chemical reactions that alter eye or skin tissue molecules.1

Thermal Absorption of light is transformed into heat, raising temperature of tissue and potentially causing denaturation of proteins, vaporization, and cell death.1,2

Acoustic Non-thermal effect associated with incident light on the eye resulting in shockwaves that rupture the retinal tissue and could lead to retinal detachment.2

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Maximum Permissible Exposure (MPE)

MPE is established as 1/10 the ED-50 limit, which is the effective dose that results in injury to 50% of the normal population

Separate MPEs for skin and eyes MPE is based on maximum exposure times

UV: ~8 hrs due to cumulative photochemical damage

VIS (eye): 0.25 s due to body’s natural aversion responses (e.g., blink response)

VIS (skin): 10 s due to thermal characteristics of skin

IR (eye): 10 s due to maximum possible stare time IR (skin): 10 s due to thermal characteristics of skin

Studies and analysis indicate that microscopic damage occurs in some cases between ¼ and ½ ED-50, but never below 1/10 ED-50

MPE does not account for special cases Eye irregularity (e.g., no lens) Impacts of medication (photosensitizers)

Source: Trager, Frank , Editor (2007). Springer Handbook of Lasers and Optics. Chapter 21: pp. 1251-1276.

ED-50 Laser Damage(dose = exposed time x irradiance)

No Damage Damage• Population of 20 exposed to a radiant exposure (J/cm2)

equal to the effective dose 50% (ED-50) level• Statistically, half of the individuals subjected to the ED-50

laser exposure will be injured; thus, ten individuals in the above population are expected to sustain laser damage

• By convention, a safety factor is applied to this ED-50 radiant exposure level to specify the maximum permissible exposure (MPE)

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Spectral Dependence of Laser Damage

Safety factors for skin and cornea are the same

Lens of the eye concentrates energy onto the retina by as much as 100,000 times in the range of 380 – 1400 nm

Cornea and lens absorb nearly all incident ultraviolet (UV) photons

Greater penetration in visible and near-infrared (NIR) parts of the spectrum, potentially leading to retinal burns

Mid-infrared and far-infrared wavelengths absorbed in the cornea

RetinaIncident UV(<380 nm)

Reaches Lens

Cornea

Simplified Eye

Lens Cornea

Simplified Eye

Lens

Incident Visible and near-IR(380 - 780 nm) and (780 - 1400 nm)

Reaches Retina without Aversion

RetinaCornea

Simplified Eye

Lens

Incident mid-IR and far-IR(>1400 nm)

Absorbed in Cornea

Retina

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Nominal Ocular Hazard Distance (NOHD) ANSI definition – “the distance along the axis

of the unobstructed beam from a laser … to the human eye, beyond which the irradiance or radiant exposure is not expected to exceed the MPE.”

Areas within NOHD must be “controlled” for safe laser operation

NOHD & Optical Density (filtration) Calculations

NOHD:Irradiance

Exceeds MPE

Beyond NOHD:Irradiance not

Expected to Exceed MPE

(2) Determine MPE using ANSI rules 1, 2, & 3Rule 1 – Singse pulse (t = 10ns)No single pulse in a train of pulses shall exceed the single pulse MPE [ANSI Std. Z136.1 8.2.3 rule 1].

MPEsingle pulse = 1 J/cm2

Rule 2 – Average power (t = 10s)The exposure from any group of pulses delivered in time T shall not exceed the MPE for time T [ANSI Std. Z136.1 8.2.3 rule 2].

MPE/pulse = 1.0 x 10-4 J/cm2

Rule 3 – Multiple pulse (Tmin = 10s)Rule 3 protects against sub-threshold pulse cumulative thermal injury [ANSI Std. Z136.1 8.2.3 rule 3].

MPE/pulse = 0.1 J/cm2

Select smallest value

(3) Calculate safety factors of interest based on MPENOHD = 460.38 m AEL = 9.6 x 10-6 J ODmin = 2.62

(1) Identify laser parameter values

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Optical density (OD)

OD 1 OD 2 OD 3

10% 1% 0.1%

Optical Density

% Radiation Transmission

1 10%2 1%3 0.1%4 0.01%5 0.001%6 0.0001%7 0.00001%

• Filtration is wavelength/wavelength range specific

• Maximize visible light transmittance

• Inspect for damage• Store properly to minimize

damaged• Can degrades over time

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Laser Classification

Lasers categorized by operational safety Six classes (1, 1M, 2, 2M, 3, and 4), one of which

is broken into two subclasses (3R & 3B) Class 1 is “safest” with Class 4 “most

dangerous” “M” designation indicates hazards increase

with use of magnifying optical aids Classification scheme used before March 2007

by ANSI: Classes I, II, IIa, IIIa, IIIb, and IV

In 2007 ANSI Z136.1 standard was harmonizedwith the international standard (IEC 60825-1) which established the current classes

Class Description

1 Considered incapable of producing damage during operation

1M Same as 1, except damage could arise when using optical instruments

2 Visible laser where eye protection is afforded by the aversion response

2M Same as 2, except damage could arise when using optical instruments

3R Medium power (<5 mW) laser may be hazardous under direct & specular reflection viewing conditions

3B Medium power (5-500 mW) laser may be hazardous under direct & specular reflection viewing conditions

4 High power (>500 mW) laser hazardous to eye and skin, may pose a diffuse reflection hazard, and can create laser generated air contaminates

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Control Measures ANSI Z136.1 Section 4.1 -“Control Measures shall be devised to reduce the possibility of exposure of the eye and skin to hazardous levels of laser radiation and other hazards associated with the operation of laser devices during operation and maintenance.” Hazard Control Priorities:

1. Substitution – Less hazardous equipment / process2. Engineering Controls – Shields, enclosures, interlocks…3. Administrative Controls – Rules, procedures…4. Personal Protective Equipment – Eyewear, …

Engineering Controls:1. Entryway controls (interlocks required for class

4 systems and unattended 3b use)2. Warning signs (illuminated signs and postings)3. Beam Enclosures4. Ventilation when Laser generated air

contaminants are possible5. Personal Protective Equipment

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Administrative & Procedural ControlsAdministrative: Authorization prior to use Authorization prior to

Maintenance and service(defeated interlocks on a Class 1 System could create a much high class hazard)

Purchases / Inventory Control (Class 3B & 4)

Procedural: Approved “SOPs” (alignment,

service…) Approved operators Laser Safety Training Medical Surveillance*

ANSI “Shall” <2007 “Should” > 2007 *APL using DOT eye screening vs full

ophthalmological exams

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Examples of Beam Pipes & Guards

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Example Curtain & Warning Sign

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Example Laser Control Area (LCA)

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Temporary LCA