Lasers & Health Physics April 6, 2017 David R. Bisson, CHP JHU/APL Laser Safety Officer 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! 2 Outline Laser uses Laser characteristics Parameters affecting safety Damage mechanisms Standards and regulatory bodies Laser classification Laser safety program elements 3 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 www.madisonavesmiles.com/laser-dentistry.html – 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) commons.wikimedia.org/wiki/File:Classical_spectacular_laser_effects.jpg 4 Laser Use by Government & Industry . Law Enforcement Speed monitoring Disorient perpetrators (e.g., Dazzler) Perimeter security systems www.gilroydispatch.com/news/contentview.asp?c=207294 www.laserdazzler.net/ . Environmental Aerial mapping with LIDAR Pollution monitoring . Engineering Use Structural analysis with laser vibrometer minnesota.publicradio.org/display/web/2008/07/09/digitalmap/ Surveying (includes use of high-powered lasers operated within a controlled area) http://www.saminc.biz/land_survey.html 5 Standards and Regulatory Bodies Organization Role Purpose American National Standards Standards U.S. standards organization that recommends safety guidelines Institute (ANSI) and details methods to calculate relevant safety parameter values International Electrotechnical Standards International standards organization analogous to ANSI. The IEC Commission (IEC) and ANSI standards for safe laser use were harmonized in 2007. Food and Drug Administration Regulatory (Products) U.S. federal agency that specifies requirements for safety features (FDA) (e.g., interlock, shutter) and imposes these requirements on manufacturers Federal Aviation Administration Regulatory U.S. federal agency overseeing outdoor laser safety and (FAA) (Usage) specifically how laser use must comply with restrictions to accommodate air travel Occupational Safety and Health Regulatory U.S. federal organization that sets occupational safety rules. Organization (OSHA) (Usage) indirectly regulates laser use in the workplace through the “General Duty Clause” which mandates a workplace free of recognized hazards. State & Local Governments Regulatory Enforcement of laser safety by the user is left to state and local (Usage) governments in the U.S. These codes vary across state and municipality.* Military Services Regulatory (Exemption Military organizations can impose more stringent constraints on for DoD Products & use. Contractors supporting military can petition the FDA for Usage) 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. 6 Laser Characteristics • 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 Wave Number 50,000 cm-1 10,000 cm-1 1,000 cm-1 – Ultraviolet (100 – 380 nm) Frequency 1.5 x 1015 Hz 3 x 1014 Hz 3 x 1013 Hz – Visible (380 – 780 nm) Adapted from Trager, Frank , Editor (2007). Springer Handbook of Lasers and Optics. Chapter 21, Fig 21.3, p. 1255. – Infrared (780 – 1,000,000 nm) . Lasers are designed to emit light that is: LASER Coherent (i.e., photons are in phase) Coherent, monochromatic, & directional 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/ 7 Parameters Affecting Safety Type of Laser Transmission Irradiance Specular Reflections Continuous wave Power per beam area or Pulsed rate of exposure (W/cm2) “Mirror” Output Power Attenuation Effective Dose Diffuse Reflections Pulse width Absorption “Scattering” Radiant exposure equals Rep rate Irradiance * Time (J/cm2) “Diffuser” Distance traveled Wavelength Reflection • Direct (or intrabeam) exposure • Hazard distance “Color” • Control of hazard domain “Mirror” • Intended vs. unintended exposure Divergence Focus (Convergence) • Aversion response times • Personal protective equipment Beam spread Optical Elements • Optics in the system (e.g., magnifying optics Waist “Lens” such as binoculars) • Optically induced hazards Min. beam dia. Safety concerns arise if laser energy is concentrated in a small area 8 Laser Damage Mechanisms 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 . 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 Photochemical . Different mechanisms dominate in different regions of the spectrum (e.g., Sunburn on Eye) 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, nd specification of thresholds evolves as new data acquired Thermal (e.g., 2 Acoustic (e.g., Degree Burn) Retinal Detachment) 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. 9 Maximum Permissible Exposure (MPE) 1 . MPE is established as /10 the ED-50 limit, which is the effective dose that results in injury to 50% ED-50 Laser Damage of the normal population (dose = exposed time x irradiance) . 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 No Damage Damage IR (eye): 10 s due to maximum possible stare time • Population of 20 exposed to a radiant exposure (J/cm2) equal to the effective dose 50% (ED-50) level IR (skin): 10 s due to thermal characteristics of skin • Statistically, half of the individuals subjected to the ED-50 laser exposure will be injured; thus, ten individuals in the . Studies and analysis indicate that microscopic above population are expected to sustain laser damage • By convention, a safety factor is applied to this ED-50 damage occurs in some cases between ¼ and ½ radiant exposure level to specify the maximum permissible 1 exposure (MPE) ED-50, but never below /10 ED-50 . MPE does not account for special cases Source: Trager, Frank , Editor (2007). Springer Eye irregularity (e.g., no lens) Handbook of Lasers and Optics. Chapter 21: pp. 1251-1276. Impacts of medication (photosensitizers) 10 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 Cornea Lens Cornea Lens Cornea Lens Retina Retina Retina Incident UV Incident Visible and near-IR Incident mid-IR and far-IR (<380 nm) (380 - 780 nm) and (780 - 1400 nm) (>1400 nm) Reaches Lens Simplified Eye Reaches Retina without Aversion Simplified Eye Absorbed in Cornea Simplified Eye 11 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 (1) Identify laser parameter values (2) Determine MPE using ANSI rules 1, 2, & 3 Rule 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]. 2 MPEsingle pulse = 1 J/cm 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]. Select MPE = 1.0 x 10-4 J/cm2 /pulse smallest