Introduction to laser safety Ian Gillett Safety Director Imperial College

What is a laser? The word “laser” is an acronym which stands for Light Amplification by Stimulated Emission of Radiation. Light energy is amplified to extremely high intensity by a process called stimulated emission. The energy generated by the laser is in or near the optical portion of the electromagnetic spectrum.

How does a laser work? Electrons surrounding atoms are excited into higher energy states to create a “population inversion”. The excited electrons release their energy in the form of photons which can, in turn, collide with other excited electrons and cause them to release an identical photon - this is called stimulated emission. As more energy is put in so more photons are released. By reflected the photons back and forth in a mirrored system the amount of energy can be increased. Some of the energy is allowed to escape through a partially coated mirror to create a “laser beam”.

Types of laser Gas lasers Dye lasers Solid state lasers including He-Ne and carbon dioxide lasers Dye lasers Solid state lasers including neodymium-YAG (Nd:YAG) lasers Semiconductor lasers diode lasers Chemical lasers

Laser light Laser light has the following properties: it is monochromatic it is very intense it has low divergence it is coherent. Lasers can have different types of beam output continuous wave (CW), or pulsed. Laser “speckle” is caused by scattered laser radiation interfering with incident laser radiation.

Beam hazards The laser produces an intense, highly directional beam of light. This light will be absorbed by surfaces it is directed at, raising the temperature of that surface. The eyes and skin of the body are vulnerable to this energy. Of greatest concern is laser exposure in the retinal hazard region, but ultraviolet and infra-red light also have an effect.

The Retinal Hazard Region 400-1400nm Affects the retina

Ultraviolet light UV-C (100-280nm) Cornea surface UV-A (315-400nm) Affects the lens UV-B (280-315nm) Absorbed by the cornea

Infra-red Region Near IR (< 1400nm) Affects the retina Far IR Affects cornea and aqueous humor Near IR (< 1400nm) Affects the retina

Reflections There are two principle types of laser reflection: specular reflection - which occurs from mirror-like surfaces. The incident beam striking a specularly reflecting surface will leave it essentially unchanged. diffuse reflections - which occur from rough surfaces such as paper or matt-painted walls. These reflections bear no relation to the direction of the incident radiation.

Non-beam hazards Non-beam hazards include: electrical hazards - especially from trailing cables and risk of water spillages fumes - from vaporised target material laser dyes - many of these are carcinogenic optical hazards - arising from the flash lamps used to “pump” the lasers.

Safety Standards There are two principle international laser safety standards: International Electrotechnical Commission (IEC) 60825 - this has been adopted in the UK as British Standard BS EN 60825-1:1994 “Safety of laser products. Part 1. Equipment classification, requirements and user’s guide.” American National Standards Institute (ANSI) Z136 Both standards share a common classification system for lasers based on their power output, wavelength and pulse duration.

British Standard BS EN 60825-1:1994 is divided into three sections (General; Manufacturing Requirements; and User’s Guide) The objectives of the Standard are to: protect persons from laser radiation lay down requirements for user and manufacturer to establish procedures ensure adequate warning of hazards reduce possibility of injury, and protect persons from other hazards.

Laser Hazard Classification Exposure limits have been set for nearly all types of laser radiation. These limits are referred to as Maximum Permissible Exposures (MPEs). MPEs are those levels of radiation to which persons may be exposed without suffering adverse effects. MPEs depend upon the wavelength of the laser and the duration of the exposure. Detailed tables are found in the British Standard.

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.

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.

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 MPE for momentary exposures of 0.25 second or less - the so-called “blink reflex”. Intentional extended viewing is considered hazardous.

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.

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.

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.

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.

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.

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.

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

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.

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.

College Procedures The College’s High Power Laser Safety Policy requires all work with Class 3B and Class 4 lasers to be subject to protocols approved by Heads of Department and to comply with British Standard BS EN 60825. There is a supporting Code of Practice which describes the College’s requirements for: preparation and approval of protocols; registration of Designated Areas, users and equipment; and, the need for the lowest class of laser to be used.

Protocols Protocols should be developed and implemented before any use of Class 3B and Class 4 lasers. Protocols should describe: the use of the laser; the Designated Area being used the laser(s) being used; the training and competence of the user(s) the use of personal protective equipment; any departure from standard conditions.

Designated Areas Areas where Class 3B and Class 4 lasers are used must be registered with the College Safety Unit. A sign must be affixed near to the entrance of each Designated Area and contain the details of the person responsible for the area, emergency contacts and the name of the Departmental Laser Safety Officer. DESIGNATED AREA CLASS 4 LASER HAZARD NO ENTRY WHEN WARNING LIGHT IS ON Departmental Laser Safety Officer: Emergency contact numbers: 1) 2) PERMIT-TO-WORK REQUIRED Person Responsible for this area:

Laser equipment registration All lasers must be registered with the Departmental Laser Safety Officer and the College Laser Safety Officer. The form includes a checklist of the precautions required for each class of laser.

Laser user registration All laser users must be authorised and must be registered with the College Laser Safety Officer. Before a user is registered they should have had an eye test and attended the “Introduction to Laser Safety” course.

Eye test Current arrangements at the College require all users of Class 3B and Class 4 lasers to have a “laser eye test” before starting work with lasers. A medical examination by a qualified specialist should be carried out immediately after an apparent or suspected injurious ocular exposure.

College Laser Safety Officer The College Laser Safety Officer monitors departmental arrangements and assists departments by: providing expert advice; reviewing protocols maintaining central registers of Designated Areas, users and laser equipment; providing training; and, investigating accidents.

Accident Reporting Procedures All incidents or accidents involving the use of lasers must be reported to the Safety Unit as soon as possible. Any individual who may have been exposed to damaging laser radiation must be referred for assessment of any damage, preferably within 24 hours of the incident. Certain accidents may be reportable to the Health and Safety Executive, including: an accident involving a student which results in them being taken to hospital for treatment; an accident which causes a “specified major injury”, which includes temporary or permanent loss of sight.

Legislation There is no specific legislation in the UK regarding the use of lasers. However, the following regulations do have to be complied with: Management of Health and Safety at Work Regulations 1999 Provision and Use of Work Equipment Regulations 1998 Personal Protective Equipment Regulations 1992 Control of Substances Hazardous to Health Regulations 2002 Electricity at Work Regulations 1989 Reporting of Injuries, Diseases and Dangerous Occurrences Regulations 1995 Health and Safety (Safety Signs and Signals) Regulations 1996

Further training and supervision Please remember this course has only been an “Introduction to Laser Safety”. Further information, instruction, training and supervision needs to be given by your supervisor.