Presentation on theme: "University Of Nottingham"— Presentation transcript:
1University Of Nottingham Laser Safety TrainingProf Tony KentUniversity Of Nottingham4th. October 2007.
2Management of health and safety is based on principles of risk assessment: Hazard – the potential of a process, material, device etc. to do harm. The hazard is often quantified with regard to the severity of the damage/harm that could occur in a worst-case situation.Risk – the likelihood that the potential harm would be realized in practice.The aim is to develop a safe system of work that minimises risk.This general approach to health and safety is no different for lasers.
3Laser hazardsCompare with looking directly at the sun:Solar radiation flux density at the surface of the earth ~ 1 kW/m2.If one were silly enough to stare at the sun, the pupil would contract to about 1 mm2.Therefore 1 mW of sunlight would enter the eye.For flux density at retina, use geometrical opticsor1r2ii = (r2/r1)o ≈ 200 μm
4Therefore at retina we have ~ 25 kW/m2. Now consider a “weak” laser, 1 mW laser pointer with 1 mm2 beam.Again 1 mW of light enters the eye.However, unlike the sun, laser light is highly spatially coherent (as if from a point source) and so is focussed to the theoretical minimum spot size – d ~ fφ, where f is the focal length ( about 2 cm) and φ the beam angular divergence, typically 1 mrad.This gives d = 20 μm or 2.5 MW/m2 at the retina.100 times stronger than staring at the sun!
5• Based on published guidance, the University has adopted administrative procedures to ensure that risks associated with laser work are minimised.• Details of these administrative procedures are contained within the handout.• However, it should be remembered that lasers are being used in lots of different ways across the campuses and there is no “one size fits all” approach to laser safety – local risk assessment is essential.• In this talk I will concentrate on some of the practical aspects of laser safety at the University of Nottingham.
6Laser ClassificationIt is a legal requirement for suppliers to classify the lasers they sell.Classes 1(1M) – 2(2M) – 3R/3A – 3B – 4 (in increasing order of ability to do harm)However, some older systems may not have appropriate labels.The class can be worked out using the “yellow book” and knowing the wavelength, power and pulse width (if pulsed) of the laser. – Example (calculation on the board):
8Laser Classification Class 1 The most limiting MPE values cannot be exceeded and no specific safety controls are required.For CW visible lasers, the maximum limit is 70 microwatts.Class 1 operation cannot be claimed for a product containing an embedded laser of a higher class unless full-interlocked high-integrity enclosures using fail-safe interlocks are incorporated.Class 1M is a large diameter or widely divergent beam (302.5 – 4000 nm).Class 2Visible lasers only, for which the MPE cannot be exceeded in less than 0.25 seconds.For CW laser the limit is 1 mW.Class 2M is a large diameter or widely divergent beam.
9Laser Classification Class 3R 302.5 – 106 nm Lower risk than 3B, but direct viewing of beam potentially hazardousVisible lasers up to five times the Class 2 limits and invisible lasers up to five times the Class 1 limits, for which specific H (Jm-2) or E(Wm-2) values are not exceeded.Class 3BVisible and invisible lasers not exceeding specified limits, which are 0.5W for CW lasers and 105 Jm-2 for pulsed lasers (less for ultraviolet wavelengths).Direct beam viewing not safe to the eye, specularly reflected beams may also be harmful to the eye, diffusely reflected beam usually safe to the eye, assumed to be safe to the skin.Class 4Those that exceed the limits of Class 3B!Viewing a direct beam or a reflected beam is always harmful to the eye and skin, diffusely reflected beams should be assumed harmful to the eye or skin unless proven otherwise, both scattered and reflected beams can present a fire hazard.
10Notes on Practical Laser Safety The general safety precautions fall under very simple headings.a) Use of a remote interlock connectorb) Key controlc) Beam stop or attenuatord) Warning signse) Beam pathsf) Specular reflectionsg) Eye protection
11Laser EyewearEyewear is the most common and certainly the most important aspect of personal laser protection, wherever there is some risk of laser exposure above the specified MPEs. Protective eyewear does not, however, preclude a full safety evaluation and consideration of all alternative means of affording protections - such as total enclosure of the beam, interlocks, beam dumps etc. Laser safety glasses are the last line of defence and not a convenient alternative to avoiding any engineering controls that it may be possible to implement.
12Procedure for Selection of Eye Protection Step 1:Determine wavelength of laser (l)Determine maximum exposure duration (t) anticipated for the use of eye protectionunintentional, accidental exposure to a visible beam where the maximum exposure may be of the order of 0.25 sec (aversion response).unintentional, accidental viewing of near IR laser beams for up to 10 sec.situations where occasional viewing of diffuse visible reflections for up to 600 sec is anticipated.4 to 8 hour occupational viewing of a diffuse reflection (generally from an invisible beam).
13Procedure for Selection of Eye Protection Step 2:Determine Maximum Permissible Exposure (MPE) for desired laserDetermine MPE from l, maximum exposure duration (t), and viewing conditions determined in Step 1. MPE will be in units of [J/cm2] for pulsed lasers and [W/cm2] for CW lasers. Example (calculation on the board):
15Procedure for Selection of Eye Protection Step 3:Determine the desired optical densityREMEMBER: MPE was determined in Step 2!Calculate Optical Density for a CW laser:Dl = Optical Density for CW laser= log10(H/MPE)Calculate Optical Density for a pulsed laser:Dl = Optical Density for pulsed laser= log10(E/MPE)
16Procedure for Selection of Eye Protection Step 4:Choose laser eye protection that meets the Optical Density requirements for the laserCompare the calculated requirements with manufacturer's specifications and find eyewear with an optical density value equal to or greater than the calculated value.Additional factors in choosing laser eyewearside-shield protectionperipheral vision requirementneed for prescription glassescomfort and fitdegradation of absorbing media (photo bleaching)strength of materialsanti-fogimpact requirements
18Limitations of Eye Protection GeneralIn general, eye protection will afford adequate protection against medium power, Class 3 lasers but will seldom provide sufficient protection against direct beam viewing of CW lasers exceeding 10 W in power or pulsed lasers exceeding 10 to 100 J in output energy. Obviously, for the higher power lasers, if a plastic frame or lens bursts into flames the wearer is going to move out of the beam path very rapidly. In these situations, the laser user should attempt to eliminate the need for eye protection when using such high power lasers by using engineering controls.Multiple WavelengthsOne pair of laser eyewear may not provide adequate protection from all multiple or tunable wavelengths produced by the laser. The laser user must be very conscious of which type of eye protection is appropriate for each different wavelength which may be used in the operation of the laser. It is the responsibility of the laser equipment supervisor to assure that the appropriate eyewear (for each wavelength) is provided for all users of the laser.
19Who’s who in laser safety: University laser safety adviserUniversity safety officerSchool/Dept. laser safety officerLaser lab/project supervisorLaser workers
20Breakdown of Responsibilities UniversitySafety OfficeTo keep a register of all laser users and all lasers.To carry out periodic checks on designated laser areas in departments and the records kept..To provide DLSO’s with adequate support in their roles.To provide yearly a training course for all new laser usersDLSOTo register new usersTo provide users with the CVCP Yellow BookTo carry out yearly audits of designated laser areasTo follow up on any problem areas identified in the auditsTo give advice on appropriate training for users where requested by either the user or a supervisor
21Breakdown of Responsibilities SupervisorsTo write a protocol for work to be carried out in any area where Class IIIR, Class IIIb and Class IV lasers are used.To provide adequate personal safety equipment for usersTo act promptly on the advice of the DLSO following an audit of the DLA(Undergraduates only) To have provide a copy of the ‘Approved Scheme of Work’ for a project(Postgraduate/post doctoral only) To have ensured that the Project Supervisory Requirements Form’ has been updated and carried entries of risk assessments associated with the use of lasers.
22Breakdown of Responsibilities - Individuals To complete the medical eye survey form if required.To view the laser safety videoTo read and have a working knowledge of the CVCP yellow book and to know the location of the laboratory copyTo understand access restrictions in designated laser areas and the operation of any laboratory door interlocksTo know the location and capabilities of laser safety equipmentTo calculate representative MPE figures for the system(s) being used(Undergraduates only) To have read, signed, and approved a copy of an ‘Approved Scheme of Work’ written by the supervisor for the project(Postgraduate/post doctoral only) To have ensured that the Project Supervisory Requirements Form’ has been updated and carried entries of risk assessments associated with the use of lasers.
24Requirements for all new research workers who will be registered laser users If the research worker arrives after the start of the academic year they must obtain the Laser Safety Video from DLSO and, as above, sign a statement indicating if they did understand the video and the documentation given to them.If at any time a research worker feels that they have not understood the laser safety protocol and the general recommendations outlined, or that they are unsure about these recommendations as they pertain to the designated laser area in which they work, they should approach their supervisor who will discuss with the DLSO what further training is appropriate for the situation.