Laser Safety Training Dr Katy Voisey University Of Nottingham Faculty of Engineering University Of Nottingham

Management 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.

• 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 at http://www.nottingham.ac.uk/safety/laser-index.htm • 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.

Laser Classification The hazard presented by a particular laser is reflected in its class. It is a legal requirement for suppliers to classify the lasers they sell. Classes 1(1M) – 2(2M) – 3R (formerly 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. http://www.aurpo.org/images/documents/guideance/aurpogn7.pdf

Laser Classification Safety of Laser Products – Part 14: A user’s guide, PD IEC/TR 60825‐14

Laser Classification Class 1: Safe – very low power or enclosed system Class 2: Low power (<= 1mW) visible lasers – protection afforded by natural aversion (blink response) Class 3R: Medium power (<= 5mW) visible lasers – as class 2, but intrabeam viewing via optical instruments may damage sight. Class 3B: Hazard from direct beam viewing and specular reflections. Class 4: Hazard from direct beam viewing and viewing of specular and diffuse reflections. Hazard to skin and fire hazard.

Who’s who in laser safety: University laser safety adviser University safety officer School/Dept. laser safety officer Laser lab/project supervisor Laser workers

Breakdown of Responsibilities University Safety Office To keep a register of 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 users DLSO To register new users To provide users with the CVCP Yellow Book/AURPO Guidelines To carry out yearly audits of designated laser areas To follow up on any problem areas identified in the audits To give advice on appropriate training for users where requested by either the user or a supervisor

Breakdown of Responsibilities Supervisors To write a protocol for work to be carried out in any area where Class 3R, Class 3b and Class 4 lasers are used. To provide adequate personal safety equipment for users To act promptly on the advice of the DLSO following an audit (Undergraduates only) To 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.

Breakdown of Responsibilities - Users To complete the medical eye survey form if required. To view the laser safety video To read and have a working knowledge of the CVCP yellow book/AURPO Guidelines and to know the location of the laboratory copy To understand access restrictions in designated laser areas and the operation of any laboratory door interlocks To know the location and capabilities of laser safety equipment To be aware of the 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.

laser film

Notes on Practical Laser Safety The general safety precautions fall under very simple headings. a) Use of a remote interlock connector b) Key control c) Beam stop or attenuator d) Warning signs e) Beam paths f) Specular reflections g) Eye protection

Laser Eyewear Eyewear 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.

Procedure for Selection of Eye Protection Step 1: Determine wavelength of laser (l) Determine maximum exposure duration (t) anticipated for the use of eye protection unintentional, 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).

Procedure for Selection of Eye Protection Step 2: Determine Maximum Permissible Exposure (MPE) for desired laser Determine 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.

Procedure for Selection of Eye Protection

Procedure for Selection of Eye Protection Step 3: Determine the desired optical density REMEMBER: 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)

Procedure for Selection of Eye Protection Step 4: Choose laser eye protection that meets the Optical Density requirements for the laser Compare 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 eyewear side-shield protection peripheral vision requirement need for prescription glasses comfort and fit degradation of absorbing media (photo bleaching) strength of materials anti-fog impact requirements

Limitations of Eye Protection General In 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 Wavelengths One 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.

Laser hazards in context Compare with looking directly at the sun: Solar radiation flux density at the surface of the earth ~ 1 kW/m2. If you stare at the sun (don’t do this), the pupil would contract to about 1 mm2. Therefore 1 mW of sunlight would enter the eye. For flux density at retina, use geometrical optics o r1 r2 i o=7x108m r1= 1.5x1011m r2= 2.5x10-2m i = (r2/r1)o ≈ 200 μm

Laser hazards in context Compare with looking directly at the sun: Solar radiation flux density at the surface of the earth ~ 1 kW/m2. If you stare at the sun (don’t do this), the pupil would contract to about 1 mm2. Therefore 1 mW of sunlight would enter the eye. For flux density at retina, use geometrical optics o r1 r2 i i = (r2/r1)o ≈ 200 μm

Therefore 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!