University Of Nottingham Laser Safety Training Prof Tony Kent University Of Nottingham 4th. October 2007.

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.

Laser hazards Compare 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 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!

• 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.

Laser Classification It 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):

Laser Classification

Laser 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 2 Visible 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.

Laser Classification Class 3R 302.5 – 106 nm Lower risk than 3B, but direct viewing of beam potentially hazardous Visible 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 3B Visible 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 4 Those 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.

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. Example (calculation on the board):

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.

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 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 users DLSO To register new users To provide users with the CVCP Yellow Book 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 IIIR, Class IIIb and Class IV lasers are used. To provide adequate personal safety equipment for users To 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.

Breakdown of Responsibilities - Individuals 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 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 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.

Requirements 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.