ICFO 2005

Laser Safety for Laboratory Users

Dr Mike Green D Phil (Oxon) F Inst P Member of BSI EPL/76 ‘laser products’,

PH2/3 ‘laser eye protection’ and IEC TC/76 ‘laser products’.

Pro Laser Consultants

Pro Laser Oxford House 100 Ock Street Abingdon Oxon OX14 5DH UK

= +44 1235 550 522 = +44 1235 550 499 email= mikeg@prolaser. co.uk

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__________________________________________________________________________________ 050329 2 SIRA laser safety

Contents Contents .................................................................................................................... 2

Overview................................................................................................................... 4

Legislation............................................................................................................. 4

Biological hazard....................................................................................................... 5

Radiation hazards in general .................................................................................. 5

Injuries to the eye .................................................................................................. 6

Effects of different wavelengths on the eyes .......................................................... 8

Safety calculations................................................................................................... 10

Maximum Permissible Exposure.......................................................................... 10

Assessing the level of laser exposure ................................................................... 11

Hazard distance ................................................................................................... 14

Nominal ocular hazard distance........................................................................ 14

Extended Nominal ocular hazard distance ........................................................ 15

The laser hazard classification scheme..................................................................... 17

Defining the problem ....................................................................................... 17

Laser-related Workplace Legislation........................................................................ 21

The Workplace Directive ..................................................................................... 21

The Provision and Use of Work Equipment Directive .......................................... 21

Personal Protective Equipment at Work Regulations............................................ 22

Associated Hazards.................................................................................................. 24

Indoor use of lasers.................................................................................................. 28

TR 60825-14 User Guide..................................................................................... 28

Administration of laser safety .......................................................................... 28

Control recommendations by Class .................................................................. 29

Control measures ............................................................................................. 32

Maintenance of safe operation.......................................................................... 36

Medical inspections ......................................................................................... 37

Personal protective equipment ................................................................................. 38

When to use eye protection .............................................................................. 38

How to choose protective eyewear ................................................................... 39

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Selecting eyewear using standards EN207 and EN208 ..................................... 40

Protective eyewear aftercare............................................................................. 41

Protective clothing ........................................................................................... 41

On-site laser maintenance and servicing .................................................................. 42

Increased risks during laser equipment servicing.................................................. 42

Temporary laser controlled areas ......................................................................... 43

Controls during servicing..................................................................................... 43

Visiting service engineers ................................................................................ 44

Fibre optic systems .................................................................................................. 45

Fibre Optic Communications Products................................................................. 45

Manufacturers requirements (subject to revision of 60825-2) ........................... 46

User’s Guide for fibre optic work......................................................................... 46

Live working practices......................................................................................... 48

Working codes for laboratory laser work ................................................................. 49

A R&D Working Code ........................................................................................ 49

Risk assessment ....................................................................................................... 51

General methodology........................................................................................... 51

Step 1 - Identify the potentially injurious situations.......................................... 51

Step 2 - Assess the risk for potentially injurious situations ............................... 51

Step 3 - Select control measures, and repeat from Step 1 process ..................... 52

Dealing with residual risks................................................................................... 53

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Overview Whilst the potential for injury exists, lasers have an excellent track record for safety. The risk of ocular injury has always been well appreciated and a cautious approach has been adopted in the development of laser safety standards. However, as laser technology develops, into such areas as high divergence semiconductor sources, laser diode arrays and femtosecond pulse lasers, the setting of safe exposure levels and control measures continues to be a challenge.

Legislation There are no specific legal requirements related to the use of lasers in Europe. The selection of appropriate safety measures is a matter for the employer, who is legally obliged under the Management of Health and Safety at Work Directive to adopt a risk assessment approach. In this context, the guidance provided in the laser safety standard TR60825-14 ‘Safety of laser products - Part 14 - A user’s guide’ has the status of a Highway Code. The CVCP laser working code ‘Safety in Universities - Notes of Guidance - Part 2:1 Lasers’ is a working code for laser use, a prescriptive implementation of the EN 60825-1 user guidance, but is currently in great need of updating.

The risk assessment approach apportions the risk of what are identified as ‘reasonably foreseeable’ injurious events and goes on to select ‘reasonably practicable’ precautions. Risk is a ‘product’ of two factors, the severity of injury and the probability of exposure. If the risk is above what is regarded as a tolerable level then a combination of engineering and administrative control measures must be introduced to reduce the risk. Personal protective equipment (e.g. protective eyewear) is used as a last resort if there remains a ‘residual risk’.

The simplest situation to deal with is where the laser product is Class 1 i.e. the laser light is properly guarded to prevent human access. In this case, the user need not be concerned with the laser light hazard during normal use of the product. However, if the embedded laser is Class 3B or Class 4, then the user may have to incorporate additional measures to address the laser light hazard during some on-site service activities.

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Biological hazard

Radiation hazards in general Over the wavelength range that different lasers operate, the skin is a strong absorber and thereby protecting all the organs of the body except for the eyes. For the eyes (but not for the skin) the wavelength of the radiation has a strong effect on the injury it can cause.

One can distinguish between two principal types of competing tissue damage mechanisms for radiation in the ultraviolet to visible: thermal and photochemical. When absorption of radiation occurs, the temperature of the absorbing tissue rises: if the temperature exceeds a temperature of approximately 60 °C then proteins in cells denature and the cell dies; if the temperature rises above 100 °C, water in the tissue begins to boil and further temperature increases lead to carbonisation of the tissue. This is the thermal damage process. It is characterised by a sharp threshold for injury and is non-cumulative in that if, over prolonged exposure, thermal injury does not occur within about 10s (during which the temperature of the exposed tissue will be rising to some steady-state value), then it will not occur at all.

For prolonged exposure to radiation at wavelengths in the ultraviolet and the blue end of the visible spectrum, a photochemical injury mechanism can dominate. The capability of radiation to initiate photochemical effects is strongly wavelength dependent and generally increases with decreasing wavelength. Chemical changes in the exposed tissue induced by this shorter wavelength radiation are cumulative, certainly over a working day, so the degree of damage depends on the radiant exposure (J m-2 on the exposed surface) i.e. the accumulated energy. Unlike the

WAVELENGTH

GAMMA X-RAY UV VISIBLE INFRA-RED MICROWAVE RADIOWAVE

0.1 µm 1000 µm

penetration penetration

Non-ionising radiation

thermal burn risk

Ionising radiation

cancer risk The wavelength of the radiation determines its biological effects

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threshold for thermal damage, which varies with wavelength only in response to the changes in the absorption depth in the tissue, the photochemical ‘action curve’ has a strong wavelength dependence and dominates only for prolonged or repeated exposures at levels which are too small to cause a sufficient temperature rise for thermal damage. For short time (less than 10s) single exposures, thermal damage will always be the more important.

Injuries to the eye The human eye is equipped to transmit electromagnetic radiation in the restricted wavelength range of 400 nm (blue light) through to 1400 nm (near infrared):

• through the cornea, which provides the primary means of focusing of the radiation;

• through the pupil at the centre of the iris, whose partial closure in response to bright light provides a degree of regulation;

• through the jelly-like lens, which under muscular control provides the remaining amount of focusing of the radiation;

• creating an image of the radiation source on the foveal region of the retina, where photoreceptors provide a signal to the brain from which the structure and colour of the image are interpreted.

Fovea

Retina

Choroid

Sclera

Visual axis

Symmetry axis

Optic nerve

Optic disc

Vitreous

Aqueous

Lens

Cornea

Iris

Conjunctiva

Ciliarybody

Pupil

The absorption of radiation incident on the eye has a strong and complex wavelength dependence. Various wavelength bands can be discerned over which the cornea, the anterior region (aqueous and lens), the posterior (vitreous) and the retina is at risk of injury. This is set out in the table below, which delineates the principal types and locations of injury according to spectral band and exposure duration. Note that

Schematic horizontal cross section of a human eye. Central sharp vision is only possible in the central part of the retina, the macula (yellow spot), and

particularly in the even smaller centre of the macula, the fovea. The main refractive power of the eye is provided by the cornea, while the lens is needed for adaption (imaging) of distant and close objects.

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damage to the cornea or lens can result in serious, though in principle correctable, vision loss. In the retinal hazard range injuries are not reversible.

Within the retinal hazard range (0.4 to 1.4 µm) a laser beam entering the eye and passing through the pupil can be transmitted focused to a spot of the order of 10 - 20 µm diameter on the retina, resulting in a concentration of up to 100 000 times (ratio of areas of a 7mm diameter pupil to a 20 µm image) over the exposure level at the cornea. In practice, factors during laser exposure including pupil size and accommodation of the eye (i.e. distance of focus), combined with involuntary eye movements and, in the wavelengths range of light (0.4 – 0.7 µm), aversion responses (including the blink reflex)), lessen the retinal exposure. Nevertheless, it remains the case that the maximum safe exposure (measured at the cornea) for, say, a 1ms duration is 100Wm-2 for visible radiation and 1MWm-2 at wavelengths just outside the retinal hazard range. Wavelength range Tissue

affected Single-pulse injury Long exposure several seconds or

more) CW and repetitively pulsed lasers

Ultraviolet

(180 nm to 400 nm)

Cornea lens Thermal damage dominates. Denaturation (clouding) of cornea and (0.28 – 0.4µm) lens.

Short pulses: Photoablation of corneal tissue with high power pulses.

Photochemical damage dominates.

Photokeratitis (‘arc-eye’ or ‘snow blindness’) of the cornea.

Photochemical cataract. (0.28 – 0.4µm)

Visible and near infrared

(400 nm to 1400 nm)

Retina Thermal damage dominates.

Burn (protein denaturation) with severe vision loss when damage in the foveal region.

Short pulses: photomechanical damage i.e. rupture of tissue, bleeding into inner eye.

400 nm – 550 nm (blue to green) Photochemical damage for exposure for more than several seconds.

Visible exposure for thermal damage normally limited to one second by aversion response to bright light. Eye movement reduces hazard for longer durations

Far Infrared

(1400 nm to 1 mm)

Cornea lens Thermal damage dominates.

Denaturation (clouding) of cornea and (1.4-1.9µm) lens.

Short pulses: photoablation of corneal tissue with high power pulses.

Exposure for thermal damage normally limited to a few seconds by reaction to pain due to heating of cornea.

Long term infrared cataract (1.4-3µm)

Summary of principal ocular injuries for ultraviolet to infrared radiation The cornea has a highly effective repair mechanism for minor injuries, but for the lens and retina the injury is permanent. Note that the injury mechanisms apply to both laser and non-laser sources, except for short pulse (typically q-switched lasers with pulse durations less than 1 µs) injury mechanisms, which are unique to lasers. devised with Dr Karl Schulmeister, Osterreichisches Forschungszentrum, Seibersdorf, Austria

The retina as sensitive only to wavelengths in the approximate range 0.4 - 0.7 µm whereas wavelengths out to 1.4 µm are transmitted. The near infrared (0.7 – 1.4 µm) reaches the retina but is not sensed directly, neither as light nor as pain, so damage can accumulate without the victim being aware of it at the time. As confirmed by accident statistics, the near infrared is the most hazardous wavelength range for lasers in general and near infrared pulsed lasers are the most hazardous sources.

For energetic laser pulses of duration less than 1 µs, rapid heating of the absorbing tissue can cause vaporisation, resulting in a micro explosion at the back of the eye, possibly leading to damage extending well beyond the irradiated area and

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haemorrhaging. The flow of blood and debris into the vitreous humour greatly increases the impairment of vision.

Effects of different wavelengths on the eyes

Ocular Exposure to Ultraviolet Radiation UV radiation can cause photochemical damage to the cornea. Arc-eye suffered by welders is an example of this injury, which can be very painful, but heals in about two days. Short wavelength UV is stopped by the cornea, longer wavelengths can reach as far as the lens, where cumulative exposure can produce a cataract. The 308 nm excimer laser is at about the worst wavelength for causing a photochemical cataract.

Ocular Exposure to Visible and Near Infrared Radiation In the 0.4 - 1.4 µm wavelength range the eye transmits, and can focus the radiation onto the retina. Short exposures to the sun and some powerful arc lamps can produce retinal burns, as can low power (several mW) laser beams in this wavelength region.

In the 0.4 - 0.7 µm wavelength range the retina senses the exposure, and attempts to regulate the level of exposure by a relatively slow partial closure of the iris, down to a pupil size of about 2 mm diameter, but no further. For exposure to potentially harmful levels of bright light the blink reflex is the eye’s natural aversion response.

Optical power density on retina

TV

100 W

Light bulb Sun1 mW laser

1 100 1,000,000 100,000,00010,000

Safe for prolonged viewing

Safe for accidental

viewing (1/4 second)

Comparison of magnitudes of retinal irradiance for visible radiation A person accidentally looking into a bright light, such as the sun, will blink and turn his head away in less than 1/4 second. The blink reflex of the eye does not, however, provide adequate defence against: near infrared radiation; laser pulses of less than 1/4s duration; very bright sources, for which even 1/4s is too long; purposeful staring (e.g. during solar eclipses)

Radiation in the 0.7 - 1.4 µm wavelength range, the near infrared, reaches the retina but the retina does not sense the radiation directly, neither as light nor as pain, and no damage can be done without the victim being aware. Many diode lasers and solid state lasers including the Nd:YAG laser operate in this most hazardous wavelength range.

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0 500 1000 1500 nm

Fraction of incident power reaching retina

Retinal response

blu

e

green

yello

woran

ge

red invisible retinal

hazard

lin

ear

scal

e

The eye-transmitting wavelength band and the limited range of retinal response

Ocular Exposure to Medium and Far Infrared Radiation Radiation of wavelength longer than 1.4 µm but less than 4 mm is fully absorbed in the volume of the anterior of the eye. At longer wavelengths the cornea is a strong absorber and thermal burns are a possibility. For severe injuries, a corneal graft may be necessary.

Pulsed laser injuries The one exception, all the injuries addressed above can be inflicted by conventional radiation sources as well as by lasers. The exception is the thermo-mechanical injury from exposure to high power laser pulses of duration 1 µs or less. If such pulses contain sufficient energy, the rapid heating of the absorbing tissue causes vaporisation and creates strong mechanical forces. Retinal injuries of this type are characterised by the victim hearing a ‘popping sound’ caused by a micro explosion at the back of the eye. Such injuries cause damage over an area of tissue much greater than that irradiated by the laser and the flow of blood and debris into the vitreous humour greatly increases the impairment of vision.

Injuries to the skin Everyone has experienced skin damage, ranging from photochemical damage by UV from the sun (sunburn) to thermal burns to the legs from sitting too close to electric bar fires, where the damage is caused by infrared radiation.

The very thin (10-20 µm thick) outer layer of dead cells (the epidermis) strongly absorbs far infrared wavelengths, such as that emitted from a CO2 laser. Similarly, in the ultraviolet this outer layer of dead cells absorbs virtually all the radiation. At intermediate wavelengths the penetration increases, peaking at 0.7 µm (red), but is always less than 1 mm. Skin cancer can be caused by over exposure to UV radiation, including sun radiation.

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Safety calculations

Maximum Permissible Exposure One of the principal aims of a laser safety programme is to ensure that any exposure to laser radiation that might occur is within safe limits. It is therefore often necessary to assess the maximum level of exposure that could arise under all foreseeable conditions and then to relate this to the maximum permissible exposure (MPE). The Maximum Permissible Exposure (MPE) can be thought of as setting the boundary between safe and potentially harmful level of exposure or irradiance. However, it is not to be thought of as a sharp boundary: MPE values are derived primarily from animal experiments in which a number of exposures are delivered to the eye or the skin for some fixed set of conditions (laser wavelength, exposure duration, spot size) and examined afterwards for detectable lesions. For obvious reasons, the exposed site can only be exposed once and therefore a range of sites per animal and also a number of animals have to be exposed. As a result, and in combination with experimental uncertainties, there is not a sharply defined threshold exposure value below which no lesions are found and above which all exposures lead to damage. Values of MPE are given for eye and skin exposure in EN 60825-1 and (identical) TR 60825-14. They are expressed as functions of the laser emission wavelength and exposure duration. The International Commission on Non-Ionising Radiation Protection (ICNIRP) develops these values. They are set below known damage thresholds and are based on the best available information. The MPE values should be used as guides in the control of exposure and should not be regarded as precisely defined dividing lines between safe and hazardous levels. Because exposure to laser radiation below the MPE can still be uncomfortable in certain circumstances and may cause secondary hazards (as explained below), exposure should in any case be kept as low as reasonably practicable. The exposure dose at which 50 % of the exposures lead to a lesion, ED-50, is generally referred to as the threshold, even though there is a finite probability for damage at energies somewhat below the ED-50. The MPE is set well below the ED-50, often a factor between by a factor of 10. However, where there is less variability and small experimental uncertainty, such as for corneal photochemical injury in the UV range, a safety factor less than 10 has been agreed by the international committee. To cover bands of wavelength and pulse duration, MPE values are expressed as single values or by a simple analytical expression and as a result safety margins greater than 10 are often encountered.

MPEs vary widely with both wavelength and exposure duration and the same values are broadly accepted world-wide. MPE values are provided for:

exposure of the skin and eyes

wavelengths from 0.18 µm (UV) to 1 mm (onset of microwaves)

exposures from less than 1 ps to 30,000 s (about 8 hours) duration.

‘point’ and ‘extended’ laser sources

exposure to ‘trains’ of pulses

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simultaneous exposure to more than one wavelength

Other points to bear in mind in this context are:

MPEs for ocular exposure in the eye-transmission range include assumptions about the accommodation of the eye (i.e. that the lens images the light onto the retina) and the size of the pupil (i.e. that the pupil is fully dilated to 7 mm).

MPEs take no account of the severity of injury. For example, exceeding the MPE by around ten times for a skin exposure may cause a mild reddening which soon heals, whereas the same exposure to the eye may be a permanent retinal burn.

MPEs relate only to acute injuries. Chronic (i.e. long term, cumulative, multiple exposure) effects such as cataracts and the blue light hazard are not included.

Ocular MPEs include a factor for the image size, to take into account direct viewing of very poor quality laser beams or diffuse reflections.

An MPE has associated with it a limiting aperture, a circular area over which the power or energy in the beam is averaged. An MPE of 25 Wm-2 with a limiting aperture of 7 mm (this figure depends on wavelength, exposure duration and whether the MPE is for skin or eye) means 1 mW through a 7 mm diameter circle.

Assessing the level of laser exposure An assessment of laser exposure may be needed in order to determine the boundary of the laser hazard zone, or to specify the level of protection that is necessary (for example, with the use of laser protective eyewear or protective viewing windows). Measurement, calculation or both can be used for this assessment. It is generally straightforward to do a first order calculation of exposure under foreseeable conditions assessment using the maximum output data for the laser source supplied by the manufacturer; for greater accuracy a measurement is generally required. On the other hand, measurements can be hazardous operations, there are many pitfalls in conducting and interpreting the data, and it can be difficult to be sure that the measurement does indeed represent the maximum level of exposure under all foreseeable conditions.

The effective exposure The level of human exposure arising from a laser product should be determined at the positions at which it is reasonably foreseeable that a person might be located and where the highest levels of exposure can occur. This evaluation should take into account all reasonably foreseeable conditions of direct beam emission and beam reflection. For CW (continuous wave) lasers, the exposure will normally be expressed in terms of the incident irradiance (Wm-2). With pulsed lasers, both the average irradiance (Wm-2) and the radiant exposure due to a single pulse (Jm-2) will usually need to be

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known. In all cases, the measurement must be made through the specified limiting aperture as given below:

Aperture diameter for Spectral region nm Eye mm Skin mm

180 to 400 1 3.5 ≥400 to 1400 7 3.5 ≥1400 to 105 1 for t ≤ 0.35 s

1.5 t 3/8 for 0.35 s < t < 10 s 3.5 for t = 10 s

3.5

105 to 106 11 11 The diameter of the limiting aperture applicable to measurements of irradiance and radiant exposure (t is the exposure duration)

Note that the irradiance (Wm-2) or radiant exposure (Jm-2) must be calculated by measuring the power or energy through the appropriate limiting aperture and then dividing the measured value by the area of the limiting aperture and not by the actual area of the beam. There is also a special procedure for dealing with large (extended) laser sources These considerations can mean that the value of the applicable exposure (called the effective exposure) that must be used for comparison with the MPE may not be the same as the exposure that would actually arise. The main parameters that may be needed for exposure assessment are:

emission wavelength; beam dimensions at laser output; beam divergence and position of the beam waist; beam profile (power or energy distribution across the beam); maximum reasonably foreseeable exposure duration; minimum reasonably foreseeable exposure distance; angular subtense of apparent source (this is usually only needed for laser

arrays and for the assessment of diffuse, i.e. non-specular, beam reflections); for scanning beams, the scanning characteristics and scan geometry.

In addition, for continuous (CW) emission: beam power;

and for pulsed emission: pulse energy; pulse duration; pulse repetition frequency; pulse shape and pulse distribution in time (if complex).

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Angle of acceptance for the assessment of exposure from extended sources The majority of single lasers represent “small” sources, since the angular subtense of the apparent source is less than αmin (1.5 mrad). Where the emission from such sources is within the retinal hazard region (i.e. between 400 nm and 1 400 nm), the eye can focus it to form an effective point image on the retina. This is not possible with larger apparent sources (often called extended sources), which, therefore, for a given level of exposure at the surface the eye, may be less hazardous. Extended-source exposure conditions may be applicable to diffuse reflections, laser arrays or laser products employing a diffuser, when these are viewed at a sufficiently close distance. When determining the level of the effective exposure arising from an extended laser source (that is, any source subtending more than 1.5 mrad at the position at which the exposure is being assessed), there are stipulated angles of acceptance that should be used: any contribution to the exposure that is due to the source’s emission arising from outside the angle of acceptance should be excluded from the assessment of the effective exposure. The angular subtense of the apparent source is measured at the distance at which the exposure is being assessed, but not at a distance less than 100 mm.

a) For the determination of the level of exposure to be evaluated against photochemical MPEs (400 nm to 600 nm), the limiting angle of acceptance .γ � is

• for 10 s < t ≤ 100 s: γ � = 11 mrad • for 100 s < t ≤ 104 s: γ � = 1.1 t0.5 mrad • for 104 s < t ≤ 3 104 s: γ � = 110 mrad

If the angular subtense of the source α is larger than the specified limiting angle of acceptance γ � , the angle of acceptance should not be larger than the values specified for γ � . If the angular subtense α of the source is smaller than the specified limiting angle of acceptance γ � , the angle of acceptance should fully encompass the source under consideration but otherwise need not be well defined (i.e. the angle of acceptance need not be restricted to γ � ).

b) For the determination of the level of exposure to be evaluated against all MPEs other than the retinal photochemical hazard limit, the angle of acceptance should fully encompass the source under consideration (i.e. the angle of acceptance shall be at least as large as the angular subtense of the source α). However, if α > αmax, in the wavelength range of 302.5 nm to 4000 nm, the limiting angle of acceptance should not be larger than αmax (0,1 rad) for the thermal hazard limits. Within the wavelength range of 400 nm to 1 400 nm for thermal hazard limits, for the evaluation of an apparent source which consists of multiple points, the angle of acceptance shall be in the range of αmin ≤ γ ≤ αmax.

For the determination of the MPE for non-circular sources, the value of the angular subtense of a rectangular or linear source is determined by the arithmetic mean of the two angular dimensions of the source. Any angular dimension that is greater than αmax or less than αmin should be limited to αmax or αmin respectively, prior to calculating the mean. The retinal photochemical MPEs do not depend on the

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angular subtense of the source, and the exposure is determined using the angle of acceptance specified above.

Hazard distance Laser beams normally have a low divergence, a narrow "pencil like" spread of angles. In such cases, the hazard to eyes and skin is approximately independent of distance. This is in sharp contrast to conventional sources, where the skin and corneal hazards decrease rapidly with distance.

Knowledge of hazard distance can be especially useful in the case of divergent-beam lasers, where the hazard distance can be relatively short and the hazard therefore limited to the immediate vicinity of the laser aperture. It can also be important for collimated beams from lasers that are used over long distances, such as out-of-doors, where hazard distances can be considerable. Particular care needs to be taken with the out-door use of collimated-beam Class 1M and Class 2M laser products: although these lasers present no hazard to the unaided eye, the distance over which the use of magnifying viewing aids could be hazardous may be very large. If the beam extends into public areas it cannot be assumed that magnifying aids such as binoculars will not be used. As a laser beam travels it expands, perhaps after having first passed through a focus. At a sufficiently great distance from the laser output, the effect of this expansion (aided by absorption and scattering in the medium that the beam is travelling through) is that the MPE is not exceeded in the beam. The closest distance where this occurs is the Nominal Ocular Hazard Distance (NOHD). Another term sometimes used is the Extended Nominal Ocular Hazard Distance (ENOHD), the point at which the level is at the Class 1 AEL. From this point onwards, about 7x NOHD from the laser, the beam is deemed safe to use with optical viewing aids.

Nominal ocular hazard distance The distance at which the level of exposure has dropped to the level of the MPE (for the eye) is known as the nominal ocular hazard distance (NOHD). Beyond this distance there is no hazard to the unaided eye, although there may be a hazard if magnifying viewing aids are used. A simple ‘back of the envelope’ calculation of NOHD is given below. Simple calculation of NOHD Step 1 Decide on the acceptable level of exposure laser radiation (generally the MPE).

Step 2 Calculate the area over which the laser's power or pulse energy would have to be uniformly spread for the power or energy density to be at the level derived in Step 1.

Step 3 Measure or estimate the full angle beam divergence and calculate how far the laser beam would have to travel before it had expanded to the safe area estimated in Step 2.

Step 4 If appropriate, allow for any non-uniformity over beam diameter by multiplying the value in Step 3:

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• If the beam diameter is specified at the '1/e2' points*, multiply hazard distance by 1.4.

• If it is suspected that there are ‘hot spots’ in the beam, multiply hazard distance by 1.6.

For extended sources the calculation is more complicated. At Step 1 it may well be best to ignore the for angular subtense and then confirm is this simplification is valid after a value is arrived at in Step 4. If the NOHD is so small that the C6 correction factor needs to be taken into account than the MPE must be expressed as a algebraic function of distance (using the fact that α is inversely proportional to distance) and then solve the quadratic equation at step 3. * For circular beams having an approximately Gaussian profile, the on-axis value is equal to the total beam power or energy divided by the area of the beam determined on the basis of its 1/e diameter. This area contains 63 % of the total beam power or energy. The 1/e diameter is the diameter at which the beam irradiance, radiant exposure or radiant intensity has decreased to 1/e, or 0.37, of the peak, on-axis value. In many cases, however, the manufacturer will specify the diameter of the beam in terms of the 1/e2 value. The 1/e2 diameter is equal to the 1/e diameter multiplied by 1.4.

Extended Nominal ocular hazard distance To take account of the possible use of magnifying aids, where this is reasonably foreseeable, the extended nominal ocular hazard distance can be used. This distance is determined on the basis of the increase in exposure (at the surface of the eye, within the relevant limiting aperture) that could arise through the use of magnifying instruments. The extended nominal ocular hazard distance (ENOHD) is therefore that distance beyond which magnifying instruments can be safely used. In this regard, TR60825-14 provides the following useful information: Use of binoculars If a laser source is viewed through binoculars, then the increase in the effective exposure at the surface of the eye will be the smaller of either M2 or (D/d)2, where M is the angular magnification of the binoculars, D is the diameter of the objective (i.e. outer) lenses and d is the diameter of the relevant limiting aperture. (Binoculars are normally specified in the form M x D, e.g., 7 x 50.) Allowance can be made for transmission losses through the binoculars at the laser wavelength. Typical transmission percentages for binoculars are: Wavelength % Transmission 0.18-0.302 µm <2% 0.302-0.4 µm 70% 0.4-0.7 µm 90% 0.7-2 .8 µm 70% 2.8-103 µm <2% The angular subtense of an extended source viewed though the binoculars will be increased by a factor M. Both the NOHD and ENOHD depend critically on the beam geometry as well as on the magnitude of the laser output. It can be possible, for example, to refocus or

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collimate the beam, even by means of an optical component positioned some distance from the source, and thereby increase both the NOHD and ENOHD. In some applications it can be useful to determine the skin-hazard distance in an analogous manner to NOHD. Nominal ocular hazard area From knowledge of the NOHD and ENOHD, and of the way in which the laser is positioned and secured, and also of the circumstances of its use, it is possible to define an area or 3- dimensional space around the laser aperture within which exposure hazards can arise. This region, the hazard zone, is called the nominal ocular hazard area (NOHA) if it is based on the criterion for the NOHD, or the extended nominal ocular hazard area (ENOHA) if it is based on the ENOHD. Because of the possible use of magnifying aids by people unconnected with the laser operation, especially where lasers are used out-of-doors, it is important to recognise that the laser hazard can extend over the full area of the ENOHA, and not just that of the NOHA. For outdoor applications, if the beam is terminated by the ground, a tree-line or other terrain features, the NOHD can not exceed the line of sight to this opaque feature. Provided that access into the ENOHA can be restricted and reliably controlled, however, it is not always necessary to enclose the hazard area.

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The laser hazard classification scheme

Defining the problem Without specifying how a laser is to be used, what is its maximum safe level of output? This question is the of defining the maximum output of a safe (Class 1) laser.

The safety standard stipulates 100s is to be taken for assessing exposure appropriate to Class 1 ‘unless intentional viewing is inherent in the design or function of the laser products’, in which case the time period is 30,000s. With this in mind, an appropriate MPE can be deduced. This value (in Wm-2 or Jm-2), multiplied by the area of the corresponding limiting aperture, gives a value (in Watts or Joules, respectively) which is referred to as the Class 1 Accessible Emission Limit (AEL).

The safety standard goes on to specify the conditions of measurement, addressing in particular the measurement distance, the collection aperture and location of the detector; and the state of the laser (including worst single fault conditions).

In essence, the classification scheme divides laser products into four major classes: Class 1: No risk to eyes or skin Lasers that are safe in normal operations under reasonably foreseeable conditions, including direct intrabeam viewing. Class 2: Low risk to eyes. No risk to skin Lasers emitting visible radiation in the wavelength range 400 – 700 nm for which the natural aversion response to bright light (including the blink reflex) prevents retinal injury for direct intrabeam viewing. These lasers do, however, present a dazzle hazard. Class 3: Medium risk to eyes. Low risk to skin Lasers for which intrabeam viewing is hazardous, but for which the viewing of diffuse reflections is normally safe. Natural aversion response to localised heating prevents serious skin injury. Class 4: High risk to eyes and skin Lasers for which intrabeam viewing and skin exposure is hazardous and for which the viewing of diffuse reflections may be hazardous. These lasers are also a fire hazard.

Each Class has associated with it an Accessible Emission Limit, generally expressed in Watts and Joules, except Class 4 for which there is no upper limit. The AEL is a maximum level of ‘accessible’ laser radiation for the class, according to prescribed measurement conditions and measurement detector positions and collection aperture sizes. The measurement procedures are intended to take account of many of the worst-case assumptions of exposure conditions for which the product could be used i.e. exposure duration, closeness of viewing and the use of optical instruments.

The current internationally approved classification scheme represents a major refinement. It includes relaxed forms of Class 1, 2 and 3, taking into account both large beam diameter and high beam divergence sources.

The table below summarises the current classification scheme. A sub division of classes has been included to highlight how, on specifics of warning signs,

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manufacturer-installed safety measures and user guidance, a total 7 classes and 11 ‘sub-classes’ can now be identified from the primary four.

Included in the table is a listing of the general manufacturers’ requirements, Section 2 of EN60825-1. There are additional requirements for certain classes of product including medical laser equipment, optical fibre communication equipment and machines that use lasers to process materials.

Protective housing All classes of laser product are required to have a protective housing to limit human access to levels of laser radiation below the AEL assigned to the product. This is the primary engineered safety feature on a laser product and its design becomes particularly important for Class 1 (embedded) laser products, particularly those enclosing Class 4 laser sources.

Laser safety standards include requirements for labelling on access and service panels that form part of the protective housing and provide access to hazardous levels of laser radiation and, in addition, interlocking for such panels that are intended to be removed during normal use or maintenance.

Thin walled enclosures will “keep fingers out” and block any weakly scattered laser radiation but insofar as they are incapable of withstanding a high power laser beam they rely heavily on the maintaining the direction of the beam path. Regular alignment checks and the maintenance of optics and the environment (e.g. vibration, humidity, temperature, dust) can therefore be important safety considerations in the context of high power laser beams. Indeed, the subject of laser guards, a ‘machine’ term but essentially simply a component of a protective housing, is the subject of a separate standard [IEC 60825-4: Laser guards].

Class 1 laser products are safe under reasonably foreseeable circumstances, either because their output of the laser is so low or because of engineering safeguards. Many commercial laser systems that are sold as Class 1 products contain higher power laser products, where the laser radiation is completely contained. Such a laser is referred to as an embedded laser, and is Class 1 by engineering design. The main benefit of making a product Class 1 is that it can be used without implementing laser safety precautions. Laser Printers and CD players are examples of such devices.

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Qualitative description of Laser Safety Classes devised with Dr Karl Schulmeister, Osterreichisches Forschungszentrum, Seibersdorf, Austria

Class Sub division Meaning Warning label and safety features Intrinsic

Safe by virtue of the intrinsic low power of the laser, even with the use of optical instruments

No warning label. No additional safety features

Class 1

Engineering Embedded laser products, safe by virtue of engineering controls e.g. total enclosure guarding, scan failure mechanism.

No warning label. No additional safety features (but see requirements for protective housings in 6.4.3)

Collimated Well collimated beam, output in range 302.5 – 4000 nm, with large diameter that is safe for unaided viewing but potentially hazardous when a telescope or binoculars are used.

LASER RADIATION DO NOT VIEW DIRECTLY WITH BIONOCULARS OR TELESCOPES* No additional safety features

Class 1M

High divergence Output in range 302.5 – 4000 nm, Safe for unaided viewing but potentially hazardous when an eye loupe or magnifier is used.

LASER RADIATION DO NOT VIEW DIRECTLY WITH MAGNIFIERS* No additional safety features

Class 2 - Output in range 400 – 700 nm. Safe for unintended exposure, even with the use of optical instruments, by virtue of natural aversion response to bright light

DO NOT STARE INTO THE BEAM No additional safety features

Collimated Well collimated beam in range 400 – 700 n, with large diameter that is safe for unaided viewing by virtue of natural aversion response to bright light but potentially hazardous when a telescope or binoculars are used.

LASER RADIATION DO NOT STARE INTO THE BEAM OR VIEW DIRECTLY WITH BIONOCULARS OR TELESCOPES* No additional safety features

Class 2M

High divergence High divergence source with output in range 400 – 700 nm. Safe for unaided viewing by virtue of natural aversion response to bright light but potentially hazardous when an eye loupe or magnifier is used.

LASER RADIATION DO NOT STARE INTO THE BEAM OR VIEW DIRECTLY WITH MAGNIFIERS* No additional safety features

Class 3R Visible Output in range 400 – 700 nm. Direct intrabeam viewing is potentially hazardous, but by virtue of natural aversion response to bright light the risk is lower than for Class 3B.

AVOID DIRECT EYE EXPOSURE No additional safety features

Non-visible Output in UV (180 – 400 nm) or IR (700 nm - 1 mm). Direct intrabeam viewing is potentially hazardous, but the risk is lower than for Class 3B.

AVOID DIRECT EYE EXPOSURE (700 – 1400 nm) AVOID EXPOSURE TO THE BEAM (outside the range 400 – 1400nm) Emission warning device.

Class 3B - Medium power laser. Direct ocular exposure is hazardous, even taking into account aversion responses, but diffuse reflections are usually safe.

AVOID EXPOSURE TO THE BEAM Key switch, emission warning device, externaπl interlock connection and beam stop

Class 4 - High power laser. Direct exposure is hazardous to eye and diffuse reflection may also be hazardous. Skin and potential fire hazard.

AVOID EYE OR SKIN EXPOSURE TO DIRECT OR SCATTERED RADIATION Safety features as for 3B

* The general phrase ‘optical instruments’ can be used in place of ‘binoculars or telescopes’ or ‘magnifiers’. However, generally only one or other group of optical instruments leads to an increase in the hazard for a given laser product. Therefore, at the discretion of the manufacturer, a specific wording can be added to the warning label.

Visible emission from lasers can cause disturbing and potentially dangerous dazzle effects at exposure levels that are well below the maximum permissible exposure and which therefore cause no direct physiological injury. This is especially so with laser classes 2, 2M and 3R (includes laser pointers and low-power alignment lasers). These

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should not therefore be directed, whether intentionally or unintentionally, at a person's eyes. This can startle and distract the exposed person, and can cause them to lose concentration, with particularly serious consequences if the person is performing a safety-critical task, such as driving or controlling machinery. It can also produce disturbing after-images, generate fear, and induce reactions such as watering eyes and headaches if the person believes that they might have suffered injury as a consequence of exposure. Persistent rubbing of the eyes in response to a perceived injury can also result in painful corneal abrasions.

The laser hazard classification scheme has a number of limitations. In particular, the scheme relates only to the safety of the product in regard to laser radiation emitted during normal and maintenance operations. For example, embedded laser products can operate in a higher classification during maintenance and servicing operations; and, of course, to say a laser is 'Class 1' says nothing about the non-laser hazards of the product. Furthermore, the laser hazard classification scheme is only a crude guide to safe laser use. It takes no account of the variations in severity of injury with wavelengths and, within any particular Class but especially within Class 4, output power.

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Laser-related Workplace Legislation

The Workplace Directive This Directive applies to all permanent workplaces. I believe that this extends previous legislation by covering research establishments, schools and colleges etc., which were previously not covered by industrial legislation. The Directive lays down requirements and demands that minimum health and safety requirements are implemented. These requirements cover a wide range of workplace topics and include:

• Definition of escape routes; • a requirements for cleanliness; • that maintenance of the workplace shall be carried out; • that safety equipment shall be maintained and checked; • minimum safety and health requirements be established for

• workplace structural stability; • electrical installation; • emergency routes and exits; • fire precautions; • ventilation and temperature; • lighting; • minimum requirements for doors and gates; • that dangerous areas are defined and minimized; • provision of rest rooms, rest area, changing rooms and washing facilities; • toilets are provided; • first-aid equipment is provided.

The Provision and Use of Work Equipment Directive This Directive, often known as PUWE78, is applicable virtually everywhere that work is done. It very clearly makes the employer have a legal (as well as any moral) obligation for health and safety of workers and others in the workplace.

The Directive requires that equipment and machinery is

• suitable for the task envisaged; • can be used without risk to health and safety • is adequately maintained to ensure an acceptably low level of risk to users. Thus there is a requirement to carry out risk assessments of workplace activities and then to eliminate or reduce to an acceptable level all risks associated with the work activity. Control systems must be safe, breakage and damage must not result in danger, protection is required against rupture or disintegration, preventing fire or overheating, discharges of gas, dust, liquid, vapour or other substances, explosion, contact with electricity and equipment must be appropriate for use. The use of personal protective equipment is only to be considered as a last resort. This Directive thus places the responsibility on an Employer not to purchase or put into service equipment which is not in conformity with relevant product Directives i.e. CE marked by the manufacturer or supplier.

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Overview Ever since the invention of the laser in 1960 the risk posed by laser injury to the eye has been well appreciated. As a result, awareness of laser safety has in general been high, laser standards have adopted a conservative approach to safety; and lasers have maintained an excellent track record for safety. Nevertheless, the pace of development of laser technology and applications remains high and continually presents new safety challenges; certainly, there is no room for complacency.

Legislation for users There are no specific legal requirements related to the use of lasers in Europe. The selection of appropriate safety measures is a matter for the employer, who is legally obliged under the Management of Health and Safety at Work Directive to adopt a risk assessment approach. In this context, the guidance provided in the international laser safety technical report TR 60825-14 ‘Safety of laser products - Part 14 – A user’s guide has the status of a Highway Code. It is universally recognised as defining best practice in laser safety. In addition to this general document, there is more specific guidance available for some laser user ‘groups’, including HSE guidance document HS(G)95 ‘The radiation safety of lasers used for display purposes’ (also 60825-3, a technical report addressing the same subject internationally), a laser working code issued by the Committee of Vive-Chancellors and Principals of Universities in the UK ‘Safety in Universities - Notes of Guidance - Part 2:1 Lasers’, and 60825-8, a technical report, ‘Guidelines for the safe use of medical laser equipment’; all of which offer a sub-set of laser users an interpretation of the EN 60825-1user guidance. There is no widely accepted guidance document for industrial laser users.

The risk assessment approach apportions the risk of what are identified as ‘reasonably foreseeable’ injurious events and goes on to select ‘reasonably practicable’ precautions. Risk is a ‘product’ of two factors, the severity of injury and the probability of exposure. If the risk is above what is regarded as a tolerable level then a combination of engineering and administrative control measures must be introduced to reduce the risk. Personal protective equipment (e.g. protective eyewear) is used as a last resort if there remains a ‘residual risk’.

Personal Protective Equipment at Work Regulations Legislation requires all employers to following protecting the health and safety of their employees. The main requirement is for employers to assess workplace risks and introduce appropriate preventative measures. In summary these principles are:

avoid risks; evaluate the risks which cannot be avoided; combat risks at source; give collective protective measures; provide individual protective measures, such as PPE

PPE thus comes into effect only when the risks cannot be avoided or sufficiently limited by collective protection, such as engineering controls or systems of work. The PPE Directive covers all equipment designed to be worn or carried to protect against

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one or more safety or health hazard. In using the PPE Directive, the employer is required to:

assess the risks; ensure that the PPE is suitable for the worker; select the PPE which

• gives protection when fitted correctly; • complies with the PPE Product Directive;

provide the PPE free of charge; maintain PPE in clean, good working order; provide information, instruction and training on the use of PPE; involve workers in the selection of the PPE

Where personal protective equipment has been deemed to be an appropriate method of risk reduction, its use should be compulsory. PPE should ideally be issued on a person-by-person basis, and for hygiene reasons should be properly cleaned by an appropriate method before reuse by another person. Special requirements apply in Europe covering the specification, marking and testing of laser eye protection, using the concept of protective (rather than optical) density, which takes into account the ability of the protection to withstand the incident laser radiation (ref EN 207 and EN 208).

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Associated Hazards In addition to the laser radiation hazard, there are other general hazards associated with laser use. These so-called ‘non-beam’ or ‘associated’ hazards do not affect the laser classification, and so may be present with even Class 1 laser products. Some, such as electric shock, can be life-threatening. The control of associated hazards should normally be addressed by the manufacturer through appropriate design of the equipment and by written instructions for safe use supplied by the manufacturer to the user. Nevertheless, where such hazards cannot reasonably be completely eliminated through engineering design (as in the case of fume), or where the laser is being used for a purpose or in a manner other than that intended by the manufacturer, some responsibility for the control of these hazards will fall upon the user. Hazards can be divided into three categories: those generally associated with the laser source, those associated with the laser application and those associated with the laser environment. The tables below summarises the main laser-related hazards and control measures.

Users should take all reasonable steps to investigate and ensure adequate protection from all hazards that may arise from their own use of laser equipment. Advice from Competent Persons who are experienced in areas other than laser radiation safety may be beneficial.

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Hazards Arising from the laser source

Hazard Typical hazardous situation Typical control measures

High voltage Laser head and power supply exposed during servicing

Many lasers utilise high voltages, and pulsed lasers frequently employ capacitors that can store significant amounts of electric charge.

Proper screening of exposed HV. Restricted access to qualified persons.

Use of earthing stick to ensure the removal of stored energy prior to commencement of servicing work.

Fore and Explosion

Laser equipment can present a fire hazard by virtue of the flammable components, plastic parts etc. contained within it, which can overheat or catch fire in the event of a fault within the equipment.

During changes of high pressure flashlamps in optically-pumped lasers, and other internal components such as capacitor banks, can explode.

For fire: provision of a fire extinguisher; smoke alarm; training.

For explosion: gloves and face shield; training.

Noise The discharge of capacitor banks within the laser power supply can generate noise levels high enough to cause ear damage. Ultrasonic emissions and repetitive noise from pulsed lasers can also be harmful. Some air-cooled lasers produce significant noise levels.

Use of ear protectors where excessive noise levels cannot be eliminated.

Collateral radiation

Ultraviolet, visible and infrared emission can be produced from gas laser discharge tubes.

Microwave and radio frequency radiation is produced in RF-excited lasers, and can be emitted by the equipment if not properly shielded e.g. .during servicing.

X-rays can be produced by high-voltage thermionic valves within the laser power supply.

Proper screening combined with access restricted to service engineers.

Mechanical Unloading and positioning of laser power supplies and ancillary items such as gas cylinders.

Trailing cables and water-circulation tubing can present a trip hazard.

Provide attachment points for use of lifting equipment by qualified persons. Training and use of gloves.

Properly secure equipment. Cover cables in ducting or under a raised walkway.

Chemical The material used as the active medium in many lasers (especially laser dyes and the gases used in excimer lasers) can be toxic and carcinogenic.

The solvents used in many dye lasers have the ability to carry their solutes through the skin into the body. They may also be highly volatile and should not be inhaled.

The liquids used in some optically active components (e.g., for Q-switching and frequency-doubling

Proper storage, handling and disposal precautions should be adopted.

Training and use of gloves and other PPE.

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Hazards Arising from the laser process Hazard Typical hazardous situation Typical control measures

Fume Release of hazardous particulate and gaseous by-products into the atmosphere through the interaction of the laser beam with the target material during laser materials processing and laser surgery*.

General precautions include:

Fume extraction and filtration. Face mask and gloves worn during cleaning operations.

Hazardous substances

Cleaning solutions and also other materials used in conjunction with the laser (e.g., zinc selenide lenses) may be hazardous..

Use proper storage and implement handling and disposal precautions.

Fire and explosion

The laser emission from high-power (Class 4) lasers can ignite target materials. These effects are enhanced in the oxygen-rich environment utilised in some laser processing applications.

Laser emission from even lower-class lasers, especially when concentrated over very small areas, can cause explosions in combustible gases or in high concentrations of airborne dust. (Power levels above 35 mW emerging from a single mode optical fibre can be sufficient to cause combustion in such environments.)

Improperly terminated Class 4 laser radiation

Control flammable materials and beam path. Provide a fire extinguisher in the laser area.

Secondary radiation

X-ray, UV and blue light emitted by the plasma that can be generated by interaction of the laser beam (particularly those containing short, high power pulses) with target materials. Such emissions can include x-rays, ultraviolet radiation (UV), visible light, infrared radiation (IR), microwave radiation and radio-frequency (RF) radiation.

Enclose target area and monitor hazard.

Wear PPE for exposure to UV and blue light.

Mechanical Beam delivery arms and robotic systems that move under remote control can cause serious injury. Large work-pieces (such as sheet metal) can present manual handling problems such as cuts, strain, and crush injuries.

Guard traps and add warning signs.

Restrict access to moving parts.

*A characteristic of fume created during laser materials processing is a relatively high concentration of small particles, of diameter less than 0.1 µm diameter; these can be inhaled into the lungs and retained. However, apart from a shift to small particle size, the fume produced during laser cutting and welding of metals appears very similar to that produced during other forms of thermal processing, such as flame cutting.

Dust is generally a problem when cutting such inorganic materials as alumina, quartz, marble and limestone. In some applications, such as the cutting of glass, small inert fibrous dust can be produced which if inhaled may pose a long-term health risk. The current evidence points to the need for effective fume extraction. This is especially true during the processing of organic material. This heading includes, in particular, wood and plastics. Chemical composition and particle size distribution and morphology have been quantified for some of the wide range of laser/assist gas/material/process combinations of industrial interest but the assessment of possible toxicity is incomplete. The fume may contain benzene or other known carcinogens as well as inorganic irritants. For example, the laser cutting of PVC and other halogenated polymers produces copious quantities of hydrogen chloride, a severe eye and throat irritant.

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Hazards Arising from the laser environment Hazard Typical hazardous situation Typical control measures

Temperature and humidity

Excessive high or low ambient temperatures, or high levels of ambient humidity, can affect the performance of the laser equipment, including its in-built safety features, can compromise safe operation and make the wearing of safety eyewear uncomfortable. Condensation on optical components can affect beam transmission through the system.

Provide air conditioning and humidity control of the local environment.

Install a gas purge in beam delivery line.

Mechanical shock and vibration

Misalignment of the optical path, generating hazardous errant beams.

Install a vibration isolation system on the legs of optical tables holding optical components.

Construct a floating foundation for large laser machines.

Atmospheric affects

Laser ignition of solvent vapour, dust, and inflammable gases present in the environment.

Enclose beam and process zone and add a gas purge.

Install gas sensors to detect presence of inflammable gases and vapours.

EM and RF interference

Compromised operation of control circuits caused by interfere to laser equipment caused by exposure to EM radiation and high voltage pulses conducted down supply or data cables.

Screen equipment and filter supply and data cables.

Power supply interruption or fluctuation

Interruption or fluctuation of the electrical supply can affect the operation of the laser's safety system.

Install a voltage regulation system and back-up supply.

Computer software problems

Serious and unpredictable hazards arising without warning caused by errors in computer programming of software control.

Use only approved protocols for software control of safety functions.

Ergonomic and human-factor considerations

Poor arrangement of the physical layout of the laser and its associated equipment.

Lack of space resulting in a cluttered environment.

Complex or difficult operating procedures.

Human factors, including: personal aspects (mental and physical attributes of the individual, including work ability and perception of workplace risks and attitude to safety); job aspects (tasks or functions to be performed; influence on human performance of the equipment that has to be used); organisational aspects ("safety culture" of the organisation, including the framework within which an individual has to work and the influences and pressures (real or imagined) that the individual may be under).

Improve layout, reduce clutter and review the ergonomics of repetitive or sustained tasks.

Training to improve human factors (play some part in the majority of work-related accidents).

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Indoor use of lasers

TR 60825-14 User Guide In the absence of national guidelines, TR60825-14 ‘A User’s Guide’, takes on the status of 'highway code' in the use of lasers.

The parts of the technical report directly related to control measures are addressed below.

Administration of laser safety This section addresses general issues of safely responsibility and its delegation.

Under ‘safety responsibilities it reminds employers and employees, and all users of lasers (including students) and those supervising or overseeing them, that they have a role to play in maintaining a safe place of work and in ensuring that their activities do not present unacceptable levels of risk to themselves or to others.

It is the employer's responsibility to ensure that the risks to health arising from the use and reasonably foreseeable misuse of laser equipment are properly assessed. The employer must take all necessary steps to ensure that these risks are either eliminated or, where this is not reasonably practicable, reduced to an acceptably low level. Significant findings of this assessment should be documented and appropriate protective measures implemented wherever necessary to reduce the identified health and safety risks. The effectiveness of such protective measures should be reviewed regularly.

The TR recommends that wherever potentially hazardous lasers are in use, the employer should establish a general policy for the safe management of these hazards, although specific safety tasks may be delegated to others. This policy should:

• Be an integral part of the organisation's overall safety policy.

• Should require that all reasonably foreseeable hazards arising from laser use are identified and that steps are taken to control them so far as is reasonably practicable.

However, the TR advises that a specific safety policy for lasers are not normally necessary where only laser products in Class 1 or Class 2 are in use (excluding embedded lasers and, for Class 2, locations where dazzle could be a problem), and may not always be necessary for laser products in Class 1M or Class 2M.

Laser Safety Officer The TR recommends that a Laser Safety Officer should be appointed in organizations where:

• Class 3B or Class 4 laser products are in use.

• Class 1M and Class 2M laser products generating well-collimated beams are in use.

• Embedded lasers are in use and serviced on site.

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• The use of lasers of a lower class than 3B or 4 may nevertheless still introduce a significant risk, perhaps through the involvement of untrained people or because of the existence of associated laser hazards.

The Laser Safety Officer should take responsibility, on behalf of the employer, for the administration of day-to-day matters of laser safety. It is the employer's responsibility to ensure that the person appointed as Laser Safety Officer has sufficient competence and capability to perform this role satisfactorily. Suitable training should be provided if necessary.

The duties of the Laser Safety Officer should be agreed with the employer (or with the employer's delegated representative) and documented. These duties should be those necessary to ensure the continuing safe use of lasers within the organization concerned, but are likely to include as a minimum:

a) being aware of and, if appropriate, maintaining records of, all potentially-hazardous laser products;

b) responsibility for monitoring compliance with the organisation's procedures for ensuring safe laser use, for maintaining appropriate written records, and for taking immediate and appropriate action in respect of any non-compliance or apparent inadequacy in such procedures.

The role of Laser Safety Officer rarely needs to be a full-time appointment and in addition to a LSO a Competent Person may be appointed to provide specialist assistance in hazard determination, risk assessment, and protective control and procedure provision. The Competent Person need not be an employee of the organisation concerned, but may instead be an external adviser.

Information and training All employees should, where relevant, be made aware of any hazards (including associated hazards) to which they may be exposed during the use of laser equipment, and of the procedures necessary to ensure protection. Sufficient instruction or training should be given in order that employees have the necessary understanding to avoid placing themselves and others at unacceptable risk. Safety training is especially important for those who work with Class 3B or Class 4 laser products.

Control recommendations by Class The classification of a laser gives an indication of its potential hazard. Laser product classification is based on the maximum level of laser radiation that is accessible during conditions of normal operation. Associated hazards that may also be present during use of the laser do not affect the laser classification.

Protection requirements for Class 1: Ensure that the conditions for Class 1 operation are maintained. If access to levels of laser radiation in excess of the limits for Class 1 could occur, for example during servicing of an embedded laser product.

Protection requirements for Class 1M: Avoid the use of magnifying viewing aids or instruments (such as binoculars, telescopes, microscopes and magnifying lenses, but not spectacles or contact lenses). Avoid placing optical devices in the emitted beam that could cause the concentration of the laser radiation to be increased. Do not direct the beam into areas where other people may be present if there is a likelihood of the people in those areas using telescopes or binoculars to look directly into the beam.

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Protection requirements for Class 2: Avoid staring into the beam (i.e. deliberate viewing of the laser source) or pointing the beam at other people.

Protection requirements for Class 2M: Avoid the use of magnifying viewing aids or instruments (such as binoculars, telescopes, microscopes and magnifying lenses, but not spectacles or contact lenses). Avoid placing optical devices in the emitted beam, which could cause the concentration of the laser radiation to be increased. Avoid staring into the beam (i.e. deliberate viewing of the laser source) or pointing the beam at other people.

Protection requirements for Class 3R: Prevent direct eye exposure to the beam or pointing the beam at other people.

Protection requirements for Class 3B: Prevent eye (and in some cases skin) exposure to the beam. Guard against unintentional beam reflections.

Protection requirements for Class 4: Prevent eye and skin exposure to the beam, and to diffuse reflections (scattering) of the beam. Protect against beam interaction hazards such as fire and fume.

Default control measures The essential difference between the user guidance in EN 60825-1 and TR 60825-14 is that the latter is risk assessment based. However, in many applications where the laser products in use are no higher than Class 3R (i.e. they are Class 1, 1M, 2, 2M or 3R), the user may implement control measures based on the highest class of laser product in use without any need to undertake a detailed risk assessment or to evaluate possible levels of human exposure. These so-called default control measures are summarized in the table below.

However a more detailed analysis to be undertaken in order to determine the protective measures that are appropriate in the case of:

• All uses of laser products in Class 3B or 4.

• The use of protective eyewear.

• Reliance for protection on the concept of a minimum safe distance from the laser.

• Other situations where the controls specified in Table 1 may be inappropriate, insufficient or unreasonably restrictive given the actual degree of risk.

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Table 1 – Default protective control measures for laser products CLASS PROTECTIVE CONTROL MEASURES

These should be implemented unless a risk assessment justifying the adoption of alternative protective control measures has been undertaken.

1 No protective control measures are necessary under conditions of normal operation. (This may not be the case under conditions of maintenance or service.)

In the case of embedded laser products containing a laser of higher power, follow instructions given on warning labels and supplied by the manufacturer.

Special precautions may be needed for on-site servicing of embedded laser products.

1M Prevent direct viewing of the laser source through magnifying viewing instruments, such as binoculars, telescopes, microscopes, optical sights or magnifying lenses, unless these incorporate adequate levels of protectiona.

Prevent the use of any external optics that could decrease the beam divergence or its diameter.

2 Do not stare into the beam. Do not direct the beam at other people or into areas where other people unconnected with the laser work might be present.

2M Do not stare into the beam.

Do not direct the beam at other people or into areas where other people unconnected with the laser work might be present.

Ensure the beam is always terminated at a suitable non-specular (i.e. non mirror-like) surface.

Prevent direct viewing of the laser source through magnifying viewing instruments, such as binoculars, telescopes, microscopes, optical sights or magnifying lenses, unless these incorporate adequate levels of protectiona.

Prevent the use of any external optics that could decrease the beam divergence or its diameter.

3R Prevent direct eye exposure to the beam.

Do not direct the beam at other people or into areas where other people unconnected with the laser work may be present.

3B and 4

Class 3B and Class 4 laser products should not be used without first carrying out a risk assessment to determine the protective control measures necessary to ensure safe operation.

Where reasonably practicable, use engineering means, as specified in IEC 60825-1, to reduce the class of the laser to below Class 3B. (This will normally mean completely enclosing the laser radiation to form a Class 1 laser product.)

a The type of viewing instrument that could be hazardous may be indicated on the warning label or in the user information supplied by the manufacturer.

Embedded lasers Since laser products are classified based on the level of laser radiation that is accessible during normal operation. In the case of an embedded laser product, opening, removal or displacement of any part of an enclosure that is not designed to be opened, removed or displaced during operation may therefore give access to harmful levels of laser radiation.

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Some products, within any class other than Class 4, may incorporate a laser having accessible emission that is constrained within that class by the design of the electronic drive circuitry or by other means, even though the laser itself is capable of generating a level of emission that would place it in a higher class. Users of such products should therefore be aware that under a combination of fault conditions, or when used in a manner other than that intended by the manufacturer, higher levels of laser radiation can become accessible. The user should refer to the manufacturer’s operating instructions in order to avoid exposure to potentially hazardous laser radiation.

Control measures

access interlocks

key controlsafety shutter

eye protection

skin protection

rigid mounting of

optical components

beam enclosure

beam path

termination

Essential tools of User Hazard Control in 60825-14

Where a risk assessment has shown that an unacceptable degree of risk exists, the feasibility of using a laser of a lower class should always be considered as the first option in controlling hazards. The need to use a hazardous laser should therefore be justified prior to purchase and use.

Control measures should be considered under three headings, covering engineering controls, administrative controls and personal protective equipment. Wherever reasonably practicable, however, laser hazards should be eliminated completely at source by the use of engineering controls (e.g., by total enclosure of the beam).

• Engineering controls include features incorporated into the laser equipment and around the laser beam by the manufacturer or user, in particular the fixture of protective barriers and guards to prevent human access to laser radiation.

• Administrative controls cover overall policy, procedural issues (the "local rules" governing laser use), and the use and display of hazard warning signs, training and instructions, assignment of responsibilities and prohibitions.

• Personal protective equipment is that protection worn by an individual. In the context of laser safety it refers primarily to the use of laser protective eyewear,

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but can also include items of special clothing (e.g., gloves and face-masks) to protect the skin, as well as respirators to protect against dust and fume and earplugs to protect against excessive noise.

Engineering controls should be given primary consideration as the means for reducing the risk of laser-related injury. Administrative controls covering procedural issues and safe systems of work should then be considered. Personal protective equipment should only be used as a last resort where a combination of engineering and administrative controls cannot reasonably provide a sufficient level of protection. Where personal protective equipment is employed it should be supported with an adequate level of administrative control governing its use.

Enclosing the hazard Beam enclosures The best way of dealing with the laser hazard is to enclose the beam and its reflections. A thin-walled metal protective housing will “keep fingers out” and block any weakly scattered laser light. The use of such enclosures to completely contain the laser beam should always be considered as a means of preventing human access to hazardous levels of laser radiation.

All enclosures need to be of appropriate material, robust, secure and fit for purpose in the context of their intended use and under the impact of their local environment.

High-power Class 4 lasers, such as those used for cutting, welding and other forms of materials processing, present an additional enclosure problem by virtue of the ability of the laser beam to penetrate an opaque material through melting, burning, vaporisation or ablation. For guidance on assessing the suitability of materials of construction for high-power laser radiation the user should refer to IEC 60825-4. In general, enclosures must be sufficient to adequately contain the laser radiation that could impinge upon its inner surface for as long as necessary.

For all protective enclosures the means of preventing unintended or unauthorised removal of all or part of the enclosure, thereby gaining access to the laser radiation, is an important consideration.

Viewing windows While observation (viewing) windows can be employed to allow for inspection of the inside of a laser enclosure during laser operation, their use is not an ideal solution and the adoption of remote viewing (TV) systems should be considered as an alternative. Where viewing windows are used, they need to be fabricated of suitable material to permit viewing of the inside of the enclosure without compromising its protective properties.

The method of calculating the required optical density of the window material at the wavelength(s) of the laser radiation that is enclosed is the same as for laser protective eyewear, but the assessment of maximum foreseeable exposure will be different. In particular, since a viewing window is not worn, accidental exposure may be of much longer duration than is the case for eyewear. (See IEC 60825-4).

Interlock protection For an interlock that performs a safety critical function it is recommended that the following criteria should be considered.

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a) Mechanical switches should be of "positive break" design. These have contacts that spring apart when the interlock switch is released, so preventing arcing or the risk of intermittent operation.

b) Proximity switches should be coded (that is, require two matched parts to be brought together) to prevent casual override.

c) Interlock systems should be so designed that a single fault in any part of the circuit does not lead to the loss of its protective function. The single fault should be detected before the system can be reset.

It is good practice for interlock systems to be designed so that, after operation, the system can only be reset by a deliberate action (e.g., reset button).

Interlock systems that have provision for an override facility should meet the following requirements.

• It should not be possible for the interlock to remain overridden when the enclosure has been reinstated. This requirement can be achieved, for example, by limiting the duration of override operation or by mechanical design of the override mechanism.

• There should be a distinct visible or audible warning whenever the override is in operation.

• Where interlocks can be overridden from outside a laser-controlled area this should only be possible by means of a coded or key-operated switch to prevent activation of the override by unauthorised persons.

Hazard mitigation Preventing access Human access to a laser hazard should be prevented by engineering means as far as is reasonably practicable.

Laser controlled areas At its simplest, a laser-controlled area is an area within which laser beam hazards can exist and over which there is some level of effective hazard control. Such areas should be clearly delineated, and access to them limited to nominated persons who have received adequate safety training and to persons under their control.

The boundaries of the area should enclose the hazards associated with the use of the laser under all reasonably foreseeable conditions of use (including reasonably foreseeable faults occurring with the laser or associated equipment, and reasonably foreseeable failures to follow correct procedures).

Warning signs should be clearly displayed on the outside of all laser-controlled areas. Such signs should include the laser hazard symbol and should indicate the type of hazard(s), the restrictions on access that are in force, and the precautions to be adopted on entry. It can also be useful to include the name of the person responsible for the area from whom further information can be obtained.

Illuminated warning signs may be used on the outside of laser controlled areas to indicate when the laser is in use and the door interlocks (if fitted) are operational. These signs should clearly indicate when it is safe, and when it is not safe, to enter the area.

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Associated laser hazards may require safety features needed in the laser-controlled area; including the provision of electrical isolation points, ventilation (fume extraction) and fire control. As in all work areas, the area layout must allow for rapid egress and admittance in the event of an emergency. Information about the hazards present and how to isolate them should be provided (for example, on a door sign) for the benefit of patrol and emergency services. The installation of conventional, red, emergency-stop buttons on the inside of laser areas to terminate hazardous laser emission in the event of an emergency should be considered in relation to other risks.

Local rules and procedures Administrative controls should be implemented in the form of documented local rules and procedures. These may be drawn up specifically for the particular organisation, location or equipment concerned, or may be based on a suitable standard model. They should include:

a) description and purpose of the equipment or process;

b) the name and contact point of the Laser Safety Officer and of the person responsible for the laser equipment;

c) the names of personnel authorised to operate, maintain or service the laser equipment;

d) the procedures to be adopted for laser operation, maintenance and service (where relevant), and of all precautions to be followed, including, where applicable, the use of personal protection and the use and secure storage of laser control keys;

e) action to be taken in the event of specified equipment failure or other emergencies;

f) the incident reporting procedure and the action to be taken in the event of a suspected accident;

g) details of requirements, if any, for authorisation for hazardous operations, e.g., procedures for approval of servicing (permit to work).

Local rules should be reviewed regularly to ensure their continued relevance to requirements.

Localised risk reduction Within all laser controlled areas, steps should be taken to reduce the risk of injury to persons authorised to work within them. These steps should include:

b) adequate training of all personnel involved;

c) sufficient levels of room illumination;

d) uncluttered environment and well-organised working layout;

e) secure control of laser operating keys;

f) the secure fixing of the laser and all components along the path of the beam;

g) safe method of beam alignment;

h) design beam paths to be horizontal or pointing vertically downwards - not upwards;

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i) a beam stop at the end of the useful path of the laser beam, where appropriate;

j) use of the beam attenuator or beam stop fitted to Class 3B and Class 4 laser products to temporarily terminate laser emission whenever such emission is not required for short periods. Whenever laser emission is not required for longer periods, the laser should be turned off;

k) enclosure of as much of the beam as is reasonably practicable;

l) keeping the beam above or below eye level where practicable;

m) confinement of the beam within well-defined areas, which are as small as reasonably practicable (e.g., keeping the beam within the confines of an optical table; placing barriers preventing human access where an open beam crosses the floor);

n) the use of screens, blinds, or curtains to contain the laser radiation (see IEC 60825-4 for guidance on selection of suitable materials);

o) use of checklists where appropriate.

Particular care should be taken to prevent the unintentional specular reflection of laser radiation. Mirrors, lenses, and beam splitters should be rigidly mounted and should be subject to only controlled movements while the laser is emitting. Note in particular that:

• reflecting surfaces that appear to be diffuse may actually reflect a considerable part of the radiation beam specularly, especially in the infrared spectral range.

• Potentially hazardous specular reflections occur at all surfaces of transmissive optical components such as lenses, prisms, windows and beam splitters.

• Potentially hazardous radiation can be transmitted through some reflective optical components such as mirrors (for example, infra-red radiation passing through a reflector of visible radiation).

• Many surfaces become specularly reflecting at grazing incidence.

Personal protection Except for the high power lasers used for materials processing, laser burns are generally small area and, like soldering iron burns, rank only as a minor injury. Moreover, skin protection can significantly hamper movement and produce significant discomfort, giving rise to the possibility of trips, bumps and other non-laser accidents. For these reasons, protective clothing is rarely used or expected, except for gloves where UV beams are present and masks where fume is generated. Very occasionally the use of whole body protection may be required.

See next section ‘Personal Protective Equipment’.

Maintenance of safe operation TR 60825-14 recommends that regular monitoring of laser working areas should be carried out, and such monitoring recorded, to ensure that the control procedures that have been adopted remain effective and that the conditions for achieving acceptable

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risk remain satisfied. Protective procedures should be modified whenever necessary to ensure continuing safe use. The results of investigations into safety incidents and suspected accidents should be used to re-appraise the effectiveness and adequacy of the control procedures.

Circumstances that could indicate an urgent need for reassessing risk and for reviewing protective procedures and controls include the following.

a) modifications to, or relocation or replacement of, the laser equipment;

b) changed conditions of use;

c) changes to the environment in which the laser equipment is used;

d) changes to the personnel who could have access to the laser equipment or who could be exposed to laser hazards;

e) indications of any reduction in compliance with safety procedures.

Medical inspections In the event of an actual or suspected hazardous exposure to laser radiation or other laser hazard (an accident), or a possible failure of a protective measure which could have led to an accident (an incident), laser emission should be terminated immediately. The incident should be reported to the management of the facility where the incident occurred.

Where an accident has or is suspected to have occurred, medical attention should be sought as necessary. In the event of an apparent or suspected injury to the eye, a medical examination by a qualified ophthalmologist should be carried out within 24 h of the event. It is useful to have a summary of the laser beam characteristics to accompany the casualty and assist the ophthalmologist. In all cases where a hazardous exposure is suspected, a full investigation to ascertain the circumstances surrounding the event and the likely magnitude of the exposure should be undertaken, and the conclusions of this investigation documented. In the case of an accident, the reason for the possible failure should be determined, and any necessary changes to the system of protective controls should be introduced before re-use of the laser.

Routine ophthalmic examinations of employees working with laser equipment have no value as part of a health surveillance programme. Ophthalmic examinations are sometimes carried out for other (e.g., medico-legal) reasons; in particular, as a precaution against later claims of laser damage.

Some of the investigative procedures used are themselves hazardous, and these should therefore only be carried out when medically advisable, and not used for routine screening. Even a simple visual acuity check can pick up, for example, if a potential new laser user had a diseased or damaged retina.. Only in cases where 20-20 vision cannot be achieved with spectacles should further investigations be carried out.

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Personal protective equipment Use of personal protective equipment (PPE) PPE (such as laser protective eyewear) should be worn, where appropriate, by individuals working in laser controlled areas in order to provide protection against laser hazards. Such protection should, however, only be used where it is not reasonably practicable to ensure adequate protection by other means, preferably by total enclosure of the laser radiation, and where it has been ascertained that personal protective equipment is able to provide sufficient protection.

Where personal protective equipment has been deemed to be an appropriate method of risk reduction, its use should be compulsory and not left as a matter of individual choice. PPE should ideally be issued on a person-by-person basis, and for hygiene reasons should be properly cleaned by an appropriate method before reuse by another person. Additional national requirements covering the design, specification and use of PPE may exist.

Standards EN207 and EN208 cover the specification, marking and testing of laser eye protection, using the concept of protective (rather than optical) density, which takes into account the ability of the protection to withstand the incident laser radiation.

Specifying eye protection Eye protection can be in the form of spectacles (having frames which rest on the ears) or goggles (secured by a band around the head). Such protection incorporates optical filters to reduce the transmission of laser radiation to the eye, and may be employed as a protective measure within a laser controlled area. Total beam enclosure combined where necessary with the use of remote viewing (e.g., television) systems should, however, always be considered first as an alternative to reliance on personal eye protection.

When to use eye protection Eye protection should only be used if all of the following conditions are satisfied.

There exists a non-trivial risk of injury arising from the accidental exposure of the eyes to levels of laser radiation above the MPE.

There is no serious co-existent risk of skin injury arising from laser exposure (but see 8.4.5.3). Such a risk is likely to exist where high levels of filter density would be necessary to protect the eyes.

It is not reasonably practicable to ensure adequate protection entirely by the use of engineering and/or administrative controls.

The protective eyewear has the necessary performance specification with regard to:

the reduction in the maximum reasonably foreseeable laser exposure to safe levels,

the capability of the eyewear for withstanding the maximum reasonably foreseeable laser exposure long enough for corrective action to be taken to terminate exposure, and

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the ability of the wearer to be able to use the eyewear without discomfort and without any significant degradation in vision.

How to choose protective eyewear When choosing appropriate eyewear the following should be considered:

the wavelength of operation. Laser eyewear utilises filter materials to provide protection over certain defined wavelength ranges. Use of the incorrect eyewear will usually mean that insufficient protection is provided;

the reasonably foreseeable worst-case effective exposure, expressed in terms of either the incident irradiance (W•m-2) or the incident radiant exposure (Jm-2);

the applicable value of the ocular MPE (determined in accordance with Clause 4), and specified in the same units as the effective exposure;

the actual exposure and the beam diameter (these parameters enable the ability of the eyewear to withstand the incident laser radiation to be established);

the optical density D. of the eyewear at the laser wavelength. The optical density should be sufficient to reduce the transmitted radiation to below the MPE applicable for the maximum reasonably foreseeable exposure time. The value of D. required to give the minimum necessary level of eye protection can be calculated from the formula: D. =log10[(maximum reasonably foreseeable exposure)/(MPE)]

Other important factors include:

visible light transmission, and the ability to see warning lights or other indicators through the filters;

general design, comfort, ventilation, peripheral vision, and provision for spectacle correction (either by using goggle-style protectors which fit over normal spectacles, or protective spectacles which incorporate the wearer's own optical correction);

degradation or modification of the absorbing material of the filter, including radiation-induced transparency;

mechanical strength of materials and resistance to shock; any relevant national requirements or regulations.

Eyewear should be permanently marked to indicate:

the operating wavelength; the optical density at the operating wavelength.

Other eyewear considerations include the following:

Any limitation on the maximum level of laser exposure to which the eyewear should be subjected (due to the possibility of damage to the filter material at high levels of exposure) should also be known.

Where different kinds of protective eyewear are in use it can be helpful to use colour-coding or other means to link each pair with its particular laser.

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For work with visible laser emission it can sometimes be desirable to be able to see the laser beam for alignment purposes or other operational reasons. In this case the eye protective filters should be specified on the basis of reducing an accidental exposure to the equivalent of Class 2, where protection is afforded by the natural aversion response. This is done by using a time base of 0,25 s for defining the MPE in the equation given above for D. .

Protective eyewear is designed to protect against accidental exposure to laser radiation. It should not be used to protect against deliberate exposure or the intentional viewing of a laser beam. Protective eyewear should be checked periodically for signs of wear or damage. The date of checking should be recorded and the eyewear replaced when necessary. Protective eyewear should also be examined for suitability on each occasion prior to use.

At high incident power or energy levels, absorption of the incident radiation in the filter material can result in severe stress build-up and sudden failure of the filter. For this reason, protective eyewear, which has been subjected to a single incident of accidental exposure at a high level of exposure, should be replaced.

Selecting eyewear using standards EN207 and EN208 Standards EN207 and EN208 are mandated under the PPE Directive for Laser protective eyewear. EN207 is used for protection again laser radiation in the spectral range 180nm to 1000µm. EN208 applies for laser adjustment protective eyewear. These EN standards define the protection given by filters by means of a scale number “Lxx”. The scale number is based on a combination of maximum transmittance and maximum power or energy density. In other words on the suitability of the filter to attenuate laser wavelengths and withstand the laser energy falling on it.

Before selecting eye protection a risk assessment must first be undertaken and the risk minimized by engineering and administrative controls.

It is possible to make a distinction between the different types of lasers according to their duration of operation and pulse length. The limits between the various types of lasers in EN207 Table A.1 should not be drawn too sharply either by physical or by biological factors and should therefore only be regarded as guideline values.

In the following calculations of the power density or energy density, the actual beam area (i.e. the area of the smallest circle containing 63 % of the laser power and energy) should be used. For non-circular cross-sections, a similar procedure should be employed and the smallest rectangle containing 63 % of the laser power and energy should be used.

Continuous wave laser (D) The power density E of the laser beam is calculated from the laser power P and from the beam area A as follows:

E = P/A

The required scale number can then be deduced from the appropriate table in the standard corresponding to the wavelength of the laser.

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Pulsed lasers Using to EN 60825-1, the maximum permissible exposure for wavelengths from 400 nm to 106 nm is determined by using the most restrictive of requirements as appropriate. A time base of 10s is applicable.

Pulse duration ≥ 10-9s (I and R) The energy density H of the laser beam is calculated from the pulse energy Q and the beam cross section A as follows:

H =Q/A

For lasers in the wavelength range 400 nm to 1 400 nm, this value of the energy density shall be multiplied by N1/4:

H1 = H x N1/4

Then, for H1, the necessary scale number is taken from column I or R in EN207 corresponding to the wavelength and type of laser.

Mode-coupled lasers: pulse duration < 10-9 s (M) The calculation, H = P/A, is done using the peak power of the individual pulses for laser power P. In addition, calculate H1 as described above for lasers in the wavelength range 400 nm to 1 400 nm and read off the required scale number.

Time base The laser radiation eye-protectors specified in EN207 and EN208 are not suitable for continuous exposure to a laser beam. The protection has been designed on the basis of 10 s with regard to transmission (attenuation of the laser beam) for wavelength in the range above 400 nm, otherwise on the basis of 30 000 s. The standards require that stability to laser radiation is tested for 10 s in both cases.

Protective eyewear aftercare It recommended that a register is maintained of all laser protective eyewear being used in a facility. This register should not only describe the type and suitability of the eyewear but also to whom it has been allocated. In addition, a regular inspection of the eyewear should be carried out to ensure that the PPE is of acceptable quality and still fit for purpose. Result of inspection should be recorded in the Register. Any equipment found to be defective should be removed from use until the defects have been rectified or the equipment replaced.

Protective clothing In some cases it may be necessary to provide other protective clothing for work in laser controlled areas. This is most likely to take the form of masks or gloves, but may very occasionally require the use of whole body protection.

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On-site laser maintenance and servicing Maintenance is generally the activity carried out by the operator of the equipment. It is normally assumed that the operator knows how to use the equipment but may have very limited knowledge or experience of how the equipment works and what the risks are when safeguards are removed to allow remedial work on the equipment. Thus maintenance should be limited to the essential tasks necessary to keep the equipment operational without the need for specialist expertise. With laser equipment, maintenance can often be done without the laser being energized and thus exposure to laser radiation can be eliminated.

The Service activity usually requires specialist training and skills. Because of this additional knowledge and skill, it may not be operator or normal user who carries out the service. A detailed knowledge required not only of how and why the equipment works, but also of the additional risks that may be encountered when safeguards are removed and how to minimize those risks.

Increased risks during laser equipment servicing Laser products are classified on the basis of the level of laser radiation accessible during operation. Maintenance and servicing, on the other hand, may require removal of protective covers, disabling of the product's protective features and/or a significant change to the performance of the laser product, thereby increasing the risk of injury. Additional hazards (e.g., electrical) may also be present.

Before service operations are undertaken a separate risk assessment should be undertaken. A record should be kept of all servicing operations and any resulting changes to the performance of the laser product.

The servicing of embedded lasers can greatly increase the risk of a laser radiation injury. Servicing usually includes beam alignment and other adjustment operations, and the likelihood of creating errant laser beams (that is, beams pointing in unexpected directions) is greatly increased. In order to carry out servicing in a safe manner it is often necessary to set up a temporary laser controlled area around the laser equipment, and to implement procedures and safeguards appropriate to the increased level of risk. Manufacturers are required to provide advice on safe procedures during servicing, upon request.

Laser products are classified on the basis of the level of laser radiation accessible during operation. Maintenance and servicing, on the other hand, may require removal of protective covers, disabling of the product's protective features and/or a significant change to the performance of the laser product, thereby increasing the risk of injury. Additional hazards (e.g., electrical) may also be present. Servicing and maintenance operations may require a higher level of safety training than is necessary for normal operation.

Before service operations are undertaken a separate risk assessment should be undertaken. A record should be kept of all servicing operations and any resulting changes to the performance of the laser product.

The servicing of embedded lasers can greatly increase the risk of a laser radiation injury. Servicing includes beam alignment and other adjustment operations, and the

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likelihood of creating errant laser beams (that is, beams pointing in unexpected directions) is greatly increased. In order to carry out servicing in a safe manner it is often necessary to set up a temporary laser controlled area around the laser equipment (see below), and to implement procedures and safeguards (e.g., a systematic method for beam alignment) appropriate to the increased level of risk. Manufacturers are required to provide advice on safe procedures during servicing, upon request.

Temporary laser controlled areas A temporary laser controlled area should be established whenever conditions allowing human access to hazardous levels of laser radiation are created temporarily (e.g., during servicing), and where persons who are unauthorised, unaware of the presence of the laser hazard and/or are not appropriately trained or supervised in the necessary safety procedures could be present.

The guidance for temporary controlled areas is the same as for laser-controlled areas in general. Although the normal requirement for engineering control of access may be difficult to achieve, administrative controls can have increased effectiveness when restriction of access is only temporary. If safe access to the area is not controlled by engineering means, then appropriate warning and prohibited entry signs should be posted at the points of entry to the area. In certain circumstances it may be desirable in addition to have another person present to enforce the temporary access restrictions.

Controls during servicing In establishing control measures during equipment servicing, where there is an increased risk of laser radiation injury, particular consideration should be given to the following:

a) reducing the level of emission to the maximum necessary;

b) limiting the range of movement of beam steering components to reduce uncertainty in beam position during alignment;

c) first checking beam alignment close to the laser and then progressively further away, to minimise the uncertainty in beam position;

d) placing large area beam stops behind target screens during beam alignment to stop the laser beam in the event that it misses the target;

e) providing beam visualising alignment aids (e.g., cameras, fluorescent or heat sensitive screens and viewers). These should also be used in the case of visible laser beams where there is the added benefit of countering the strong temptation to remove protective eyewear in order to clearly see the beam;

f) provision of comfortable laser safety eyewear, suitable for use over prolonged periods, where adequate protection by other means is not feasible;

g) providing an engineering means for the transfer of control of the laser beam (e.g., a handheld hold-to-fire device) where two or more persons are involved in servicing, in particular where a person remote from the laser might otherwise call to the other to fire the laser;

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h) using non-reflective coatings or diffusely-reflecting surfaces on tools, and requiring the removal or covering of jewellery, watches, etc. by those in the controlled area, in order to minimise stray reflections.

Statistically, beam alignment is the most hazardous laser operation. The reasons for this obvious: (i) it can be tempting to remove protective eyewear, especially if the beam is visible (ii) the operator's eyes may be at the level of the laser beam and (iii) there is maximum uncertainty about the position of the beam. Techniques of beam alignment include the use of fluorescent screens for UV, visible and IR, 'liquid crystal' film which changes colour (black to red to yellow to blue) as it is warmed up, heat sensitive paper to make ‘burn marks', visible Class 2 alignment lasers, TV and infra-red (thermal imaging) cameras.

Visiting service engineers If an outside agency (e.g. the laser equipment supplier) is engaged to conduct the servicing of the laser equipment, then a permit-to-work procedure should be adopted for handing the equipment over to the service engineer and accepting it back fully-restored to normal operation when the work is completed. Written procedures should be used to achieve this. Verification of safety interlock restoration should be part of the release of the equipment to the user.

A risk assessment of the service operation is required, even if the service engineer has complete control of the work. Responsibility for establishing a temporary laser controlled area prior to starting service activities, if such an area is required, may be determined by contractual arrangement. If no such contractual arrangement exists, then responsibility should be taken by the laser user to ensure that necessary servicing controls are put in place.

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Fibre optic systems

Fibre Optic Communications Products The safety of laser products, equipment classification, requirements and user’s guide are covered by IEC 60825-1. Part 2 of the series deals with optical fibre communication systems. Such systems would be safe under normal operating conditions because the optical radiation is totally enclosed under intended operation, but because the length and distribution of fibre (where optical power, under certain conditions, may be accessible many kilometres from the optical source), the precautions to minimise the hazard will be different from those concerning more conventional laser sources which are normally under local operator control.

The potential hazard of an optical fibre communication system depends on the likelihood of the protective housing being breached (e.g. a disconnected fibre connector or a broken cable) and on the nature of the optical radiation that might subsequently become accessible. 60825-2 addresses engineering requirements and user precautions in such cases. Its requirements include the necessity for assessing the potential level of accessible laser emission from an optical fibre in terms of the hazard level (e.g., hazard level 1, 1M, 2, 2M, 3R, 3B or 4), equivalent to product class. (N.B. Previously, a Class ’k x 3A’ was introduced for part 2 only, but with the new revision of part 1, ‘k x 3A’ has been withdrawn.) The hazard level applies only to a particular location at which an interruption of the fibre might reasonably foreseeably occur, rather than to the complete system or installation as a whole. It is therefore possible that different locations at which access to fibre emission could occur within the same system may be assigned different hazard levels. This is not possible for product class, which is based on the highest level of accessible emission from a complete laser product.

In summary, the primary differences between IEC 60825-1 and part 2 are as follows:

• a whole optical fibre communication system is not classified in the same way as required by IEC 60825-1. This is because, under intended operation, the optical radiation is totally enclosed, and it can be argued that a rigorous interpretation of IEC 60825-1 would give a class 1 allocation to all systems, which may not reflect the potential hazard accurately. However, if the emitter can be operated separately, it must be classified according to IEC 60825-1;

• each accessible location in the extended enclosed optical transmission system is designated by a hazard level on similar procedures as those for classification in IEC 60825-1, but this level is based not on accessible radiation but on radiation that could become accessible under reasonably foreseeable circumstances (e.g. a fibre cable break, a disconnected fibre connector etc.);

• the nature of the safety precautions required for any particular hazard level will depend on the type of location, i.e. domestic premises, industrial areas where there would be limited access, and switching centres where there would be controlled access. For example, it is specified that, in the home, a

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disconnected fibre connector should only be able to emit radiation corresponding to class 1, whilst in controlled areas it could be higher.

Manufacturers requirements (subject to revision of 60825-2) • Mechanical protection of cables (degree of depending on location)

• Cable connectors

All systems operating in unrestricted locations in which cable connectors are accessible require the use of a tool for disconnection if hazard level 1 can be exceeded.

All systems operating in restricted locations in which cable connectors are accessible require the use of a tool for disconnection if hazard level 1M or 2M can be exceeded.

All systems operating in controlled locations in which cable connectors are accessible require the use of a tool for disconnection if hazard level 1M or 2Mor 3R can be exceeded.

The positioning of the connector in a way that prevents human access to a higher hazard level is an acceptable feature to ensure that these requirements are met.

Automatic power reduction may be used to control the hazard levels.

• Labelling Optical fibre cables should carry appropriate markings to distinguish them from cable containing other services (e.g electricity) if the hazard level at the location is in excess of hazard level 1. (Labels as per 60825-1).

User’s Guide for fibre optic work Optical fibres carrying laser radiation normally provide a complete enclosure of the radiation, and so prevent access to it. However, if a fibre is disconnected or a fibre break occurs, hazardous levels of laser exposure can be present.

Safety requirements specifically applicable to optical fibre communication systems are defined in IEC 60825-2. These requirements include the necessity for assessing the potential level of accessible laser emission from an optical fibre in terms of the hazard level (e.g., hazard level 1, 1M, 2, 2M, 3R, 3B or 4), equivalent to product class. The hazard level applies only to a particular location at which an interruption of the fibre might reasonably foreseeably occur, rather than to the complete system or installation as a whole. It is therefore possible that different locations at which access to fibre emission could occur within the same system may be assigned different hazard levels. This is not possible for product class, which is based on the highest level of accessible emission from a complete laser product.

The following working practices are taken from an informative annex of 60825-2 as an example of good practice when working with any optical fibre system:

Viewing fibre Do not stare with unprotected eyes or with any unapproved collimating device at the fibre ends or connector faces, or

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point them at other people.

Viewing aids Use only approved filtered or attenuating viewing aids.

Fibre ends (single or multiple)

Any single or multiple fibre end(s) found not to be terminated (for example, matched, spliced) shall be individually or collectively covered when not being worked on. They shall not be readily visible and sharp ends shall not be exposed.

Suitable methods for covering include the use of a splice protector or tape. Always attach end caps to unmated connectors.

Ribbon fibres Do not cleave ribbon fibres as an unseparated ribbon, or use ribbon splicers, unless authorised.

Test cords When using optical test cords, the optical power source shall be the last to be connected and the first to be disconnected.

Fibre off-cuts Collect all fibre off-cuts and dispose of them in an approved container. The container itself should be disposed of in an approved manner.

Maintenance Follow only approved instructions for operating and maintaining the system being worked on.

Cleaning Use only approved methods for cleaning and preparing optical fibres and optical connectors.

Modification Do not make any unauthorised modifications to any optical fibre system or associated equipment.

Board extenders Board extenders shall not be used on optical transmitter cards. Do not power optical sources when they are outside transmitter racks.

Label damage Report damaged or missing optical safety labels to line management.

Key control For equipment with key control, the keys shall be placed under the control of a person appointed by management who shall ensure their safe use, storage and overall control.. Spare keys shall be retained under strict control procedures by a nominated line manager.

Test equipment Use test equipment of the lowest class necessary and practical for the task. Do not use test equipment of a higher class than the location hazard level.

Signs Area warning signs are required for locations exceeding hazard level !M and 2M. Area signs may be displayed in locations of lower classification.

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Live working practices

Warning signs should be erected/displayed, even at temporary work locations.

Use only approved viewing aids with valid calibration labels or test reports.

For hazard level 3B, responsible and adequate OFCS safety and training programmes should be established and maintained by management. Personnel engaged in the installation and servicing of OFCS should observe all rules, and report to management any potentially unsafe conditions or abnormal exposures to optical radiation.

Live working on hazard level 3B locations or systems is not recommended.

If live working is not permitted on hazard level 3B systems, then the following working practices should be used:

• all general practices defined above;

• power down the system;

• check that there is no optical power in the fibre using an approved optical power meter;

• cover the ends of all exposed fibres not being worked on. Always attach end caps to unmated connectors;

• use only indirect viewing aids (for example televised or shadow imaging splicing machines). Do not use microscopes without authorisation;

• when using optical test cords, the optical power source shall be the last to be connected and the first to be disconnected.

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Working codes for laboratory laser work

The international safety standard presents user control measures in the form of guidance, leaving it to the user to decide if, for example, ‘the remote interlock connector of Class 3B and 4 lasers should be connected to … a room door’ is interpreted as a requirement or not. A number of laser safety working codes set out to take away some or all of the decision, with sets of rules based entirely on the hazard classification system. The USA’s ANSI Z136 series is the most comprehensive of these, but there are other National, Profession, and Organization-based working codes that serve a similar purpose of controlling laser use. This prescriptive approach only works, however, because the scope of such codes restrict their application to specific laser applications and/or types of user.

The Health and Safety Executive regard EN60825 as a guide to what is "reasonably practicable" within the terms of the Health and Safety at Work Act. The responsibility for safety invariably lies with the line management and ultimately with the Director: the promulgation of a Working Code is one of the ways in which the Director’s responsibilities are discharged.

Working Codes of Practice are comprehensive statements describing safety precautions to be implemented. A Working Code applies the general guidance given in TR60825-14 to the specific needs of the organisation devising it, replacing the "shoulds" in the list of control measures in the User section of the Standard with "shalls".

A R&D Working Code

Introduction The first few sections can be regarded as introductory. Points include stating the scope of the code in terms of its area of applicability (e.g. on site), defining “BS TR60825-14: 2004” as applying to the current British Standard if and when BS TR60825-14: 2004 is superseded, and addressing the issue of exemptions. A general requirement is that the LSO has the power to grant exemptions from any of the requirements of the Code

Definitions Among the most important definitions are those of the Classes of laser. Other useful terms might include the defined terms for responsible officers (e.g. laser engineer, laser system supervisor), a definition of what constitutes Standing Orders, and some of the jargon such as ‘human access’, ‘specular reflection’ and ‘intrabeam viewing’ might be included.

Responsibilities A clear chain of responsibility should be established through line management to the appointed LSO and from there to those responsible for laser work. The aim should be to ensure that responsibilities fall within the envelope of authority that that person has been given e.g. for a local supervisor of laser activities they might include:

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• ensuring that this Code is complied with

• maintaining radiation safety equipment in good working order

• ensuring that Standing Orders are prepared, posted and are kept up to date.

Appointments Written appointments, signed by both parties, should define limitations of responsibility. A renewal date of appointment ensures a regular (e.g. annual) review.

Control Measures The main use of control measures arises in the context of Class 3B and Class 4 lasers. The working code will usually define that such lasers are to be used only within a Designated Laser Area. The design criteria of such areas as specified in the Code might include: (i) light tight boundaries, (ii) warning signs and lights, (ii) interlocks (with override facilities) at entrances.

Control measures for specific activities might address: alignment procedures, maintenance and service operations in the area, remote operation of a laser, multiple laser operation and open beam path operation.

Special rules might be included to address embedded lasers because of the change in radiation hazard during some servicing operations. In particular, there are now many items of office equipment, including laser printers, scanners and CD readers that are embedded laser products; these may be excluded if it is made a requirement that such items are serviced off site.

Standing Orders. The use of standing orders is the central element of a Working Code. Key points might include such items as: (i) regular revision to accommodate changes within the DLA, (ii) the means of transfer of control and the form of verbal warnings given before firing the laser, (iii) minimum reliance on personal protection, but adequate specification of when and what to use, (iv) requirements for safe alignment (e.g. reduce laser beams to Class 3R or below).

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Risk assessment Traditionally, risk has been assessed as the product of the likelihood of an event that will cause harm and the severity of harm caused.

General methodology The reduction of risk to tolerable levels is an iterative process. There is no standard approach to procedure and documentation for this process. Nevertheless, the steps involved are universal:

Step 1 - Identify the potentially injurious situations List every reasonably foreseeable injurious situation that could arise in the use of the laser equipment, including those of installation, normal operation, maintenance, service, and reasonably foreseeable misuse or failure. This list should be irrespective of the prevention/precautions included and should be applied in all cases whether assessing a workplace, a work activity, a product or a service provision.

There are three key types of issue to consider when drawing up the list of potentially injurious situations:

The hazards involved Make a list of most of the hazards that may be involved in the use of the laser product. Almost inevitably, control measures already be in place at the time of the risk assessment will effectively isolate some of these hazards (except, perhaps, during servicing). The extent to which such controls are taken into account when drawing up the initial list is a matter of judgement.

The laser environment The laser environment in the present context includes the location of the laser equipment (e.g. inside within an enclosed and dedicated laser working area; inside within an open-plan working area; outside), the state of the working area from an equipment viewpoint (e.g. the influence on equipment of temperature, humidity, vibration, dust etc. and the possibility of disturbances or damage by collisions with persons or moving equipment), the state of the working area from a personnel viewpoint (e.g. cluttered; clean or dirty; well-lit or dark), and the level of access (e.g. localised restricted area within premises having no public access; unrestricted area within premises having no public access; public access areas.)

The personnel at risk Issues relating to persons at risk include their level of awareness, protection and training. The list may include trained operators, service personnel, employees unaware of the hazards, visitors, children and other members of the public who may not fully understand warning signs.

Step 2 - Assess the risk for potentially injurious situations For each item on the list of potentially injurious situations, separately assessed the likelihood of injury and severity of injury.

It is usually difficult or impossible to quantify these elements. One way of dealing with this is (i) to place the likelihood of injury into one of five categories from likely

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to remote, taking account of the frequency of exposure to the hazard, the time of exposure to the hazard and the probability that, when exposed, the hazard will not be avoided, (ii) place the severity of injury into one of four categories from fatal to no injury with perhaps a fifth category for damage to plant or environment, then (iii) use the table below to classify the resultant risk into one of four ranks from high to tolerable.

Likely Probable Possible Improbable Remote

Fatal High High Medium Low tolerable

Major injury/perm. disability

Medium Medium Low tolerable tolerable

Minor injury

Low Low tolerable tolerable tolerable

No injury tolerable tolerable tolerable tolerable tolerable

In the above table the following descriptions are applied to the Probability/likelihood:

Likely - Occurs repeatedly/ event only to be expected

Probable - Not surprised - will occur several times.

Possible - Could occur sometimes

Improbable - Unlikely - through conceivable

Remote - so unlikely that probability is close to zero.

Actions necessary

High Highest priority for immediate action to reduce risk to an acceptable level. If the risk constitutes a “Serious and Imminent Danger” after considering the existing controls, implement withdrawal procedures.

Medium Urgent action required to analyse existing controls urgently to determine what action is required to reduce the risk to an acceptable level.

Low Prompt action required to analyse existing controls as soon as possible.

Tolerable No action required.

Step 3 - Select control measures, and repeat from Step 1 process Consider each hazard and risk, even where control measures already exist, and decide what existing or additional measures are needed to comply with relevant legislation and/ or Company Safety Policy. It is important at this stage to ensure that all the assessments and the conclusions are recorded as this is the “what to do and why we do it” stage.

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In selecting appropriate controls, engineering controls should be given primary consideration as the means for reducing the risk of injury. Personal protective equipment should only be used as a last resort where a combination of engineering and administrative controls cannot reasonably provide a sufficient level of protection.

When assessing the various options, the basic priorities are:

remove the danger;

guard against the danger;

warn about the danger.

Where the danger cannot be removed, as much as possible should be done on the second option to minimise what requires to be done on the third.

These three descending priorities can be explained to a more refined scale with the following safety precedence sequence:

Eliminate the risk (is there an alternative method? can the design be changed?);

Substitute the hazard for a lesser one (replace the risk activity, use a safer chemical process);

Isolate the hazard and thus the risk to persons or environment (make it difficult to get at);

Control making it difficult for the person to get at the hazard;

Personal Protection giving the person protection from the hazard;

Discipline/training may be provided to both combat latent risks and also to ensure that controls, instructions, warning are recognised and understood by all those who are likely to come into contact with the hazard.

After control measures for reducing the risk have been determined, the risk assessment procedure outlined above should be repeated, and if necessary a further iteration carried out, until the risk from all potentially injurious situations has been reduced to a tolerable level. It is recommended that these iterations are carried out before the proposed controls are implemented and the laser equipment is used, in order to confirm that the once the control measures have been adopted the risk is tolerable.

The final stage of any risk assessment is to establish reviews to ensure that the implemented actions taken to eliminate or reduce risks are really put into practice and are effective. This stage is equally as important as any of the others to ensure effectively of the assessment and to establish a structured monitoring process to recognise inappropriate measures, misuse and abuse, changes in equipment/ process/ environment, etc., all of which may make previous assessments redundant and obsolete.

Dealing with residual risks The hierarchy of approaches is as follows:

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1 Warn of the residual risks This step is necessary to ensure that persons are aware and are reminded that additional actions are or may be required to contain the residual risks.

2. Permit to Work The second approach may be to introduce permits to work. These permits are to cover specific actions that must be carried out to make a task safe. They are normally recorded in writing before further work is permitted to take place. The safe systems of work are specified in documents such as: standard operating procedures or safety policy arrangements. Permits to work supplement these safe working practices. They are documented procedures that require people to carry out and sign for defined actions to make a task safe. A permit system is only part of a system of work and must be carefully set up and properly managed to be successful. Permits to work are not the answer to every safety problem and must be confined to those clearly specified situation where there is a need to ensure tight control over certain action to be carried out. They require a high degree of training and supervision to ensure compliance. The weakness of this system of working is that the procedure covered by the permit must be as simple as possible and should not take too long to complete. Often lack of time for the user of the permit as well as those supervising the use of the permit lead to short cuts being taken and the process becoming totally ineffective.

3. Training Training is necessary to implement the objectives or either warnings or permits to work. European Directives issued under Article 137 demand that workers are trained so that they are capable of carrying out the tasks they have been entrusted with.

4. Information Information is of prime importance. Information about risks and control measures must be supplied to all users. The information should be comprehensible and relevant information based on the risks identified by the assessment. It should include, the preventative and protective measures, emergency procedures and the staff involved in them. It is common for information of this kind to be written in rule books or safety manuals. This is a good method but they must be written with the workforce in mind, taking into account factors such as language and reading ability. Information may sometimes be in the form of signs, such as warning signs.

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ICFO 2005

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