Laser Safety for Laboratory Users

Transcription

1 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 = = = mikeg@prolaser. co.uk

2 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 Maximum Permissible Exposure Assessing the level of laser exposure Hazard distance Nominal ocular hazard distance Extended Nominal ocular hazard distance The laser hazard classification scheme Defining the problem Laser-related Workplace Legislation The Workplace Directive The Provision and Use of Work Equipment Directive Personal Protective Equipment at Work Regulations Associated Hazards Indoor use of lasers TR User Guide Administration of laser safety Control recommendations by Class Control measures Maintenance of safe operation Medical inspections Personal protective equipment When to use eye protection How to choose protective eyewear SIRA laser safety

3 Selecting eyewear using standards EN207 and EN Protective eyewear aftercare Protective clothing On-site laser maintenance and servicing Increased risks during laser equipment servicing Temporary laser controlled areas Controls during servicing Visiting service engineers Fibre optic systems Fibre Optic Communications Products Manufacturers requirements (subject to revision of ) User s Guide for fibre optic work Live working practices Working codes for laboratory laser work A R&D Working Code Risk assessment General methodology Step 1 - Identify the potentially injurious situations Step 2 - Assess the risk for potentially injurious situations Step 3 - Select control measures, and repeat from Step 1 process Dealing with residual risks SIRA laser safety

4 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 TR 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 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 SIRA laser safety

5 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. GAMMA X-RAY UV VISIBLE INFRA-RED MICROWAVE RADIOWAVE 0.1 µm 1000 µm WAVELENGTH penetration penetration Ionising radiation cancer risk Non-ionising radiation thermal burn risk The wavelength of the radiation determines its biological effects 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 SIRA laser safety

6 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. Conjunctiva Iris Cornea Lens Pupil Aqueous Fovea Optic disc Retina Choroid Sclera Visual axis Symmetry axis Optic nerve Ciliary body Vitreous 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. 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 SIRA laser safety

7 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 µm diameter on the retina, resulting in a concentration of up to 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 ( 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 ( µm) 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. ( µm) 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 ( µm) 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 µm whereas wavelengths out to 1.4 µm are transmitted. The near infrared ( 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 SIRA laser safety

8 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 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 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. TV Safe for prolonged viewing Safe for accidental viewing (1/4 second) 100 W Light bulb Sun 1 mw laser Optical power density on retina ,000 1,000, ,000,000 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 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 SIRA laser safety

9 linear scale Fraction of incident power reaching retina Retinal response nm red orange yellow green blue invisible retinal hazard 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 CO 2 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 SIRA laser safety

10 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 and (identical) TR 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 SIRA laser safety

11 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 SIRA laser safety

12 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 to to for t 0.35 s 1.5 t 3/8 for 0.35 s < t < 10 s 3.5 for t = 10 s 10 5 to The diameter of the limiting aperture applicable to measurements of irradiance and radiant exposure (t is the exposure duration) 3.5 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) SIRA laser safety

13 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 α min (1.5 mrad). Where the emission from such sources is within the retinal hazard region (i.e. between 400 nm and 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. Extendedsource 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 10 4 s: γ = 1.1 t 0.5 mrad for 10 4 s < t 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 > α max, in the wavelength range of nm to 4000 nm, the limiting angle of acceptance should not be larger than α max (0,1 rad) for the thermal hazard limits. Within the wavelength range of 400 nm to 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 α min γ α max. 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 α max or less than α min should be limited to α max or α min respectively, prior to calculating the mean. The retinal photochemical MPEs do not depend on the SIRA laser safety

14 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: SIRA laser safety

15 If the beam diameter is specified at the '1/e 2 ' 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 C 6 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/e 2 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, TR 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 M 2 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 µm <2% µm 70% µm 90% µm 70% µm <2% % Transmission 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 SIRA laser safety

16 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 treeline 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 SIRA laser safety

17 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 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 worstcase 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, SIRA laser safety

18 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 EN 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 : 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 SIRA laser safety

19 Qualitative description of Laser Safety Classes devised with Dr Karl Schulmeister, Osterreichisches Forschungszentrum, Seibersdorf, Austria Class Sub division Meaning Warning label and safety features Class 1 Class 1M Intrinsic Engineering Safe by virtue of the intrinsic low power of the laser, even with the use of optical instruments Embedded laser products, safe by virtue of engineering controls e.g. total enclosure guarding, scan failure mechanism. Collimated Well collimated beam, output in range nm, with large diameter that is safe for unaided viewing but potentially hazardous when a telescope or binoculars are used. High divergence Output in range nm, Safe for unaided viewing but potentially hazardous when an eye loupe or magnifier is used. Class 2 - Output in range nm. Safe for unintended exposure, even with the use of optical instruments, by virtue of natural aversion response to bright light Class 2M Collimated Well collimated beam in range 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. High divergence High divergence source with output in range 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. Class 3R Visible Output in range 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. Non-visible Output in UV ( nm) or IR (700 nm - 1 mm). Direct intrabeam viewing is potentially hazardous, but the risk is lower than for Class 3B. Class 3B - Medium power laser. Direct ocular exposure is hazardous, even taking into account aversion responses, but diffuse reflections are usually safe. Class 4 - High power laser. Direct exposure is hazardous to eye and diffuse reflection may also be hazardous. Skin and potential fire hazard. No warning label. No additional safety features No warning label. No additional safety features (but see requirements for protective housings in 6.4.3) LASER RADIATION DO NOT VIEW DIRECTLY WITH BIONOCULARS OR TELESCOPES* No additional safety features LASER RADIATION DO NOT VIEW DIRECTLY WITH MAGNIFIERS* No additional safety features DO NOT STARE INTO THE BEAM No additional safety features LASER RADIATION DO NOT STARE INTO THE BEAM OR VIEW DIRECTLY WITH BIONOCULARS OR TELESCOPES* No additional safety features LASER RADIATION DO NOT STARE INTO THE BEAM OR VIEW DIRECTLY WITH MAGNIFIERS* No additional safety features AVOID DIRECT EYE EXPOSURE No additional safety features AVOID DIRECT EYE EXPOSURE ( nm) AVOID EXPOSURE TO THE BEAM (outside the range nm) Emission warning device. AVOID EXPOSURE TO THE BEAM Key switch, emission warning device, externaπl interlock connection and beam stop 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 SIRA laser safety

20 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 SIRA laser safety

21 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 SIRA laser safety

22 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 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 , 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 , 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 user 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 SIRA laser safety

23 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) SIRA laser safety