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Safety aspects in the use of outdoor and surveying lasers
Safety aspects in the use of outdoor and surveying lasers B. J. GORHAM This paper reviews the variety of laser instruments currently used in the surveying and construction industry. Their manner of use and individual function are described and an indication provided of the sources of ocular hazard they present to users and others in the vicinity of their beams. Methods of making quantitative assessment of the laser ocular hazards are then introduced in relation to the current European Standard of Laser Safety EN 60825: 1991. This is followed by a discussion of practicalities in applying administrative and engineering methods of hazard control. KEYWORDS:
Laser
laser safety,
surveying,
hazard
laser
The prime laser instrument used in surveying is probably the alignment laser. This generates a visible laser beam, usually from a helium-neon gas laser, and produces a weakly convergent beam of light of initial diameter some 10 to I5 mm. The convergence is introduced into the laser beam by some form of focusing beam expander in order to limit the beam cross-section as it propagates under natural diffraction. If the maximum working range of the alignment laser is R metres then a symmetrical beam can be propagated which minimizes the beam cross-section over this distance by setting the initial beam diameter at the laser to be D metres where, for a Gaussian beam output profile, this is related to the working range by the formula: D = ~(;.R/z)‘.~ where i is the wavelength of the laser light. By focusing the beam to a point at mid-range, the natural expansion of the beam diameter can be postponed until the distance R has been traversed. An exaggerated view of the designed beam geometry for an alignment laser is shown in Fig. 1. The main function of an alignment laser is to provide a straight line reference for the purposes of setting-out; this may be performed by observing the laser beam when it impinges on an appropriate target or by the use of some form of electronic detector. In either case it is likely that accidental viewing of the beam can occur by the naked eye and, in view of the working environment normally associated with civil
The author IS in the industrial Metrology Research Unit, University of East London, Longbridge Road, Dagenham, Essex RM8 2AS. UK. Received 4 January 1994. Revised 2 April 1994.
0030-3992/95/$10.00 Optics & Laser Technology Vol 27 No 1 1995
safety
standards
engineering operations, also likely that accidental exposure of the eye could occur when using an optical instrument.
instruments
Alignment
assessment,
@ 1995
However, alignment lasers are normally used in fixed and pre-determined locations and often below normal eye-level. In these circumstances it follows that both the assessment of potential laser eye hazards as well as implementation of corresponding safety measures can be performed without undue difficulty. The degree of observance of such safety measures by the site personnel and operators of the laser equipment may still remain questionable however. Whut we the most signl_fisant IJ,LVhuzardy assoc~i~~tt~d ndh the KW oj’alignmmt lasers? The highest threshold of hazard is that corresponding to accidental viewing of the direct beam. Since alignment lasers emit visible beams, the chances of incurring an accidental exposure of the eye are reduced, but unfortunately visible laser radiation offers a greater hazard to the eye than does invisible radiation in the infra-red; a feature of many other surveying instruments. It can be seen from the figure that the region where the beam is most concentrated occurs near to the middle of its working trajectory. This is potentially the most hazardous region for exposure of the eye.
--R----------_) Fig. 1 Beam geometry
Elsevier
Science.
for an alignment
All rights
laser
reserved 19
Safety
aspects
of outdoor
and surveying
lasers:
B. J. Gorham
The safety threshold for accidental viewing of such a visible laser beam corresponds to the irradiance value in an): part of the beam not exceeding 25 W m ’ of beam cross-section. This value is equivalent to I mW of visible laser light entering the pupil of the eye (always assumed to be of 7 mm diameter) in an exposure period of 0.25 s (the nomial period of time required for the body’s natural aversion responses, such as the involuntary blinking of the eye, to take place). If the particular alignment laser is a Class 2 laser product, its maximum radiant output power will be 1 mW; thus the maximum beam power which could enter the eye in this case, irrespective of any beam focusing, is that which is considered safe for accidental exposure. This also applies to Class 3A visible lasers provided their beams everywhere remain sufficiently expanded so that not more than 1 mW can enter the largest nominal eye pupil diameter of 7 mm. To maintain, for example, the safe accidental exposure level of beam irradiance for a Class 3A visible alignment laser to within its limit of 25 W mm2 no more than 1 mW of radiant power can enter a circular diameter of 7 mm in any part of the beam; thus, for a Class 3A laser with a radiant output power of 5 mW, the circular section beam should nowhere reduce in diameter below 16 mm. NE. In the context of laser eye hazards, normal spectacles do not constitute optical instruments. Once it is established that the accidental exposure threshold is not exceeded. the next consideration is the possible need for the operator to view a beam deliberately which may be reflected from the face of a detector or scale. Here, it is necessary to establish the radiation level involved and the maximum time of deliberate exposure required for the particular operation to take place. The manner of undertaking such an assessment in relation to the current European Standard EN 60825: 1991 is covered later in this paper. Finally, it must not be forgotten that beams from alignment lasers are highly collimated and designed to be effective at ranges of several hundred metres. Their use may have significant implications for other operations on the construction site as well as in public areas which are off-site. In particular, the effects of accidental irradiation of the eyes of a vehicle driver by an alignment laser, which is being moved whilst switched on, can obviously be extremely serious. In the UK, at least, such an occurrence would have significant implications for the laser operator under the Health & Safety at Work Act. Laser
theodolite
Another kind of laser surveying instrument which is becoming very popular due to its flexibility and versatility as a measuring tool is the laser theodohte. This may be realized by the process of attaching to the eyepiece of a conventional theodolite a glass-fibre coupler fed by a visible laser, usually HeNe. which is attached to the tripod leg. Alternatively, the device is realized by a purpose-built theodolite which has an integral visible laser source. This latter design is shown in diagrammatic form in Fig. 2. The main difference in
4I
-
:e ---
L -
’ =r __piJ
-
-
-
-,
-
-
I
!
- -
_-)
Fig. 2 A laser theodollte
use of this type of device and an ordinary alignment laser is that (a) the beam issuing from the theodolite may be directed by movements of the telescope and (b) the beam may be focused using the telescope controls of the theodolite. In use as an alignment laser, the beam is directed either along a horizontal line or at a particular desired inclination for interception by eye or electronic detector as before. Another very important use of such a device is to provide an illuminated target for observation by a second non-laser theodolite. The intersection measurements from both ends of the defined fixed baseline enable three-dimensional co-ordination of a number of points on a structure to be determined and thus enable its surface shape to be mapped. It is clear from the extra degrees of freedom available for the laser beam when projected by a laser theodolite that all of the hazards described earlier for the alignment lasers are present here and more besides. First, the pointing of the beam is not static now and so the area of ocular hazard can change instantly, and secondly, the ability of the operator to focus the laser beam can both move the region of hazard and exacerbate it. At any particular instant the most hazardous region of the beam will be that of maximum focus. Laser
level
So far we have discussed those surveying instruments which use visible laser beams for essentially defining a line reference in space, but there is a category of laser instruments that represents the largest volume of laser use in surveying; these are the rotary laser levels. They divide easily into two types; one of which emits visible beams and whose rotation may be stopped by the operator without extinguishing the beam, and the other which emits laser radiation in the near infra-red (invisible) and whose scanning beams cannot be stopped without being extinguished.
The principle of operation of a rotary laser level is particularly simple and is illustrated in Fig. 3. An essentially collimated beam of laser light is projected in a horizontal direction from the laser projector and this beam is then caused to rotate about a vertical axis through the projector so as to sweep out a horizontal plane. Typically, the instrument provides a height reference which can be accessed by any operator around the instrument equipped with an appropriate electronic detector over a radius of some 300 metres. All normal commercial infra-red laser levels are Class 1 laser products and represent no significant eye hazard to anyone in the vicinity. Should their scanning Optics
20
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& Laser Technology Vol27 No 1 1995
Safety aspects of outdoor
Horizontal
and surveying
lasers: B. J. Gorham
reflection from a distant target. Some of these devices, used in land surveying, require co-operative targets such as glass corner-cube retro-reflecting prisms to return the moderately powerful signals to the transmitter; others, which emit much more powerful signals will operate even with uncooperative targets such as a rock face or even the hull of a ship. The latter type of range-finder when not used by the military is usually employed in hydrographic work rather than on a construction site. Most of the range-finders used in land surveying are Class 1 laser products and no specific precautions are necessary for ocular hazard control when using these invisible beam devices. In contrast, many of the second variety of range-finder are Class 3B laser products and substantial precautions may be necessary in their use. Quantitative
assessment
of laser
hazards
The process of determining the extent of any hazardous areas in a laser beam is made easier if a portable laser radiation power meter IS used; if one is not available however, it is still possible to make a reasonable estimate of hazard in the field although with increased difficulty. Fig. 3 A laser level
drive fail for any reason the laser beam will automatically be extinguished. Visible beam laser levels, however, may introduce some hazards to their operators and others in the vicinity of the beam. Most of these devices are capable of being operated in a manual mode with the scanning drive turned off. This provides a whole range of measurement capability both in a horizontal plane or even in a local vertical plane produced by setting the level on its side. In this ‘stopped’ mode the beam may be pointed by the operator to define a set of illuminated targets on a structure which all lie in one plane; be it horizontal or vertical. The potential hazards to the eye represented by these operations are identical to those considered in relation to laser theodolites. The most hazardous mode of operation is always the static beam mode, and the manufacturer’s classification should reflect this. A Class 2 laser level will produce no more than 1 mW in the beam and, even if accidentally viewed through an optical instrument, will not produce a significant risk of eye injury. A Class 3A laser level, however, can generate a significant risk of eye injury for an accidental exposure where an optical instrument is involved. Laser range-finder Normal electronic distance meters (EDMs) which rely for their measurements on the modulation of cw emissions from semiconductor laser diodes are invariably Class 1 laser products and no specific precautions are necessary in their use for the control of laser eye hazards. However, there is a type of range measuring instrument which generates very short pulses of intense laser radiation and measures the time for return of the transmitted pulse signal after Optics & Laser Technology Vol 27 No 1 1995
The Lasersafe hand-held laser power meter produced by the Industrial Metrology Research Unit of the University of East London is designed for the rapid assessment of laser hazards where surveying-type laser instruments are used. It uses a 7 mm diameter entrance aperture to match the human eye as well as a further similar aperture to monitor continuously ambient light levels. It can provide rapid and accurate determinations of laser radiant power for static beams as well as for those produced by scanning lasers: it operates with HeNe lasers at 633 nm as well as lasers emitting in the near infra-red between 700 and 900 nm. We will firstly consider the use of a Lasersafe meter or one similar to it for assessing the eye hazards presented by the types of laser surveying instruments already discussed. The risk of eye injury from exposure to the static beam of an alignment laser depends, in the first instance, on the maximum irradiance in the beam and, more particularly, on whether this value anywhere exceeds 25 W mm2. This irradiance level represents the threshold for safe accidental exposure of the eye to visible laser radiation and is derived from the Maximum Permissible Exposure (Level) or MPE(L) recommended for the eye by the current European Standard on laser safety EN 60825: 1991. The appropriate MPE is given in Table VI of the Standard for the blink-reflex time period of 0.25 s in respect of visible laser radiation by 18 to-75 J mm ’ where t = 0.25 s. In terms of irradiance in watts per square metre, this translates into 18 (O.25)mo,25 or 25 W m-‘. As stated earlier, if the alignment laser is a Class 2 laser product, the maximum emitted radiant power is 1 mW and in that event there is no possibility of exceeding the 25 W mm2 limit when averaged over the nominal eye-pupil of 7 mm at any point in the beam. If
21
Safety
aspects
of outdoor
and surveying
lasers:
8. J. Gorham
the laser product is Class 3A and classified under a European laser safety Standard then provided the beam has not been subject to focusing after leaving the exit aperture of the laser again the accidental exposure threshold will not be exceeded. In this connection it is worth pointing out that some surveying laser instruments from the USA are provided with classification labels showing them to be Class 3A and, as expected, their total radiant power output is not in excess of 5 mW. However, the beam diameter of some of these lasers is such that there are regions of the beam where the local irradiance value averaged over a 7 mm diameter circle exceeds 100 W m 2 ! These products are correctly classified under existing American laser safety Standards but would be invalid for that classification under any European Standard currently in force. Thus, the first task with a Lasersafe, or similar type of laser power meter which has a 7 mm diameter aperture in front of its integral photocell, is to measure the radiant power entering this aperture of the meter and to ensure that nowhere does the meter reading exceed 1 mW. Any regions of the beam which exhibit a higher reading should be designated hazardous areas since any exposure of the eye to these parts of the laser beam carries with it a significant risk of eye injury. Class 3B laser products can emit radiation which results in exposure of the eye beyond the safe accidental viewing threshold, and if this condition is seen to apply along the beam of a particular laser being assessed that laser product should rightly carry a 3B classification. For this reason it is common to refer to these hazardous areas of a laser beam as 3B areas. Laser theodolites are assessed in a similar manner by a laser power meter but, in the case of this type of instrument, it is essential that it is not above a Class 2 laser product. The reason for this is that, if the instrument emits a laser beam of power greater than 1 mW but nevertheless carries a classification label stating that it is a Class 3A product, the act of bringing the laser beam to a focus will invariably result in the accidental exposure threshold being exceeded. One is then presented with essentially a Class 3B laser beam in terms of hazard which can be moved around in space with ease. Controlling the hazard from such a device on site is virtually impossible. Laser levels present more problems for eye hazard assessment than alignment lasers and less than fat laser theodolites; and this only when their scanning is stopped. No Class 3B laser instrument should be used on an open site as a laser level; hazard control would be virtually impossible. The stopped laser level beam may be rotated by hand or by motorized hand control and brought to rest in any direction in a full circle. The prime concern for the site laser safety officer is to determine before operations commence that there are no 3B regions in the long-range beam. In this connection, it should be remembered from Fig. 1 that the beam geometry is usually designed to produce a maximum concentration of light at some considerable distance from the laser source.
22
After identifying any 3B areas present in the laser beam, the next consideration is to ascertain whether normal operations of the equipment would entail a direct viewing of a reflected beam from the laser, and if so the period of time that would be needed for this deliberate viewing to occur. The laser power meter should then be used to replace the human eye in order to determine the maximum radiant power which could enter the 7 mm diameter eye pupil. If. for example, the time required to view the reflected laser beam is t seconds, then the formula quoted earlier for MPE from the European laser safety Standard, 18 t o.25 W m - ’ may be used to determine the maximum safe radiant power to be received by the meter. The meter aperture is 7 mm diameter, equivalent to an area of 3.85 x 10m5 m2; if the maximum meter reading for safe viewing for the period t is P mW, then: P = 1000 x 18 x 3.85 x lo-”
x tm”.25 mW
or P = 0.7 x fmO.”
mW
If a suitable laser power meter is not available then assessment can still be made but here the maximum radiant output power of the laser should be obtained from the classification label or manufacturer’s literature and the geometry of the laser beam must be known or measured directly. The beam diameter may be measured in one location by viewing the beam transmitted through mm grid graph paper; the apparent diameter should be reduced by a factor of 0.7 if the ambient light levels are low during the measurement but taken as seen for high background illumination. If the laser beam power from the laser is P mW and the estimated beam size is D mm then the irradiance is given by radiant power/area, namely (P,ilOOO) + (7cD2/4 x 106) W mm2. This is to be compared with 25 W mm2 to define the 3B areas. If the divergence of a laser beam is required then the simplest way is to measure the beam diameter at two locations in the beam at a known separation and determine the ratio of change in diameter per separation length to give the divergence in radians. For example, if the two beam diameters are D, mm and D2 mm which are separated by X m, the beam divergence in radians (4) is given by (i, = (D, - D,)/(lOOO X) If the divergence angle 4 is known, then for a range R the beam cross-sectional area A is given by .-1 = (n!4)(cl + Rq+)’ The appropriate
beam irradiance
is P/A (see Fig. 4).
The types of laser surveying instrument that are most difficult to assess for ocular safety are the pulsed range-finders. In addition to requiring a knowlege of the radiation wavelength, the maximum beam power during transmission and the beam energy distribution in space; the beam geometry and time pattern for the beam transmission, is also needed. EN 60825: 1991 provides tables of MPEs for ocular exposure at various wavelengths and for a number of time ranges; Optics & Laser Technology Vol 27 No 1 1995
Safety aspects of outdoor
Hazard
VRFig. 4 Typical
conical
form
laser beam
Table VI has the appropriate list of MPEs where direct exposure of the eye to laser beams is involved. Section 13.3 of the Standard deals with exposures from repetitively pulsed or modulated lasers, and pulsed laser range-finders fall into this category. The method of assessment is essentially the same as before, in that an estimate should be made of the MPE for the particular laser being used in relation to the regions where persons who may be exposed to the hazard from the beam may be found. It is the method of assessing the MPE for these instruments that is more complex. Section 13.3 of the Standard sets out three criteria which should be used in assessing MPE for pulsed laser sources. Before using these it is necessary to estimate a reasonable maximum time (T seconds) over which a person could suffer exposure from the laser radiation. For example, if frequent ranges are taken over a period of half an hour to a vessel being berthed into a dock and personnel are situated on the far side of the dock but in positions where they will be exposed to the radiation for at least some of this time, then the assumed time T should be 1800 seconds (12 hour). The three criteria for estimating the MPE to be applied to each individual pulse from the range-finder are as follows: (i) knowing the laser wavelength and the time duration of a single pulse, use Table VI to obtain the MPE (single pulse), say MPE,; (ii) compute from Table VI the MPE corresponding to a continuous exposure over the estimated period T seconds and divide this result by the number of pulses which could actually occur during this period: this provides an estimated average MPE per pulse. say MPE,,; (iii) finally, the effects of the repetition of the pulsed exposure should be taken into account and this is achieved by estimating an MPE per pulse which allows for the whole of the pulse train and is given by MPE,,,;, = MPE, t No.25 where N is the total number of pulses occurring in the total period of T seconds. There are thus three assessments to be made of the MPE value and the one which should be applied is the most restrictive; that is, the least value for the MPE per pulse. Optics 81 Laser Technology Vol 27 No 1 1995
and surveying
controls
lasers: B. J. Gorham
in practice
The Standard recommends that where Class 3B or Class 4 lasers are being used, a Laser Safety Officer should be in attendance on site. In fact it is rare to see powerful lasers which are classified in the upper 3B band being used in construction or surveying; the exceptions are usually ex-military range-finders and these require very special attention. The normal laser equipment used on site is therefore unlikely to be hazardous beyond that corresponding to the lower band of 3B. It is essential that the operators of Class 3A lasers and those at the lower end of Class 3B are well aware of the hazards to the eye which their lasers may present and are equally aware of the legal obligations which they inherit when they operate the laser equipment. In the UK, the controlling legislation for laser use is the Health & Safety at Work Act which places a duty upon laser operators to ensure the safety of their own eyes as well as the eyes and general safety of any others who may be affected by the laser. Ignorance of the potential hazards from using the laser do not relieve these legal responsibilities. The first job of the site safety manager is to determine the classification of all of the laser equipment to be used on the site. The only advice s/he needs to provide to the operators of Class 1 laser equipment, and then onlv if it emits a visible beam, is to guard against direciing it into the eyes of personnel who may, as a consequence, become distracted from their own operations amd thereby cause an accident. Class 2 (visible) lasers are more susceptible to this kind of indirect hazard since they produce more powerful light beams. It is worth considering the placing of warning signs in the vicinity of the laser beam pointing out the potential hazard of looking into the beam and also placing an opaque obstacle in the beam at the end of its useful path a beam stop. Although there is not considered to be a significant risk of irreversible eye injury resulting from a brief exposure to a Class 2 laser beam, there can be severe impairment of vision in the exposed eye for a considerable time and construction sites are often hazardous places even with perfect vision. Class 3A lasers deserve more attention from the site safety manager. Any exposure of the e,ye to the laser beams via an optical instrument, such as a telescope or binoculars, is likely to be hazardous. The operator should be fully aware of this fact since such instruments are very common where surveying or construction is practised. The operator should also be sensitive to any conditions on the site where the beam from the laser could become focused, perhaps after reflection from a curved mirror surface, so that the safe exposure level for the eye could become exceeded. There are two types of hazard control which can be put into effect: there are the administrative controls which involve the training of personnel who are responsible for the operation of laser equipmentwhich involves raising the level of awareness of laser hazards and some guidance as to the
23
Safery aspects of outdoor and surveying
lasers: B. J. Gorham
implementation of safety measures in practice. These controls also include the siting of appropriate warning signs in the regions where exposure to laser beams can occur and ensuring that all personnel potentially at risk from the laser are aware of established effective accident reporting procedures. Engineering controls refer to physical barriers which limit the degree of hazard. These may be shields fitted close to the laser beam source where the beam is strongly divergent, so that the radiant power is safely diluted at any point where it is possible to place an eye in the beam. A more common form is provision of rope barriers which carry closely-spaced signs which warn of the hazards of exposure to an open laser beam. Most common of all is a simple beam stop. Controlling the hazards where Class 3B lasers are being used is usually more difficult and more important than for those of a lower classification. The first requirement in the hazard assessment is to determine the region where even an accidental exposure of the eye may be hazardous. This is best performed by using an appropriate laser power meter as described earlier but may be achieved by calculation
if the laser beam power and geometry are known. The ‘3B’ areas which are identified by such an assessment must be isolated by physical barriers wherever possible and suitably posted with warning signs. Any personnel who need to work within these hazardous areas should be fully aware of the eye hazards presented by the laser beams and, if consonant with other hazards within that environment, should wear appropriate laser eye protectors. Notwithstanding the careful measurements of laser power and geometry and the local controls which are put in place, the best insurance against eye injury from the use of laser equipment is provided by a high level of awareness of laser hazards, at least among those who are responsible for their operation. References British Standard BS EN 60825: 1992 Radiation safety of laser products. equipment classification, requirements and user’s guide. British Standards Institution, 2 Park Street, London W 14 2BS, UK International Electrochemical Commission Standard IEC 825-l : 1993 A Guide to the Safe Use of Lasers in Surveying and Construction The Royal Institution of Chartered Surveyors, 12 Great George Street. London SWlP 3AD. UK
Optics & Laser Technology
Optics
24
& Laser Technology Vol 27 No 1 1995
Laser
laser safety,
surveying,
hazard
laser
The prime laser instrument used in surveying is probably the alignment laser. This generates a visible laser beam, usually from a helium-neon gas laser, and produces a weakly convergent beam of light of initial diameter some 10 to I5 mm. The convergence is introduced into the laser beam by some form of focusing beam expander in order to limit the beam cross-section as it propagates under natural diffraction. If the maximum working range of the alignment laser is R metres then a symmetrical beam can be propagated which minimizes the beam cross-section over this distance by setting the initial beam diameter at the laser to be D metres where, for a Gaussian beam output profile, this is related to the working range by the formula: D = ~(;.R/z)‘.~ where i is the wavelength of the laser light. By focusing the beam to a point at mid-range, the natural expansion of the beam diameter can be postponed until the distance R has been traversed. An exaggerated view of the designed beam geometry for an alignment laser is shown in Fig. 1. The main function of an alignment laser is to provide a straight line reference for the purposes of setting-out; this may be performed by observing the laser beam when it impinges on an appropriate target or by the use of some form of electronic detector. In either case it is likely that accidental viewing of the beam can occur by the naked eye and, in view of the working environment normally associated with civil
The author IS in the industrial Metrology Research Unit, University of East London, Longbridge Road, Dagenham, Essex RM8 2AS. UK. Received 4 January 1994. Revised 2 April 1994.
0030-3992/95/$10.00 Optics & Laser Technology Vol 27 No 1 1995
safety
standards
engineering operations, also likely that accidental exposure of the eye could occur when using an optical instrument.
instruments
Alignment
assessment,
@ 1995
However, alignment lasers are normally used in fixed and pre-determined locations and often below normal eye-level. In these circumstances it follows that both the assessment of potential laser eye hazards as well as implementation of corresponding safety measures can be performed without undue difficulty. The degree of observance of such safety measures by the site personnel and operators of the laser equipment may still remain questionable however. Whut we the most signl_fisant IJ,LVhuzardy assoc~i~~tt~d ndh the KW oj’alignmmt lasers? The highest threshold of hazard is that corresponding to accidental viewing of the direct beam. Since alignment lasers emit visible beams, the chances of incurring an accidental exposure of the eye are reduced, but unfortunately visible laser radiation offers a greater hazard to the eye than does invisible radiation in the infra-red; a feature of many other surveying instruments. It can be seen from the figure that the region where the beam is most concentrated occurs near to the middle of its working trajectory. This is potentially the most hazardous region for exposure of the eye.
--R----------_) Fig. 1 Beam geometry
Elsevier
Science.
for an alignment
All rights
laser
reserved 19
Safety
aspects
of outdoor
and surveying
lasers:
B. J. Gorham
The safety threshold for accidental viewing of such a visible laser beam corresponds to the irradiance value in an): part of the beam not exceeding 25 W m ’ of beam cross-section. This value is equivalent to I mW of visible laser light entering the pupil of the eye (always assumed to be of 7 mm diameter) in an exposure period of 0.25 s (the nomial period of time required for the body’s natural aversion responses, such as the involuntary blinking of the eye, to take place). If the particular alignment laser is a Class 2 laser product, its maximum radiant output power will be 1 mW; thus the maximum beam power which could enter the eye in this case, irrespective of any beam focusing, is that which is considered safe for accidental exposure. This also applies to Class 3A visible lasers provided their beams everywhere remain sufficiently expanded so that not more than 1 mW can enter the largest nominal eye pupil diameter of 7 mm. To maintain, for example, the safe accidental exposure level of beam irradiance for a Class 3A visible alignment laser to within its limit of 25 W mm2 no more than 1 mW of radiant power can enter a circular diameter of 7 mm in any part of the beam; thus, for a Class 3A laser with a radiant output power of 5 mW, the circular section beam should nowhere reduce in diameter below 16 mm. NE. In the context of laser eye hazards, normal spectacles do not constitute optical instruments. Once it is established that the accidental exposure threshold is not exceeded. the next consideration is the possible need for the operator to view a beam deliberately which may be reflected from the face of a detector or scale. Here, it is necessary to establish the radiation level involved and the maximum time of deliberate exposure required for the particular operation to take place. The manner of undertaking such an assessment in relation to the current European Standard EN 60825: 1991 is covered later in this paper. Finally, it must not be forgotten that beams from alignment lasers are highly collimated and designed to be effective at ranges of several hundred metres. Their use may have significant implications for other operations on the construction site as well as in public areas which are off-site. In particular, the effects of accidental irradiation of the eyes of a vehicle driver by an alignment laser, which is being moved whilst switched on, can obviously be extremely serious. In the UK, at least, such an occurrence would have significant implications for the laser operator under the Health & Safety at Work Act. Laser
theodolite
Another kind of laser surveying instrument which is becoming very popular due to its flexibility and versatility as a measuring tool is the laser theodohte. This may be realized by the process of attaching to the eyepiece of a conventional theodolite a glass-fibre coupler fed by a visible laser, usually HeNe. which is attached to the tripod leg. Alternatively, the device is realized by a purpose-built theodolite which has an integral visible laser source. This latter design is shown in diagrammatic form in Fig. 2. The main difference in
4I
-
:e ---
L -
’ =r __piJ
-
-
-
-,
-
-
I
!
- -
_-)
Fig. 2 A laser theodollte
use of this type of device and an ordinary alignment laser is that (a) the beam issuing from the theodolite may be directed by movements of the telescope and (b) the beam may be focused using the telescope controls of the theodolite. In use as an alignment laser, the beam is directed either along a horizontal line or at a particular desired inclination for interception by eye or electronic detector as before. Another very important use of such a device is to provide an illuminated target for observation by a second non-laser theodolite. The intersection measurements from both ends of the defined fixed baseline enable three-dimensional co-ordination of a number of points on a structure to be determined and thus enable its surface shape to be mapped. It is clear from the extra degrees of freedom available for the laser beam when projected by a laser theodolite that all of the hazards described earlier for the alignment lasers are present here and more besides. First, the pointing of the beam is not static now and so the area of ocular hazard can change instantly, and secondly, the ability of the operator to focus the laser beam can both move the region of hazard and exacerbate it. At any particular instant the most hazardous region of the beam will be that of maximum focus. Laser
level
So far we have discussed those surveying instruments which use visible laser beams for essentially defining a line reference in space, but there is a category of laser instruments that represents the largest volume of laser use in surveying; these are the rotary laser levels. They divide easily into two types; one of which emits visible beams and whose rotation may be stopped by the operator without extinguishing the beam, and the other which emits laser radiation in the near infra-red (invisible) and whose scanning beams cannot be stopped without being extinguished.
The principle of operation of a rotary laser level is particularly simple and is illustrated in Fig. 3. An essentially collimated beam of laser light is projected in a horizontal direction from the laser projector and this beam is then caused to rotate about a vertical axis through the projector so as to sweep out a horizontal plane. Typically, the instrument provides a height reference which can be accessed by any operator around the instrument equipped with an appropriate electronic detector over a radius of some 300 metres. All normal commercial infra-red laser levels are Class 1 laser products and represent no significant eye hazard to anyone in the vicinity. Should their scanning Optics
20
-
& Laser Technology Vol27 No 1 1995
Safety aspects of outdoor
Horizontal
and surveying
lasers: B. J. Gorham
reflection from a distant target. Some of these devices, used in land surveying, require co-operative targets such as glass corner-cube retro-reflecting prisms to return the moderately powerful signals to the transmitter; others, which emit much more powerful signals will operate even with uncooperative targets such as a rock face or even the hull of a ship. The latter type of range-finder when not used by the military is usually employed in hydrographic work rather than on a construction site. Most of the range-finders used in land surveying are Class 1 laser products and no specific precautions are necessary for ocular hazard control when using these invisible beam devices. In contrast, many of the second variety of range-finder are Class 3B laser products and substantial precautions may be necessary in their use. Quantitative
assessment
of laser
hazards
The process of determining the extent of any hazardous areas in a laser beam is made easier if a portable laser radiation power meter IS used; if one is not available however, it is still possible to make a reasonable estimate of hazard in the field although with increased difficulty. Fig. 3 A laser level
drive fail for any reason the laser beam will automatically be extinguished. Visible beam laser levels, however, may introduce some hazards to their operators and others in the vicinity of the beam. Most of these devices are capable of being operated in a manual mode with the scanning drive turned off. This provides a whole range of measurement capability both in a horizontal plane or even in a local vertical plane produced by setting the level on its side. In this ‘stopped’ mode the beam may be pointed by the operator to define a set of illuminated targets on a structure which all lie in one plane; be it horizontal or vertical. The potential hazards to the eye represented by these operations are identical to those considered in relation to laser theodolites. The most hazardous mode of operation is always the static beam mode, and the manufacturer’s classification should reflect this. A Class 2 laser level will produce no more than 1 mW in the beam and, even if accidentally viewed through an optical instrument, will not produce a significant risk of eye injury. A Class 3A laser level, however, can generate a significant risk of eye injury for an accidental exposure where an optical instrument is involved. Laser range-finder Normal electronic distance meters (EDMs) which rely for their measurements on the modulation of cw emissions from semiconductor laser diodes are invariably Class 1 laser products and no specific precautions are necessary in their use for the control of laser eye hazards. However, there is a type of range measuring instrument which generates very short pulses of intense laser radiation and measures the time for return of the transmitted pulse signal after Optics & Laser Technology Vol 27 No 1 1995
The Lasersafe hand-held laser power meter produced by the Industrial Metrology Research Unit of the University of East London is designed for the rapid assessment of laser hazards where surveying-type laser instruments are used. It uses a 7 mm diameter entrance aperture to match the human eye as well as a further similar aperture to monitor continuously ambient light levels. It can provide rapid and accurate determinations of laser radiant power for static beams as well as for those produced by scanning lasers: it operates with HeNe lasers at 633 nm as well as lasers emitting in the near infra-red between 700 and 900 nm. We will firstly consider the use of a Lasersafe meter or one similar to it for assessing the eye hazards presented by the types of laser surveying instruments already discussed. The risk of eye injury from exposure to the static beam of an alignment laser depends, in the first instance, on the maximum irradiance in the beam and, more particularly, on whether this value anywhere exceeds 25 W mm2. This irradiance level represents the threshold for safe accidental exposure of the eye to visible laser radiation and is derived from the Maximum Permissible Exposure (Level) or MPE(L) recommended for the eye by the current European Standard on laser safety EN 60825: 1991. The appropriate MPE is given in Table VI of the Standard for the blink-reflex time period of 0.25 s in respect of visible laser radiation by 18 to-75 J mm ’ where t = 0.25 s. In terms of irradiance in watts per square metre, this translates into 18 (O.25)mo,25 or 25 W m-‘. As stated earlier, if the alignment laser is a Class 2 laser product, the maximum emitted radiant power is 1 mW and in that event there is no possibility of exceeding the 25 W mm2 limit when averaged over the nominal eye-pupil of 7 mm at any point in the beam. If
21
Safety
aspects
of outdoor
and surveying
lasers:
8. J. Gorham
the laser product is Class 3A and classified under a European laser safety Standard then provided the beam has not been subject to focusing after leaving the exit aperture of the laser again the accidental exposure threshold will not be exceeded. In this connection it is worth pointing out that some surveying laser instruments from the USA are provided with classification labels showing them to be Class 3A and, as expected, their total radiant power output is not in excess of 5 mW. However, the beam diameter of some of these lasers is such that there are regions of the beam where the local irradiance value averaged over a 7 mm diameter circle exceeds 100 W m 2 ! These products are correctly classified under existing American laser safety Standards but would be invalid for that classification under any European Standard currently in force. Thus, the first task with a Lasersafe, or similar type of laser power meter which has a 7 mm diameter aperture in front of its integral photocell, is to measure the radiant power entering this aperture of the meter and to ensure that nowhere does the meter reading exceed 1 mW. Any regions of the beam which exhibit a higher reading should be designated hazardous areas since any exposure of the eye to these parts of the laser beam carries with it a significant risk of eye injury. Class 3B laser products can emit radiation which results in exposure of the eye beyond the safe accidental viewing threshold, and if this condition is seen to apply along the beam of a particular laser being assessed that laser product should rightly carry a 3B classification. For this reason it is common to refer to these hazardous areas of a laser beam as 3B areas. Laser theodolites are assessed in a similar manner by a laser power meter but, in the case of this type of instrument, it is essential that it is not above a Class 2 laser product. The reason for this is that, if the instrument emits a laser beam of power greater than 1 mW but nevertheless carries a classification label stating that it is a Class 3A product, the act of bringing the laser beam to a focus will invariably result in the accidental exposure threshold being exceeded. One is then presented with essentially a Class 3B laser beam in terms of hazard which can be moved around in space with ease. Controlling the hazard from such a device on site is virtually impossible. Laser levels present more problems for eye hazard assessment than alignment lasers and less than fat laser theodolites; and this only when their scanning is stopped. No Class 3B laser instrument should be used on an open site as a laser level; hazard control would be virtually impossible. The stopped laser level beam may be rotated by hand or by motorized hand control and brought to rest in any direction in a full circle. The prime concern for the site laser safety officer is to determine before operations commence that there are no 3B regions in the long-range beam. In this connection, it should be remembered from Fig. 1 that the beam geometry is usually designed to produce a maximum concentration of light at some considerable distance from the laser source.
22
After identifying any 3B areas present in the laser beam, the next consideration is to ascertain whether normal operations of the equipment would entail a direct viewing of a reflected beam from the laser, and if so the period of time that would be needed for this deliberate viewing to occur. The laser power meter should then be used to replace the human eye in order to determine the maximum radiant power which could enter the 7 mm diameter eye pupil. If. for example, the time required to view the reflected laser beam is t seconds, then the formula quoted earlier for MPE from the European laser safety Standard, 18 t o.25 W m - ’ may be used to determine the maximum safe radiant power to be received by the meter. The meter aperture is 7 mm diameter, equivalent to an area of 3.85 x 10m5 m2; if the maximum meter reading for safe viewing for the period t is P mW, then: P = 1000 x 18 x 3.85 x lo-”
x tm”.25 mW
or P = 0.7 x fmO.”
mW
If a suitable laser power meter is not available then assessment can still be made but here the maximum radiant output power of the laser should be obtained from the classification label or manufacturer’s literature and the geometry of the laser beam must be known or measured directly. The beam diameter may be measured in one location by viewing the beam transmitted through mm grid graph paper; the apparent diameter should be reduced by a factor of 0.7 if the ambient light levels are low during the measurement but taken as seen for high background illumination. If the laser beam power from the laser is P mW and the estimated beam size is D mm then the irradiance is given by radiant power/area, namely (P,ilOOO) + (7cD2/4 x 106) W mm2. This is to be compared with 25 W mm2 to define the 3B areas. If the divergence of a laser beam is required then the simplest way is to measure the beam diameter at two locations in the beam at a known separation and determine the ratio of change in diameter per separation length to give the divergence in radians. For example, if the two beam diameters are D, mm and D2 mm which are separated by X m, the beam divergence in radians (4) is given by (i, = (D, - D,)/(lOOO X) If the divergence angle 4 is known, then for a range R the beam cross-sectional area A is given by .-1 = (n!4)(cl + Rq+)’ The appropriate
beam irradiance
is P/A (see Fig. 4).
The types of laser surveying instrument that are most difficult to assess for ocular safety are the pulsed range-finders. In addition to requiring a knowlege of the radiation wavelength, the maximum beam power during transmission and the beam energy distribution in space; the beam geometry and time pattern for the beam transmission, is also needed. EN 60825: 1991 provides tables of MPEs for ocular exposure at various wavelengths and for a number of time ranges; Optics & Laser Technology Vol 27 No 1 1995
Safety aspects of outdoor
Hazard
VRFig. 4 Typical
conical
form
laser beam
Table VI has the appropriate list of MPEs where direct exposure of the eye to laser beams is involved. Section 13.3 of the Standard deals with exposures from repetitively pulsed or modulated lasers, and pulsed laser range-finders fall into this category. The method of assessment is essentially the same as before, in that an estimate should be made of the MPE for the particular laser being used in relation to the regions where persons who may be exposed to the hazard from the beam may be found. It is the method of assessing the MPE for these instruments that is more complex. Section 13.3 of the Standard sets out three criteria which should be used in assessing MPE for pulsed laser sources. Before using these it is necessary to estimate a reasonable maximum time (T seconds) over which a person could suffer exposure from the laser radiation. For example, if frequent ranges are taken over a period of half an hour to a vessel being berthed into a dock and personnel are situated on the far side of the dock but in positions where they will be exposed to the radiation for at least some of this time, then the assumed time T should be 1800 seconds (12 hour). The three criteria for estimating the MPE to be applied to each individual pulse from the range-finder are as follows: (i) knowing the laser wavelength and the time duration of a single pulse, use Table VI to obtain the MPE (single pulse), say MPE,; (ii) compute from Table VI the MPE corresponding to a continuous exposure over the estimated period T seconds and divide this result by the number of pulses which could actually occur during this period: this provides an estimated average MPE per pulse. say MPE,,; (iii) finally, the effects of the repetition of the pulsed exposure should be taken into account and this is achieved by estimating an MPE per pulse which allows for the whole of the pulse train and is given by MPE,,,;, = MPE, t No.25 where N is the total number of pulses occurring in the total period of T seconds. There are thus three assessments to be made of the MPE value and the one which should be applied is the most restrictive; that is, the least value for the MPE per pulse. Optics 81 Laser Technology Vol 27 No 1 1995
and surveying
controls
lasers: B. J. Gorham
in practice
The Standard recommends that where Class 3B or Class 4 lasers are being used, a Laser Safety Officer should be in attendance on site. In fact it is rare to see powerful lasers which are classified in the upper 3B band being used in construction or surveying; the exceptions are usually ex-military range-finders and these require very special attention. The normal laser equipment used on site is therefore unlikely to be hazardous beyond that corresponding to the lower band of 3B. It is essential that the operators of Class 3A lasers and those at the lower end of Class 3B are well aware of the hazards to the eye which their lasers may present and are equally aware of the legal obligations which they inherit when they operate the laser equipment. In the UK, the controlling legislation for laser use is the Health & Safety at Work Act which places a duty upon laser operators to ensure the safety of their own eyes as well as the eyes and general safety of any others who may be affected by the laser. Ignorance of the potential hazards from using the laser do not relieve these legal responsibilities. The first job of the site safety manager is to determine the classification of all of the laser equipment to be used on the site. The only advice s/he needs to provide to the operators of Class 1 laser equipment, and then onlv if it emits a visible beam, is to guard against direciing it into the eyes of personnel who may, as a consequence, become distracted from their own operations amd thereby cause an accident. Class 2 (visible) lasers are more susceptible to this kind of indirect hazard since they produce more powerful light beams. It is worth considering the placing of warning signs in the vicinity of the laser beam pointing out the potential hazard of looking into the beam and also placing an opaque obstacle in the beam at the end of its useful path a beam stop. Although there is not considered to be a significant risk of irreversible eye injury resulting from a brief exposure to a Class 2 laser beam, there can be severe impairment of vision in the exposed eye for a considerable time and construction sites are often hazardous places even with perfect vision. Class 3A lasers deserve more attention from the site safety manager. Any exposure of the e,ye to the laser beams via an optical instrument, such as a telescope or binoculars, is likely to be hazardous. The operator should be fully aware of this fact since such instruments are very common where surveying or construction is practised. The operator should also be sensitive to any conditions on the site where the beam from the laser could become focused, perhaps after reflection from a curved mirror surface, so that the safe exposure level for the eye could become exceeded. There are two types of hazard control which can be put into effect: there are the administrative controls which involve the training of personnel who are responsible for the operation of laser equipmentwhich involves raising the level of awareness of laser hazards and some guidance as to the
23
Safery aspects of outdoor and surveying
lasers: B. J. Gorham
implementation of safety measures in practice. These controls also include the siting of appropriate warning signs in the regions where exposure to laser beams can occur and ensuring that all personnel potentially at risk from the laser are aware of established effective accident reporting procedures. Engineering controls refer to physical barriers which limit the degree of hazard. These may be shields fitted close to the laser beam source where the beam is strongly divergent, so that the radiant power is safely diluted at any point where it is possible to place an eye in the beam. A more common form is provision of rope barriers which carry closely-spaced signs which warn of the hazards of exposure to an open laser beam. Most common of all is a simple beam stop. Controlling the hazards where Class 3B lasers are being used is usually more difficult and more important than for those of a lower classification. The first requirement in the hazard assessment is to determine the region where even an accidental exposure of the eye may be hazardous. This is best performed by using an appropriate laser power meter as described earlier but may be achieved by calculation
if the laser beam power and geometry are known. The ‘3B’ areas which are identified by such an assessment must be isolated by physical barriers wherever possible and suitably posted with warning signs. Any personnel who need to work within these hazardous areas should be fully aware of the eye hazards presented by the laser beams and, if consonant with other hazards within that environment, should wear appropriate laser eye protectors. Notwithstanding the careful measurements of laser power and geometry and the local controls which are put in place, the best insurance against eye injury from the use of laser equipment is provided by a high level of awareness of laser hazards, at least among those who are responsible for their operation. References British Standard BS EN 60825: 1992 Radiation safety of laser products. equipment classification, requirements and user’s guide. British Standards Institution, 2 Park Street, London W 14 2BS, UK International Electrochemical Commission Standard IEC 825-l : 1993 A Guide to the Safe Use of Lasers in Surveying and Construction The Royal Institution of Chartered Surveyors, 12 Great George Street. London SWlP 3AD. UK
Optics & Laser Technology
Optics
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& Laser Technology Vol 27 No 1 1995