High Power hand-held Lasers - some Safety and Legal considerations

I don’t like it when people play with Class 3B handheld lasers anywhere near me ( or my friends for that matter ). This document explains why.

1 Laser Classifications

Laser Classes are a way of distinguishing between lasers on the basis of their power, although the underlying purpose of the classification system is to draw 'lines in the sand' in terms of safety. The Health Protection Agency ( HPA ) is an agency of the UK Government, whose remit is "to protect the community (or any part of the community) against infectious diseases and other dangers to health" (HPA Act 2004).

This HPA page describes the basic characteristics of the various Classes of laser :

http://www.hpa.org.uk/webw/HPAweb&HPAwebStandard/HPAweb_C/1197021719447?p=1158934607766

This information is available in several alternative presentations, e.g.

http://www.lasermet.com/resources/classification_overview.htm

Historically, the American classification system used Classes denoted by Roman numerals ( Class I, Class II etc ), but this is no longer the case :

http://en.wikipedia.org/wiki/Laser_safety - Classification

In any event, the old American Classes are substantially similar to the new ones.

2 Safety aspects of Laser Classes

2.1 Basic principles

The basic distinction between Classes is made in terms of Power, although the risk of injury is more directly related to the intensity of the beam. Note that ‘brightness’ is not what is meant by intensity here. The perceived brightness is also a function of colour - for a given power level, a green beam will appear up to 30 times brighter than a red beam, due to differences in the sensitivity of the eye to different colours (http://members.misty.com/don/photopic.html, http://eosweb.larc.nasa.gov/EDDOCS/Wavelengths_for_Colors.html ,

http://en.wikipedia.org/wiki/Luminous_efficacy ).

Some lasers will claim a Class designation based on an average power, since they work by generating a stream of pulses in which each pulse may have a considerably greater peak power than the quoted average ( http://www.rp-photonics.com/modes_of_laser_operation.html ) .

Therefore even though the power of a laser may be small in absolute terms, the fact that laser designs almost invariably generate that power in a highly directional beam with a very small cross sectional area means that the intensity of a laser ( compared to other light sources of similar power ) is very high, and the attenuation ( decrease of intensity with distance ) is very low. Indeed, these characteristics are mainly what makes lasers useful.

Normal laser pointers are of Class 1 ( 1mW max ) or 2 ( 1mW maximum incident power ).

In such devices, accidental eye injury is unlikely, either because the device is inherently safe ( it's just not powerful enough to ever do damage ), or because the normal 'aversion response' ( also called the 'blink reflex' ) will act quickly enough to prevent damage, by interrupting or avoiding the beam.

Class 3R ( previously called Class 3A ) devices ( up to 5mW ) are capable of causing damage ( i.e. they ‘beat’ the aversion response ), but in effect the HPA simply judges the risk at this power level ( up to 5mW ) to be sufficiently low that a qualitative distinction can be made between devices in this class and devices in the next class ( 3B ).

2.2 Class 3B devices

The aversion response will typically protect humans from permenant eye damage due to accidental viewing of the sun.

For Class 3B devices, this aversion response ( normally quoted as 0.25s ) is not fast enough to prevent permenant eye damage.

This can be better appreciated by comparison of the relative intensities of sunlight and laser light.

The maximum intensity of sunlight ( that is, staring directly into the sun with no clouds etc ), is about 1mW/mm^2 ( 1kW/m^2 ) ( http://www.jgsee.kmutt.ac.th/exell/Solar/Intensity.html ). Laser pointer beam diameters are typically order of 2mm ( e.g. http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productID=2218 ).

Assuming a 2mm beam diameter, the intensity of a 100mW Class 3B laser ( commonly available, though illegal for sale to the public - see below ) is 100mW/6.3mm^2, or 15.9mW/mm^2, i.e. 16 times the intensity of the sun. Lasers with even greater powers ( 200mW, 300mW ) are also to be found for sale.

This assumes that the power is equally distributed over the area of the beam, i.e. it is an average intensity. However, power distribution is normally centre - weighted ( Gaussian - see http://www.rp-photonics.com/gaussian_beams.html, http://www.edmundoptics.com/TechSupport/DisplayArticle.cfm?articleid=240 ), meaning that the central intensity is greater than the simple calculation above suggests, and therefore that the ‘hazardous distance’ is also significantly greater.

Actually calculating the hazardous distance ( i.e. the distance below which eye damage is highly likely ) is very difficult, as many factors come into play, in particular beam pulsing, and the non-uniform power distribution across the beam. If we assume that there is a hazard up to the point where the intensity of the beam becomes equal to that of the Sun ( not that it's a good idea to look into the Sun... ), then for a 100mW laser we're looking for the distance at which the intensity has dropped by a factor of at least 16.

The divergence ( spread ) of the beams of handheld lasers is generally quoted as between 1.2 and 1.5mrad ( 0.086degrees ) , so assume 1.35mrad (0.077degrees). If we start with a 2mm beam diameter, we're looking for the distance at which the diameter of the beam has reached 8mm. Ignoring power distribution issues, this gives us a distance of about 2.25m. However, if we assume that c. 75% of the power is in the central 1mm of the beam, this corresponds to a hazardous distance of 3.25.m. For a 200mW beam the equivalent distance would be 4.75m, or 15.4ft, and for a 300mW beam then 5.9m, or 19ft. NB - these are approximate, indicative figures only, don't rely on them for safety critical purposes.

The HPA states :

"Class 3B lasers may have sufficient power to cause an eye injury, both from the direct beam and from reflections. The higher the output power of the device the greater the risk of injury. Class 3B lasers are therefore considered hazardous to the eye."

This use of the term ‘hazardous’ is the strict health and safety use - it means that there is a certain innate risk ( the 'hazard' ), which may require a variety of measures to be taken in order to reduce that risk to an ‘acceptable’ level.

The question of reflection is a little confusing. Two types of reflection are considered by the literature - 'diffuse' reflection, and 'specular' reflection. The former is reflection from a matt surface, the latter from a shiny surface.

Class 3B is hazardous when reflected from a shiny surface ( a mirror, glass, polished metal, glazed ceramic, etc ), but not when reflected from a matt surface. Class 4 is hazardous under any conditions.

Further information on the classification system and HPA's rationale, can be found here :

http://www.liv.ac.uk/radiation/pdf/laserpointers.pdf

3 Legal restrictions on Class 3B lasers

The conclusion of the above HPA report is especially pertinent :

"After seeking advice from NRPB (now the Radiation Protection Division of the HPA) the Department of Trade and Industry urged Trading Standards Authorities to use their existing powers under the General Product Safety Regulations 2005 5 to remove laser pointers of a Class higher than Class 2 (as defined in the British Standard) from the general market. Such devices are too powerful for general use as laser pointers and present an unacceptable risk in the hands of the consumer because they may cause eye injury in normal reasonably foreseeable use."

Emphasis is mine.

In other words, Class 3 lasers have been banned from being sold to the general public in the UK on safety grounds.

Infact, they have been banned since at least 1997, when the then Consumer Affairs minister Nigel Griffeths instructed that Trading Standards be used to prevent their sale (http://www.prnewswire.co.uk/cgi/news/release?id=44867 , http://archive.thisislancashire.co.uk/1998/2/11/808318.html ).

Note that 'higher than Class2' means 'higher than 5mW'. The device(s) in question are 'Class 3B' or 'Class IIIB' devices, with a power of 100mW. Therefore these devices have a power which is a factor of 20 greater than the minimum power for a device to be classified as Class 3B ( 5mW ), and are therefore very definitely in the 'hazardous' category ( see also the ‘sunlight’ calculation above ).

For example, Salford Trading Standards describe sellers responsibilties ( and penalties ) when it comes to not selling Class 3 devices to the general public :

http://www.salford.gov.uk/business/tradingstandards/business-advice-leaflets/leaflets_self_help/product-safety/laser-pens.htm

Some vendors clearly recognise the existance of this obligation not to sell to the general public, see :

http://www.megagreen.co.uk/shopdisplayproducts.asp?id=33&cat=10mW+%2B+Green+Lasers

and the extensive legal/safety terms here :

http://www.megagreen.co.uk/shopexd.asp?id=250&bc=no

It is also illegal to sell devices of this class as laser pointers or 'amusement articles' in the States. The US FDA describes such devices as follows :

"Class IIIb hand-held lasers are too dangerous for use as pointers or amusement articles" : http://www.fda.gov/cdrh/radhealth/products/lpm.html

After incidents of interference with aircraft, an Australian state is considering classifying such devices as weapons :

http://www.vnunet.com/vnunet/news/2214806/australian-state-set-ban-laser

There have been many instances of aircraft interference closer to home, eg in the last couple of months :

http://www.manchestereveningnews.co.uk/news/s/1047746_helicopter_pilots_laser_terror

http://www.towerhamlets.gov.uk/templates/news/detail.cfm?newsid=9168

Interestingly the first article notes :

"The incidents have raised fears a rogue batch of lasers are being sold in the area.

Their beams are said to be more powerful than normal laser pens."

4 Safe use of Class 3B devices

Organisations which do use such lasers for research and development have strict rules ( following HSE guidelines and international standards, in particular IEC/TR 60825-14 ) which kick in when you start dealing with Class 3B devices e.g.

http://www.dur.ac.uk/resources/physics/internal/safety/LaserSafetyPolicy_Class3Band4.pdf

http://www.eng.cam.ac.uk/safety/laser/lasers.html

Some features of systems like this which illustrate the care needed for safe operation are :

1. A requirement for key - operated on/off function

2. Door interlock control systems - so that the laser turns off if there are unexpected visitors.

3. Beam Tubes - so that nothing reflective can accidentally be put in the beam path.

4. The wearing of Laser goggles whilst the beam is in operation

Also, these scenarios tend not to envisage handheld use - they assume that the path of the beam is predictable, e.g. by virtue of the thing being bolted to a laboratory bench. It is clearly much more difficult to reduce the risk to a safe level ( in Health and Safety terms ) if the beam path is unpredictable.

It is possible to envisage measures which would reduce the risk from handheld use ( e.g. wearing of laser goggles, suitable training, site assessment, controlled access to the site, etc ), but none of these are likely to be the case if such a device is used as a laser pointer by a member of the general public.

Indeed, the HPA’s conclusion is that in these circumstances the risk cannot be reduced to an acceptable level, at least insofar as their use by the general public is concerned.