Hazard and Risk Assessment For Lasers

The hazards associated with laser radiation can be indicated by the laser classification system, which divides the lasers into 4 classes.  The class is determined by the Accessible Emission Limits (AELs) (ie. laser power output or energy) as set out in the Australian Standard on Lasers.  The AELs are related to the wavelength of light emitted.  The classes, from least to most hazardous, are:

• Class 1 - laser devices are safe for use under all conditions of exposure, based upon current medical knowledge.  This class includes all lasers or laser systems that cannot emit levels of optical radiation above the exposure limits for the eye under any exposure conditions inherent in the design of the laser product.  However, caution should be expressed when repairing a class 1 laser product, as a higher class laser may be embedded within it.
• Class 2 - laser or laser system emit a visible laser beam, which by its very bright nature will be too dazzling to stare into for extended periods. Momentary viewing is not considered hazardous, with potential damage to the eyes avoided by normal blink and aversion responses (turning head, looking away etc).  Class 2 lasers are only in the visible spectrum.  Some controls to prevent direct staring are required.
• Class 3 - laser or laser system can emit any wavelength, but it cannot produce a diffuse (scattered) reflection hazard unless focused or viewed for extended periods at close range.  These lasers are of medium to high power and risk (output power is 0.5 W or less).  They emit radiation that is hazardous to the eye, but not a serious skin hazard.  More stringent controls are required, concentrating on eliminating intra-beam viewing.  Class 3 is further divided into:
  
  • Class 3A - mainly (visible) medium power lasers, (less than 5 mW in visible).  Marginal eye hazard.  The beam must be expanded to an irradiance of less than 25 Wm-2, to ensure that only 1 mW or less can enter the 7 mm pupil.
  • Class 3B (restricted) - medium to high power visible lasers with more stringent conditions of use.  These are usually restricted to daylight conditions where the pupil size is less than 5 mm in diameter.  (eg. outdoor lasers in the construction industry).  They operate at similar power levels to a class 3A laser, but have higher levels of irradiance (up to 50 Wm-2).  If used in low light conditions, the controls for a class 3B laser are appropriate.  This classification is unique to Australia.
  • Class 3B - mainly medium to high power lasers, with many operating in the non - visible spectrum.  Significant eye hazard (up to 500 mW in visible).  Diffuse reflection viewing is only safe if exposure is less than 10 s and the distance to the reflecting surface is greater then 130 mm.
• Class 4 - laser or laser system is any that exceeds the output limits (Accessible Emission Limits, AELs) of a Class 3 device.  These lasers are of high power and risk.  They are capable of producing diffuse reflections hazardous to the eye and skin.  They may be a fire risk.  Very stringent control measures are required for a Class 4 laser or laser system.

Note: the Australian Standard on Laser Safety is undergoing review and is likely to adopt the IEC standard and classifications (which are slightly different to those above).

The use of Class 3 and 4 lasers require the overseeing by a Laser Safety Officer, and the training of staff in understanding the hazards and risks associated with the laser and its correct use.

By using the laser classification system as an indication of hazard, the need to perform calculations or measurements are greatly reduced.

Although this hazard in many cases is a real one, the actual risk of injury may or may not be possible.  If the risk of a laser accident is a real one, it can be generally be controlled.

In assessing whether a laser, laser product, or laser system is hazardous, the following should be considered -

• the laser systems capability of causing injury (laser classification),
• the environment in which the laser is operated,
• the personnel who operate, and personnel who may be exposed.

This is best summarised in the following diagram -

Laser risk and hazard assessment

The Laser System

A number of laser parameters are of importance.  These include:

• Wavelength, As already indicated, the wavelength of radiation determines the depth of penetration, and absorption by the tissue.
  
• Radiant exposure, The exposure depends on the source strength, duration, and beam characteristics.  The power output depends on the laser’s physical design characteristics.
  
  • Source strength (power or energy),
    These are the characteristics that the manufacturer applies to their product, including the power/energy output.  The laser classification system includes this information.
    
  • Duration, The exposure duration is the length of time a subject is likely to be exposed.  This can be any realistic viewing time.  In general, exposure duration for a pulsed laser is the pulse length, or pulse train duration for repeated pulses.  For a visible, continuous wave (CW) laser, accidental viewing times could be taken as 0.25 seconds - the aversion response time.  For an infrared laser, an accidental viewing time of 10 seconds may be probable, while for ultraviolet lasers any ‘on - time’ could be considered as the exposure duration.
    
  • Beam characteristics, The beam characteristics include: diameter (or source size), divergence, and repetition rate (for pulsed lasers).

Environmental Conditions

Environmental conditions include;

• Ambient light levels,
• Atmospheric conditions (clouds, smoke, convection currents, etc.), and
• How the beam is viewed.  The beam could be viewed directly (intra-beam), by a specular (mirror like) reflection, or a diffuse (scattering) reflection.  Microscopes, and other optics are commonly used to observe the target, hence laser reflections can be a concern.

Some lasers are so powerful that their reflections can cause damage. A reflection from surfaces also depends on the wavelength, for example surfaces which appear dull and diffuse for visible wavelengths, can cause specular reflections in the infrared.

Note : all polished surfaces have a reflectance close to 100 % at the grazing incidence (90°).  Hence, viewing a reflected spot while standing next to the laser, is more hazardous than viewing the spot from the side.

The hazard is increased by the use of optical instruments (microscopes, telescopes or binoculars) because both of these have the effect of gathering additional light and concentrating it in the eye.

In addition to the points above, other environmental factors include whether the lasers used are;

• Enclosed - Laser radiation is unlikely to escape and hence is of low risk.
• Indoors - Research laboratories, factories, machine shops, classrooms, etc.  To determine the risk within an indoor environment, the extent of the hazard area or zone is required.  This requires knowledge of the laser beam’s path, and any dangerous specular or diffuse reflections.  For a class 4 laser system, which is not enclosed, the hazard zone may occupy the entire room.
• Outdoors - mining, pipeline construction, highway construction, open sea, military targeting range, atmospheric research, etc.  To determine the risk in an outdoor environment, the Nominal Ocular Hazard Distance (NOHD) is important.  The extent of dangerous specular or diffuse reflections should also be considered.

Personnel

Operational personnel (using class 3 and 4 lasers) should have a high degree of knowledge, awareness, training, and employ safe operating procedures.  The risk of exposing persons to laser radiation increases when used outdoors, as the public and non-aware staff may be involved.  In the indoor environment, exposures are (generally) limited to operating personnel.

Certain physiological conditions may increase a person’s susceptibility to laser radiation.  These include: aphakic individuals (those who have had the lens of eye removed in cataract surgery), and those taking photosensitising drugs.

Further information is available in the ANU’s Laser Safety training notes and course

Top

For further information contact:
Stephen Altree-Williams, e-mail: Stephen.Altree-Williams@anu.edu.au