An introduction to laser safety
The reduction in cost of low- to medium-power industrial lasers has led to a significant increase in their use and ownership. However, understanding the safety aspects of their use has not kept pace with this proliferation.
Most industrial lasers are no more or less hazardous than any other machine tool found in a typical workshop and can be used perfectly safely if their hazards are known, understood and correctly controlled.
However, while the hazards of using, say, a lathe have been highlighted from early days at school or college, familiarity with industrial lasers is much less common.
Even amongst teachers and lecturers, the potential hazard of a large piece of spinning metal is far more obvious than those associated with a laser system.
What are the hazards of a laser system?
The type of hazard presented by a laser system depends on its power and wavelength. The greatest hazard is normally to the eyes due to their greater sensitivity to light compared with skin.
The two most common types of industrial laser are fibre and CO2. These lasers have wavelengths of around 1μm and 10μm respectively (legacy YAG and vanadate lasers have wavelengths very similar to fibre lasers). Both of these wavelengths are in the infrared and so it is not possible to see them with the naked eye. This makes them potentially more dangerous than even the much lower power laser pointers because it is not possible to see the light or to take any avoiding action.
The fundamental quality that distinguishes lasers from other sources of light is their ability to be focused to very small spots and be collected into beams that expand (diverge) very little over large distances. This means that the intensity of light can still be harmful at significant distances from the source of light.
Fibre lasers present one of the highest hazards of any laser because the light is transmitted through the cornea and lens at the front of the eye and through the vitreous liquid that fills the eyeball, and so gets all the way to the back of the eye to the light sensors, known as the retina (see Figure 1). The cornea and the lens both focus the laser light, increasing its intensity at the retina, but as it is not visible, the usual protective responses such as blinking or looking away do not occur. Therefore very high intensities of light can be present on the back of the eye without the observer being aware of the danger.
Figure 1: Anatomy of the human eye.
Just like staring at the sun or a welding arc, light entering the eye can cause permanent damage to the retina, causing blind spots, or worse still damage the optic nerve, which can lead to total blindness. There are currently no treatments to reverse retinal blind spots or optic nerve damage.
Long term exposure to the light from fibre lasers can also cause cataracts in the lens of the eye, causing the lens to become cloudy and eventually leading to total blindness. Cataract surgery is possible and involves replacing the natural lens with a clear plastic lens.
The light from CO2 lasers is heavily absorbed by the cornea, which causes the surface of the eye to heat up. Long term exposure to low levels of CO2 light causes premature ageing due to drying out of the eye’s surface. Drying the surface can cause it to crack and peel in the same way that sunburn affects the skin, causing irritation that feels like rubbing sand into the eyes. In the most extreme cases, the light can cause corneal burns. Damage to the cornea makes it less transparent and changes its shape, causing distorted and unclear vision.
Corneal surgery is the most common type of organ transplant involving removal of the damaged cornea and replacing it with a donated one.
Where are the hazards?
Because laser beams can travel large distances without expanding greatly, the intensity of the light remains high over very large distances. The laser safety standards provide methods to calculate the levels of exposure to laser light that are not hazardous based on biological observations. For example, the raw beam from a typical 20W fibre laser remains an eye hazard for up to 3km. This is known as the Nominal Ocular Hazard Distance (NOHD). The NOHD for a typical 80W CO2 laser is less than 1km because of the lower hazard of its wavelength and its greater divergence. A raw beam must therefore be controlled or contained in some way to allow the safe use of the laser.
Most lasers use a lens to focus the beam to a small spot, which also increases the effective divergence of the beam (see Figure 2). When fitted with a typical lens the NOHDs are reduced to about 25m for a 20W fibre laser and less than 5m for a 80W CO2 laser. So, the laser beam remains a hazard for several metres from its focus, meaning that either personnel access must be prevented or the beam must be blocked in some way.
Figure 2: Divergence increases when a lens is used.
Completely enclosing the laser processing volume will obviously ensure the hazard is contained, but this is often impractical or incompatible with the production process requirements.
Beam stops and diffusely reflecting panels are the most common method of creating a safe work volume. If the laser beam reflects off a part or panel in a specular (mirror-like) manner, the total beam path length would have to be the NOHD calculated above, tens of metres.
If however the reflection is diffuse, the power of the laser is distributed out in all directions, similar to the light from a light bulb. Diffusely reflected light spreads in all directions, with the highest intensity being directed normal to the surface of the reflector – irrespective of the angle with which the beam hits the surface – and lower intensities being directed at a glancing angle to the plane (see Figure 3).
Figure 3: Effect of a diffuse reflection.
The NOHDs for diffuse reflections are approximately 350mm and 200mm looking normal (straight down) onto the surface for the 20W fibre and 80W CO2 examples, falling to less than 100mm looking side-on to the point of reflection.
Therefore with carefully designed beam stops, guards and panels, combined with suitable interlocks and working procedures, it is possible to design safe working cells for most laser processes.
Laser safety classification system
International safety standards’ committees have developed a laser Class system dependent on the risks posed by the laser. In Europe this is given by EN60825-1, using Classes 1 to 4. In America, the standard ANSI Z136.1 does not provide such a qualitative guide, but essentially follows the same ideas. Following is a summary of Classes 1-4:
- Class 1, Class 1M, Class 1C: Safe under reasonably foreseeable conditions of operation. Class 1 also includes high-power lasers that are fully enclosed so that no potentially hazardous radiation is accessible during use.
- Class 2, Class 2M: Emitting only visible light. Eye protection is afforded by aversion responses including the blink reflex.
- Class 3R: Exposure could cause injury but injury is unlikely. Prolonged or deliberate ocular exposure is hazardous.
- Class 3B: Viewing raw beams is always hazardous. Viewing diffuse reflections is normally safe. May produce minor skin injuries and ignite flammable materials.
- Class 4: Even diffuse reflections are hazardous. May cause skin injuries and could be a fire hazard. Their use requires extreme caution.
Most industrial lasers are Class 4 lasers, but can be treated as Class 1 when suitable safety systems and precautions have been implemented.
Safe laser work enclosures and cells
Safety standards recommend that all lasers in excess of Class 2 are used in Class 1 enclosures. Laser system suppliers often offer Class 1 laser systems which contain a Class 4 laser, containing the hazard with suitable guards and interlocking systems.
Any company selling a Class 1 laser system takes on the responsibility of ensuring that the laser is safe to use in all reasonably foreseeable circumstances, but it is recommended that anyone purchasing a Class 1 system requests a Certificate of Conformity from the vendor that details the standards that have been considered when designing and building the laser system. If the purchaser has any doubts about the system they can also request a risk assessment from the laser system vendor, which should follow the format detailed in EC standard EN ISO 12100 or equivalent and covered by the Machinery Directive 2006/24/EC.
Many laser system suppliers offer Class 4 laser systems for integration by the purchaser into a safe working environment or system. It is essential that anyone purchasing a Class 4 system requests a Certificate of Incorporation that details the standards that have been considered, as well as a risk assessment that will allow them to incorporate the laser into a safe working system.
Note that any person, company or organisation purchasing and/or operating a Class 4 laser system must appoint a Laser Safety Officer (LSO) to oversee its operation, which can be an external person or company.
There are many important considerations when designing a suitable safe enclosure including interlocking, the integrity of the materials used and their reflectivity/diffusivity, which can be very different at the wavelength of the laser compared with what is observable in the visible wavelengths.
Laser Safety Officer and Standard Operating Procedure
The Laser Safety Officer (LSO) has, amongst others, the following responsibilities:
- Coordinate acquisition of laser devices and systems.
- Coordinate and assure compliance of all lasers with standards.
- Implement appropriate engineering and administrative controls including personal protective equipment using a Standard Operation Procedure (SOP) and laser safety manual.
- Keep records of accidents and report them.
- Carry out periodic safety inspections of laser areas and laser devices.
- Provide basic training, educational material and proficiency tests for personnel
- Maintain an up-to-date inventory of all lasers in the company.
- Participate in accident investigations involving lasers.
Other aspects of laser safety, such as fume extraction and filtration, will be covered in future articles.
Neal Croxford, of Dovecote Technologies, is the AILU representative on the BSI laser safety technical committee. He has been designing and building industrial laser systems for over 36 years and offers laser system design and safety consultancy services. Neal originally wrote this article for the Autumn 2022 issue of AILU’s own publication: The Laser User.
Lead Image: Shutterstock/FrankHH)
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