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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. 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 to 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 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
Figure 2: Divergence increases when a lens is used
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. Completely enclosing the
30 LASER SYSTEMS EUROPE THE 2023 GUIDE TO LASER SYSTEMS
Figure 3: Effect of a diffuse reflection
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
“Completely enclosing the laser process will ensure the hazard is contained, but this is often incompatible with the production”
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 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). 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. The 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 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
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