Page 110 - Lasers and IPLs in Medical/Aesthetic Applications v1.2
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Chapter 1 – Fundamentals of Medical/Aesthetic Lasers and IPLs v1.2
Nominal Ocular Hazard Distance – NOHD
The NOHD is the minimum distance from a laser aperture which may be considered ‘safe’ to the eye. At this distance the intensity of the laser beam is below the MPE for that laser, and so is considered to be below the threshold for ocular tissue damage.
Like the MPE, the NOHD depends on:
the output power of the laser unit, and the beam divergence, and
the beam diameter at the aperture.
The divergence is very important here. Any laser with a high divergence (the beam spreads out rapidly) will have a relatively low NOHD – i.e. the danger only really becomes imminent ‘near’ the laser aperture.
As an example, the NOHD for a 1000 milliwatt laser (1 Watt, so this is Class 4), with a divergence of 1 milliradian (around 0.06° – so, a highly collimated beam with minimal spread) is 225 metres. Hence, anyone outside of this distance from the laser aperture is deemed to be safe and does not require safety glasses. This is important for outdoor displays where lasers may be used. The NOHD must be calculated carefully to protect any public viewers.
Anyone inside this distance of 225m will need to wear appropriate safety glasses, to prevent eye damage. This may occur if the viewer looks directly at the laser, or if there is a reflection from a smooth surface.
The risk of injury depends on the wavelength, power output, duration of exposure, distance from the aperture and the rank stupidity of the observer! If you don’t know what is coming out of the laser, DO NOT switch it on!
Class 4 lasers are dangerous.
They can blind you - permanently.
If you do not know what is coming out of it, ask a professional!
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Chapter 1 LEVEL A Fundamentals of Lasers/IPLs
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