Laser safety management (medical and cosmetic applications)
'Core of knowledge' course notes
Maximum permissible exposure
Various exposure data has been analysed and international safety committees have been able to publish levels of intense light above which it is deemed that harm will occur.
The Maximum Permissible Exposure (MPE) is defined as,
The level of optical radiation to which, under normal circumstances, persons may be exposed without suffering effects.
The MPE varies depending upon;
a) Wavelength of light
b) Duration of exposure
c) Beam intensity
d) Target tissue (eye or skin)
MPE figures are typically given in terms of energy or power density.
Hazard Distance
All light sources spread out as they propagate away from their origin. At a certain distance the intensity of the light will not be strong enough to cause damage to the eye. This distance is known as the Nominal Ocular Hazard Distance (NOHD).
A technical definition of the NOHD is,
The Minimum distance from the light source at which the exposure to the beam falls below the Maximum Permissible Exposure (MPE)
This is useful when determining the safety controls of a particular facility. It may be of the order of a few centimetres (physiotherapy lasers) or tens of meters. It is mainly determined by the divergence of the beam but wavelength, duration of exposure and output power all have an effect.
Light from an IPL spreads out quickly. In theory, the hazard distance should be relatively short. However, in the absence of manufacturer data, we can often just assume the worst case of "greater than room dimensions".
When the laser beam hits an object, a number of things will happen. These include;
- absorption of laser engery into the material
- transmission of some light through the object (if not opaque)
- Specular reflection (e.g. mirror-like reflection)
- Diffuse reflection, e.g. from rough/matte surfaces.
We may be able to estimate the amount of reflected or absorbed light for risk assessment pourposes. In some cases, we may be able to assume a signifcant amount of absorption as the reflectance of dead-flat (no gloss/sheen) painted surfaces can be less than 1%. We may also be able to assume a fully diffuse (Lambertian) refelection.
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Revision Questions 1. Which wavelengths (colours) are most dangerous to the eye (see page 2)? 2. Would you expect the MPE to be relatively lower or higher at these wavelengths? 3. Why is useful to know the 'hazard distance'? |
