The main criterion for assessing the optical safety of a laser application is the maximum permissible exposure (MPE). MPE is the level of laser radiation to which, under normal circumstances, persons may be exposed without suffering adverse effects. MPE levels represent the maximum level to which eye or skin can be exposed without consequential injury immediately or after a long time and are related to the wavelength of the radiation, the pulse duration or exposure time, tissue at risk and, for visible and near-infra-red radiation in the range 400 nm to 1400 nm, the size of the retinal image.
Potential ocular exposure to laser light must not exceed the MPE and therefore MPE calculations should form part of any prior risk assessments carried out for a laser procedure. Software is available (Laserbee®) which will perform the calculations using data entered for various laser parameters – output, wavelength, divergence, pulse frequency if pulsed etc: see your Department H&S arrangements for names of staff with acess to Laserbee® or contact the University Laser Safety Officer j dot brannon at warwick dot ac dot uk
If there is a risk of laser exposure to levels above the specified MPE, safety eyewear is an important element of personal laser protection. But safety eyewear should only be regarded as a last line of defence against exposure to laser radiation, to be adopted only after a full safety evaluation has been carried out and other control measures have been considered. Its use should not be regarded as a convenient alternative to proper engineering controls.
Protection by safety eyewear is effected by incorporating optical filters to reduce the level of eye exposure for a specified wavelength and power or pulse energy, to below the limiting MPE. Laser safety eyewear must be tested and certified in accordance with EN 60825/207. The protection must be geared to the wavelength of the laser and be suitable for worst-case scenario i.e. the maximum power density or energy density which the user could be exposed e.g. J/m2. Eyewear is rated on an ‘L’ scale according to the exposure time (10 secs for CW or 100 pulses) and beam energy density which it can withstand. The L rating is only meaningful in conjunction with the relevant wavelength, wavelength range and the laser mode D, I, R or M; there are tables in EN 60825 / 207 listing these parameters which are used to select the correct glasses for a given laser beam. Note that eyewear which only has an OD (optical density) value do not meet the requirements of EN 60825 / 207 because OD alone does not factor in exposure time.
In addition to the tables in EN 60825 / 207, the Laserbee® software can be used to identify the type of lenses required for protection against specific laser beams: see above.