Limits

M.P.E. is the maximal radiation level one can be exposed to before undergoing immediate or long term injuries. This maximum permissible exposure was established from the energy density limits, or the power-per-surface-unit (intensity) limits, that can be admitted on the cornea and on the skin. They were obtained by extrapolating to mankind the experimental M.P.E. measured on animals.

Thus, the M.P.E. levels were calculated as functions of the radiation wavelength, of the pulse duration, of the exposure duration of the exposed tissue (skin or eye), and of the size of the image on the retina. Table 2. summarizes a few M.P.E. limits. More details and M.P.E. values can be obtained from the (NF) EN 60825-1/A2 standard in Appendix 1 (after the exercises). In order to evaluate the M.P.E. to repeated pulses, see the exercises, section B.

These limits were defined according to the EN 60825-1/A2 standard. They enable to define a laser classification according to the related hazard, depending on their characteristics. The limits were defined on the powers and energies emitted by the laser and accessible to the user – this explains the acronym A.E.L. Each laser class is labeled by a maximum accessible emission that must not be exceeded.

A.E.L. limits are based on the laser emission, while M.P.E. limits are based on the radiation received by the eye or the skin – directly from the beam or after reflection.

Because of the wide range of wavelengths, energies, pulse durations... accessible by laser technologies, they are the source of very different hazards. Due to the impossibility to treat laser sources as one unique group with common safety limits, laser sources were finally classified according to their accessible emission limits (A.E.L.). The EN 60825-1/A2 standard defines four classes, as long as the laser sources are used in their specifications.

• Class 1 : The sources of this class do not present hazard, due to their performances. The beam carries energies and intensities inferior to the lowest M.P.E. values. A class 1 laser becomes of a higher class when not used accordingly to the manufacturer's instructions (thus exceeding the M.P.E.)

• Class 1M :This class contains laser sources whose spectrum is comprised between 302.5 nm and 4000 nm. Compared to Class 1 sources, Class 1M sources may carry higher powers but still low intensities, as they are either diverging, or collimated with a large diameter – so that the energy carried through the area of a pupil is lower than class 1 limits. As any class 1 source, they are harmless in standard conditions of use, but can present a danger when the user inserts some optical elements in the beam trajectory, ether to collimate a diverging beam, or to focus a collimated source. A diverging class 1 source becomes class 1M when optical elements are inserted in the beam trajectory at less than 10 cm from the source output port.

• Class 2 : This class contains the low power sources whose spectrum is fully in the visible range (400 nm – 700 nm), with powers up to 1 mW. These sources are harmless for the eye because of the action reflex (i.e., when the eye is hit with a bright light, the eye lid will automatically blink or the person will turn their head to escape the bright light). This reaction to visible light ensures a sufficient protection in standard conditions of use, even if the user needs optical instruments to look at the beam.

• Class 2M : Like class 1M laser sources, class 2M sources may carry higher powers but still low intensities, as they are either diverging, or collimated with a large diameter – so that the energy carried through the area of a pupil is lower than class 2 limits. They are dangerous if the user inserts some optical elements in the beam trajectory, ether to collimate a diverging beam, or to focus a collimated source. A diverging Class 2 source becomes Class 2M when optical elements are inserted in the beam trajectory at less than 10 cm from the source output port.

• Class 3 : This class contains laser sources carrying medium powers. A short skin exposition does not lead to any damage. This class is divided in two subclasses :

• Class 3R : These sources emit powers between 1 and 5 mW in the wavelength range from 302.5 nm to 106 nm, where direct beam eye exposure is dangerous but still presents a risk inferior to the one related to Class 3B sources exposure. The Accessible Emission Limit is five times higher than for class 2 sources in the range 400 – 700 nm, and five times higher than for class 1 sources at any other wavelength.

• Class 3B : This class is made of medium power laser sources, from 5 mW to 500 mW. The direct vision of the beam of these lasers is always dangerous. On the contrary to class 3A, diffused radiations or diverging sources are dangerous if the exposure duration is higher than 10 s and if the eye is situated at less than 13 cm from the source. Continuous sources of this class emit powers of up to 500 mW, and the maximal energy per surface carried by a single pulse of a pulsed source must be inferior to 105 J.m-2. Compared to 3R laser sources, the manufacturers have more obligations and more control measurements must be done by the user.

• Class 4 : For every source of this class, not only the direct vision of the beam but also diffused radiations are dangerous for the eye. These laser provoke skin injuries and can light a fire. Their use thus requires to be very cautious. A continuous source of this class emits more than 500 mW.