What is the difference between M.P.E and AEL?

[jin0706]

Hi, nice to meet you guys.

I'm learning Laser Safety now, but I don't know what is the difference between MPE and AEL

If I want to use laser to human, shoild it be satisfaction MPE and AEL both?

Thank you.

 

[CurtisOliver]

Welcome to the forum. I have added some definitions for you.

Based on a glossary made by Oregon State University.

Accessible Emission Limit (AEL) The maximum accessible emission level permitted within a particular
class. The AEL is determined as a product of the maximum permissible exposure (MPE) times an area
factor called the limiting aperture (LA). The LA is dependent on laser wavelength pupil size. AEL=MPE *
area of LA.

Maximum Permissible Exposure (MPE) The level of laser radiation to which an unprotected person may
be exposed without adverse biological changes in the eye or skin.

This is some further information about the laser classification based on AEL from Optique pour l’ingénieur

A.E.L. : Accessible Emission Limit , and laser classes
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.

To make sense of all this, lasers above the power of 5mW (Class 3B) should not be shone at a living thing.

The other thing you have to watch out for with brought lasers is that, are they compliant with safety regulations and limited to it's laser class? For instance, if the laser states that it is a Class 3R with a max power of 5mW, then it should not exceed this. Cheap suppliers are often overspec and dangerous. Also if buying 532nm (green) pointers, you should also be aware of IR dangers.

For a quick guide these are the power bands for the lasers you are most likely to see:

Class 2 - <1mW
Class 2M - can have a beam intensity to the equivalent of a collimated Class 2
Class 3R - <5mW
Class 3B - <500mW
Class 4 - >500mW

What application are you using the laser for?

 

[Cyparagon]

As I understand it, AEL determines the class of laser. It is possible to meet the AEL while not meeting MPE. The AEL for a class 1 laser would meet MPE by definition, but probably not for other classes. MPE is the figure you want to monitor.