Maximum Permissible Exposure
MPE is specified for workers (including performers). There is no legal MPE for the audience in the UK! However we do use MPE to set a safe limit for audience beam exposure.
The point of MPE is to protect the eyes and skin, based on intended and accidental / unintended exposure.
It is the amount of laser energy that a person can be exposed to without experiencing any adverse effects. Protect the eyes and skin
Display lasers today are diode based and continuous power (not necessarily true for older lasers which were sometimes pulsed) . The MPE is a complex formula that is based on:
- Wavelength
- Duration
- Beam size
- Distance
Terminology that is used in relation to considering MPE
Intentional exposure
MPE for intentional exposure of workers/performers can be higher than the MPE for unintentional exposure. So a performer deliberately exposed to the laser beam, NOT for audience exposure. In this situation the MPE gives a margin of safety while still allowing for the visual effects of the laser
Unintentional exposure
Situations where a person is exposed to a laser beam without their knowledge or consent.
E.g. a person walks into the path of a beam during a show, the MPE is set much lower to protect against any possible harm.
NHZ
Nominal Hazard Zone. The area where a worker, performer or audience is
NOHD
Nominal Ocular Hazardous Distance. The distance after which the laser beam becomes safe. Calculated from:
Power
Divergence
Beam size
Simplified MPE Value
You can simplify the assessment of MPE by using an industry accepted eye MPE of:
1mW/cm²
Intentional audience exposure, 10 second duration.
2.5mW/cm²
Unintended exposure of a performer and worker, 0.25 second duration.
These apply to all wavelenghts from 400nm to 700nm. What these values basically mean is that if you ensure your laser beams entering these areas are <= the values above (by using a calibrated beam power meter) then that can be considered safe practice.
Children
If children are present then the MPE level must be reduced by a factor of 10 for wavelengths < 500nm (blue). A child is defined as a person under 16 years old in the workplace regualtions in the UK.
Skin MPE
If eye exposure cannot take place then skin MPOE value can be used instead.
200mW/cm²
Intended exposre, 10 second duration.
Time limits specified
0.25 second
You will blink and move your eye away in this time period, “eye aversions rate”
10 seconds
You will physically move away from feeling a burning sensation
Actually these times can often be ignored, they are there because the calculation needs a time base, but they are really infinite as in the person will blink or move within enough time to avoid injury. They are not actually the time you are permitted to use laser for.
Testing beam energy
1mW/cm2
You do test any beams entering the audience area (within the first 3m from the floor) for this, and any beams entering the performer area if a performer or worker will be exposed to them. You need to test a static beam (the meter won’t react fast enough to a scanned beam so you’ll get an averaged reading not the full beam energy reading you need to be checking)
2.5mW/cm2
Not something you’d typically test because it’s an unintended exposure. In the audience you ensure it can’t happen. In the performance/worker area you have eyes on to ensure the effects are kept off people (e.g. a spotter with an Estop).
To give an example of a scenario for when you might want to test:
Where the 1st and 2nd order beams from a diffuser splitting the laser beam cover a stage area where you are not expecting any stage user to be exposed intentionally, but the 3rd order is in the audience.
The 2.5mW MPE is a value above which harm can be possible in a workplace setting. So think about it like this – “can you prove the beams were below 2.5mW? If not, then the accidental exposure of any beam is presumed to be capable of harm”.
