Maximum permissible exposure

BLOG: MPE - How much laser light is safe?

Laser safety expert, James Stewart, gives an overview of how much laser light is safe to view, and why how much light is considered safe varies on who a person is.

Exposed to the Light: How much light is safe?

A question we are regularly asked is ‘how much light is safe to view?’. Any person that has been working with lasers for a while will likely have some awareness of the existence of exposure limits, and the importance of not creating laser exposures that exceed those limits. The golden rule being that if the exposure level of a laser effect is unknown, or the user is unsure of what exposure level to use, then the laser effect must not be directed anywhere it may be able to illuminate people.

So, what about the instances were deliberate laser illumination is intentional? e.g. the times when you want use a laser effect that illuminates a performer or the audience. How much light is safe to view in those circumstances? The answer may seem a little surprising in that it varies depending upon who is viewing the laser effect. The reason for this is that the dose of light someone is allowed, varies depending upon how long they may be illuminated by the light. This means that if a person could be illuminated for a longer period of time, then the light they may be illuminated by must be reduced to account for the potential longer exposure duration.

The maximum amount of laser light a person may be exposed to without harm expected to occur is a limit known as the ‘Maximum Permissible Exposure’ or ‘MPE’. Two types of MPE exist: one for eye exposure, and a separate MPE that has a higher threshold for skin exposure. Both MPEs are defined in the laser safety standards and the government regulations relating to laser use.

Because the general laser safety standards, (such as EN 60825-1:2014), and optical radiation regulations consider exposure in general terms, they don’t provide guidance on how the MPE should be applied for lightshow application. Instead, specific guidance on how much light can safely illuminate people from laser effects is provided by laser show specific guidance. In the UK the two key documents of concern are PD IEC/TR 60825-3:2022 Safety of Laser Products – Part 3: Guidance for laser displays and shows, published by the British Standards Institute, and the Safety of Display Lasers Guidance available from PLASA, the Professional Light and Sound Association.

Behavioural Expectations Influence the Applicable MPE

The MPE limit, that is, the maximum amount of light a person may be exposure to, varies depending upon their association with the laser effect. Recognising the fact that exposure potential is based upon behavioural expectations, the laser show safety guidance considers that there are three general types of person that could be present when a laser effect is in use. These are spectators (audience), ancillary personnel (employees), and possibly performer’. The people in each of these groups are expected to behave differently to direct laser exposure, meaning that different ‘applicable’ MPE exposure limits should be used.

Spectators (Audience)

This group of people will generally have no concept of the risk of excess laser exposure. In fact, quite the opposite, for they are present to enjoy watching the laser effects as part of the performance. They will assume everything is safe, and not think of taking evasive action to avoid any seemingly bright laser flashes. As spectators, they are assumed to want to deliberately watch the laser effects; exposure in inevitable and intentional. For this reason, the safety guidance specifies that the applicable MPE should be based on continuous direct eye exposure for at least 10 seconds.

Ancillary Personnel (Employees)

This group of people includes laser technicians, and other crew members and staff that could be present when the laser effects are being installed and focussed, or are present in areas where the public are not allowed during the performance, but where laser effects could be. If this group of people has been advised of the exposure risk, and instructed to look away if a laser effect strikes them, then the MPE for a 0.25 second (aversion response) exposure can be justifiable as the applicable MPE. If it’s not possible instruct people to look away, then the applicable MPE should be the same as it is for spectators. The reason it’s possible illuminate this group of workers with more light is that because of the way they have been instructed to behave, their exposure should be less than that of spectators.

Performers

This group of people includes musicians, dancers, and actors etc., essentially any person that is present and forms part of the performance that the spectators watch. The applicable MPE should be the same as for ancillary personnel, if the performers have been instructed to avoid exposure. However, if there are well choreographed cues, where direct eye/face exposure is not going to occur, for example, the laser effect makes contact with the performer passing through their legs, then the applicable MPE can be raised to allow more light by using the skin MPE. Again, the reason that more light may be present in the vicinity of the performer is that if well planned and executed, the exposure potential is limited even more so than for similar ancillary personnel, meaning more light can be used.

An important point to note with both ancillary personnel and performers is that they are both workers, which means under specific laser H&S regulations it’s a criminal offence to allow light in excess of the MPE to illuminate them, or to have not taken steps to prevent it from happening. And remember also that performers are still workers, so don’t become exempt from the scope of the regulations.

This explanation should help in understanding the reasoning for having different exposure limits based on how people are expected to behave. But as mentioned at the outset, if you are unsure of what the exposure is for an effect, or don’t have a reliable way of checking it, keep the light away from people to stay safe.

This topic and related areas are covered in greater detail on our one-day laser safety courses. More details here.