Changes to Laser MPE Limits

Updates to the visible laser radiation exposure limits

The International Commission on Non-Ionizing Radiation Protection (ICNIRP) has published updated guidance for the exposure limits relating to laser radiation. ICNIRP is responsible for periodically reviewing the exposure limits it publishes, which through the acceptance of ongoing research and evidence, can lead to changes in the understanding of how laser light interacts with human tissue. A copy of the new guidelines, published in the September 2013 edition of Health Physics, are available from www.icnirp.org.

The document is 26 pages in length, covering wavelengths from 180nm through to 1mm, meaning that ultraviolet, visible and infrared exposures are considered. Of most interest to the lightshow industry are the changes to how visible light in the time domain of laser effects exposure are to be considered in future.

There are changes to some of the limits in regions that are typical of lightshow type exposures, but before you get too excited, they do not translate to a sizable change over what is considered acceptable under the existing limits, and certainly not 10x the MPE. In fact, in some regions there is a slight reduction in what is allowed. – But, before you give up reading, there is a significant change happening to what traditionally is nearly always the most limiting factor.

Scanning laser effects (i.e. the majority of effects used in laser shows) have to be considered as pulsed exposures; that is, each time the beam crosses the eye, it is treated as producing a pulse of light that enters the pupil. This holds true even for effects that look solid or stationary.

When checking the exposure level against the MPE, three different MPEs have to be considered; Single Pulse, Average, and Multiple Pulse MPE.

Normally the Multiple Pulse MPE would be the most restrictive, for each time a new pulse is received it increases a reduction factor, leading to less light being allowed for the exposure to be considered below the MPE. However, with the changes that have been published, for many of the typical light show exposures we have seen, the reduction factor of the Multiple Pulse MPE has disappeared, and the Single Pulse MPE has become more dominant. – which is good news as it allows more light.

Summary of changes

The changes to the exposure limits in terms of MPEs that relate to the type of exposure created by a scanning laser show effect are:

Single Pulse Exposure

For a large part of the time domain this remains unchanged. However, the limit is reduced for periods less than 18us. For periods of 5us to 18us the formula 18 x CE x t0.75 J m-2 is used, and for periods less than 5us, down through the ns time domain a constant 2.0 x CE mJ m-2 is used. This is less than the 5mJ m-2 that is presently allowed.

Average Pulse Exposure

Any pulse train or sub-group of pulse trains within the exposure period must be below the average exposure limit.

Multiple Pulse Exposure

The new limits still use a Cp correction factor which is applied to the single pulse MPE, but for small sources, such as those we would see on laser shows, for pulses of 5us or more, Cp remains a constant 1.0, so is no longer a correction factor that reduces the permitted limit. This constant correction factor holds true for pulse durations less than 5us too, if the total exposure duration is less than 0.25s. If the exposure duration is greater than 0.25s and more than 600 pulses are received within the exposure duration, Cp = 5 x n-0.25, (n being the number of pulses), must be applied.

The upshot of these changes means that for typical laser light show exposures, the previously most limiting Multiple Pulse Exposure Limit is much less influential on the assessment. For most of the time its correction factor is unity, meaning that it no longer has the reduction effect. This makes the Single Pulse Exposure and Average Pulse Exposure the more dominant MPEs, which for the most part allow more light to be emitted before approaching the MPE.

Power through an Aperture

Another interesting development is in how the limits are presented; the limits are now expressed in the traditional radiant exposure J m-2 or irradiance W m-2 formats, and also as an expression of power through an aperture. For visible light the aperture is considered to 7mm in diameter. Describing the limits this way makes it more straightforward to relate the exposure limit to a power that can be measured with a laser power meter, rather than being expressed as power or energy over a 1m square area, which can be difficult to conceptualise. For example, the 0.25s MPE for visible light can be expressed as 25.4W m-2 as an irradiance, which is the equivalent to 1mW though a 7mm diameter aperture; the latter being easier to appreciate, even though they are both the same.

What happens next?

Both the IEC and ANSI laser safety committees have been working hard to formulate the next editions of IEC 60825-1 and Z136.1 respectively, which when published in the coming months will refer to the new updated exposure limits, as well as introduce several new and updated concepts to other parts of the standard.

Across the European Community the amount of laser light that a worker may be exposed to is set by exposure limits defined in the Artificial Optical Radiation Directive (Directive 2006/25/EC). The version that currently mandates workplace exposure limits refers the to the previously published ICNIRP limits. An updated version of the directive will be published that will use the newly published limits.

For the time being, carry on working to the existing limits, but be aware that over the coming months the changes will finding their way into the new editions of the laser safety standards, and also amended workplace exposure limits.

In the meantime, we will discuss both methods on all our training courses, and updates to our software tools will cover both sets of limits.

© 2013 LVR Optical