Over decades of study, safety experts have determined the Maximum Permissible Exposure (MPE) level for various types of lasers and exposure lengths. These MPEs are listed in standards such as ANSI Z136 and IEC 60825.

They also form the basis of laser safety laws in countries that regulate laser exposure. In such countries, your show cannot expose an audience to laser light that exceeds (is brighter than) the MPE. You could be prosecuted by the government, or sued by a private person or company.

MPEs are the same worldwide

MPEs are essentially the same around the world. What may differ from country to country is how strictly laser laws (if any) are enforced.

Just because a show is legal, or is not stopped, does not necessarily mean that it is safe.

ILDA does not dispute that the MPEs are valid. They have been carefully examined in laboratory situations. The MPEs are set low enough to provide a safety factor. We support the MPEs.

However, in real world situations there are other, non-laboratory factors which appear to be increasing safety. This is demonstrated by the fact that over three decades of laser show audience scanning, there have been about 110 billion pulses of light entering audiences' eyes, with "essentially no injuries" reported from continuous-wave laser shows. For this reason, ILDA believes it is acceptable under special conditions to scan audiences with light that is 10 times the MPE. (This topic is discussed in much greater detail in this paper, presented at the 2009 International Laser Safety Conference.)

Whether you choose to scan at the MPE, or (under special conditions) at 10 times the MPE, the first and most important thing is to know exactly how much light you are putting into the audience area.

For audience safety measurements, you need to accurately measure the beam's irradiance. This is the power over a given area, expressed in units such as "milliwatts per square centimeter".

For example, a 1-watt laser can put all of its power into a single thin beam that can enter the pupil. The irradiance is very high; it can instantly cause severe eye damage. Or, the same beam can be spread out using lenses or scanning, so that only part of the light enters the eye at any one time. This lowers the irradiance. If the irradiance is low enough, the beam from a 1-watt laser can be safe for audience exposure.

To measure irradiance, you MUST use an appropriate power meter. You cannot "eyeball" it. The eye is not nearly accurate enough.

There are two main parts to the meter: the measuring part, and the light detector head. The head can be a silicon detector, or a thermal detector.

     • Silicon detectors are very sensitive but are wavelength dependent. This is OK if you are measuring a single-color beam. But for projectors with two or more laser colors simultaneously, you will need a flattening filter. This assures that all wavelengths are measured equally. Greg Makhov uses an International Light model IL-1400 radiometer with a flattening filter on the head. (Sample instructions for use are here.)

     • Thermal detectors are less sensitive to low, milliwatt-levels of light. Their advantage is that they are far less sensitive to differences in visible wavelengths of light.

The detector head ideally will have an aperture (hole or active area) of one square centimeter. When this aperture is in place, the meter can then measure how much power falls on that square centimeter. For larger or smaller detectors, calculate the difference between your detector’s area and one cm², and then divide or multiply by the appropriate amount. For example, if the detector area is only 0.75 cm², then you would multiply your power readings by 1.5 to find out the irradiance in cm².

So let's measure a static beam from your laser projector and see how powerful it is. Be careful to meet all of these conditions:

     • Use an appropriate power meter and detector head. Be sure you are operating it properly. This can be tricky, so read the manual and double-check your usage.

     • The detector head must be at the same distance as the closest human exposure. This represents the maximum power an audience member or performer could receive.

     • All colors in the beam must be on, at the maximum show level. For example, if you have an RGB projector, the red, green and blue beams must all be on at the same time, and at the maximum power you are using during the show.

     • Your software must be putting out a full-power beam. Some laser software automatically adds blanking points when the beam is a dot. Check with your software company to ensure that this is NOT happening during the measurements. The beam should be "on" 100% of the time.

     • The beam must be static. It must be a single spot of light.

Once you're sure of these conditions, put the detector head into the beam and measure where the light is strongest. This is a "static laser power measurement".

In general, it is better to increase the beam’s divergence. You are spreading out the available light, not reducing the total quantity of light. Yes, the beam will be fuzzier but this is the trade-off for being able to use more total light.

Do this by putting a concave lens in front of the projector's output. You can carry lenses of various powers, so you can choose the best one to give you the desired irradiance without the beam being too fat and fuzzy.

Parallel light rays from the left side are diverged by a double-concave lens, so that they spread out on the right side. Diverging a laser beam in this way lowers the intensity of light that can enter the pupil of a viewer's eye.

An ideal solution: Variable divergence

An ideal solution is to use a software controlled "beam brush". This device is a moving lens that can quickly change the beam's divergence, making it sharp or fuzzy. The laser graphics software controls the beam brush. The beam can be tight and full power above the audience, and fuzzy with lower power when in the audience.

As of September 2015, a practical beam brush device has been demonstrated but is not yet commercially available.

Shows look best if the beam is at maximum power above the audience. Therefore, you also should consider techniques for reducing the beam power only when it is in the audience.

This reduction can be done at the projector with some type of filter or beamsplitter in the lower half of the scan field. Of course, this must be firmly affixed relative to the projector output, and the projector cannot be bumped or moved during the show.

A more flexible way is to use a software feature sometimes called a "beam attenuation map". For various parts of the scan field, you can paint the attenuation level you want. The darker the "paint" in an area, the more the laser power is reduced by software.

If this is the view from the laser projector...

 
...this is what a beam attenuation map might look like:

 
The darker the map block, the more the software reduces the beam power when the laser scans in that area. As you can see, the beam power is reduced the most for the closest audience members. The power is not reduced as much for persons in the back, since the beam is wider in the back and is thus safer. In the area above the audience there is no beam attenuation at all.

Note that in a real-life situation, it would be necessary to measure or calculate the beam irradiance for the farther-back zones, to be sure the desired MPE is not exceeded.

Also, in many countries, if you aim beams into the sky, this must be done in compliance with aviation laws or guidelines.