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Laser Safety Practices

Abstract

This articles uses pictorial diagrams to explain the basics of good laser safety practices in a variety of settings.  The information presented is generalized in accordance with international laser safety guidelines.  Laserists should check with the regulatory authorities in the show jurisdiction for the specific details of regulations that may apply to the territory where they are performing a show.

Projector

The first point to implement laser safety is at the projector itself.  Some people use a very simple projector set-up consisting of the laser on a base plate, with a shutter, PCAOM and scanner pair bolted to the end of the plate [Figure 1A].  While this may be simple and low cost to build, it poses dangers from stray reflections at optical surfaces such as the windows or crystal facets of the PCAOM.  It is also possible that due to high vibration, a components can come loose and an uncontrolled beam will be projected into the venue.
No matter how well you clean your optics, there is always a certain amount of 'splatter' - stray uncoliminated light.  This can be annoying and unaesthetic as it can find it's way onto the screen and lower the contrast ratio.  Without a cover, part of the laser beam remains visible to the audience even when the shutter is closed adding a further unaesthetic visual distraction.

In the simple shutter, PCAOM, scanner type projector we have described above, it is relatively easy to enclose all of those parts.  A simple sheet metal enclosure that coves the optics and goes over the end of the laser can be fashioned and then fastened to the baseplate.  The reason for overlapping the end of the laser is to form a 'light trap' as light travels in straight lines and very little goes around the corners formed between the cover and the body of the laser.
Since the scanners remain in a fixed position, and have a known maximum image size, it is simple to cut an opening a few millimeters larger than the maximum scan field at the appropriate place in the cover [Figure 1B].  In the drawing, the cover plate is show semi-transparent for clarity - in reality it would be made of metal with sufficient thickness to absorb the power of the laser being used.

In the case of a projector with a beam table, there is an added level of complexity.  While the size and position of the scan field in any projector is usually fixed and can be determined in advance, the position of the beams from the beam table vary from show to show.  The ideal method of solving this problem is to use a "turret" type beam table.  In this type of projector, when the actuator is engaged, the beam is projected vertically into an enclosed turret which then deflects the beam outwards into the venue.  The additional 'corners' that the light must travel around also helps to limit stray light [splatter] when the actuator is not engaged.  Figure 2 shows a drawing of this type of projector with a 3D view of a typical turret at the top right.

In the older style 'flat' beam tables, it is difficult to know where the beams will exit the projector.  The best approach is to have an opening large enough to allow the beams to exit in any possible direction that the beam table can steer them.  To prevent stray light and to have the maximum safety, one can cover the beam table opening with a sheet of "blackwrap" and punch holes for the beams to exit.  Blackwrap is a thick aluminum foil that has been anodized black.  It can be purchased from theatrical supply companies or film supply houses as it is used to control lighting particularly on movie shoots.  Since it is metal, it is not flammable although it may not be able to withstand the heat from high power lasers for long periods of time.  The use of "foamcore" or other non metallic covers is NOT recommended as a stray beam can catch them on fire causing an additional safety hazard.
Another possible solution is to obtain a sheet of the thin metal used in making air conditioning ducts, a tin snips and some black paint.  Once the beam positions have been established, the metal can be cut or drilled with exit holes for the beams and then painted black.  This solution is more viable in situations where the show will be in the same venue for a few days or weeks.

Restricted Zone

The area around the projector also needs attention.  There should be a physical barrier to cordon off the laser from the public and even the stage crew.  This can be crowd control barriers or stands with yellow caution tape to demark the "restricted zone".
The area closest the laser on the boundaries of the restricted zone should meet the vertical and lateral separation requirements outlined below.

Audience separation

International laser safety regulations require that the audience be separated 3 meters vertically and 2 meters laterally from any laser beams.  This is particularly true of static beams projected from a beam table as it is possible to project almost the full power of the laser into a single beam.  It is good practice to align your scan head in such a way that the zero position of the beam meets the audience separation guidelines.
Even in countries where audience scanning is permitted, care must be taken to insure that the effects projected below the 3 meter level are widely scanned and of low enough power to meet exposure guidelines.  Even where audience scanning is permitted, static beams must ALWAYS be projected above the 3 meter level as they are of much higher power and are much more dangerous.

Figure 3 shows a simplified diagram of a typical set up with the laser atop a 2 meter high scaffolding set on a 1 meter high stage.  The red beam is 3 meters above the floor that the audience is located on.  Note that the 3 meter vertical separation is not affected by whether the audience is seated or standing.  The couple on the floor is safe... but we have risers at the back for the venue.  The couple standing on the risers is not safe.  The 3 meter vertical separation must be measured from the highest point that the audience has access to, in this example the top of the risers, thus the green beam shows the proper placement for safe vertical separation in this example venue.

Lateral separation between the audience and the laser must be maintained at 2 meters.  This means that beams that travel near balconies or stands must be at least 2 meters from the closest point at which the audience could have access.  This often means that you can not hang mirrors on balconies on stands in arenas.  While the beam may be more then 3 meters off the highest surface the audience has access to, it may be possible for an audience member to lean out and down over the railings and come in contact with the laser beam.

In figure 4, we see the same venue as above but now with a balcony.  The orange beam targeting mirror A appears to be safe as the beam is more than 3 meters off the highest point the audience has access to.  However the balcony only has a 1 meter high railing so theoretically someone could lean out and down and come in contact with the beam.  Naturally they would be endangering themselves and risking falling off the balcony, but audiences have been known to do these kinds of things.  By hanging a pipe or truss 1 meter below the balcony Mirror B, targeted by the blue beam, is now safe as it has a 2 meter lateral separation from the balcony, and is more than 3 meters above the highest surface the audience has access to.
Note that we are not concerned in this example with keep the blue beam 3 meters above the highest point in the balcony as it terminates below the balcony.
The balcony in this example venue has now introduced another problem.  The scanned image on the screen could overshoot [dotted red lines] and the audience in the balcony would be in an unsafe area.  There are two solutions, close the balcony and post security to insure that the public does not sit there - this is not always possible.  The second solution is to use a mask on the scanners.
A mask is a metal plate with a hole cut in it just large enough to permit the image to cover the screen.  If the scanners drift off the screen for any reason, the metal mask prevents scanned beams from being projected around the edges of the screen.  The mask should be thick enough to absorb the full power of the laser used so that it is not punctured if the laser should come to rest on the back of the mask for any length of time.

In Figure 5 we see a simplified top view of a common arena or concert hall configuration with multiple balconies.  The laser is placed on scaffolding on the stage as before and the yellow area represents the "restricted zone" around the laser.  Note that the area at the boundaries of the restricted zone must also meet the 3 meter vertical and 2 meter lateral separation requirement.  In figure 4, it does not meet the 3 meter vertical separation but this could be solved by declaring the entire stage area in front of the laser to be the restricted zone.
For the sake of this example, the beams are at the same height off the floor as the blue beam shown in Figure 4 - that is 2 meters below the top of the railings at the edges of the balconies.  The red beam at the top of the diagram meets the separation requirements from the back balcony as there is more than 2 meters of lateral space.  It does not meet the separation requirements from the balcony at the top of the diagram as there is less than 2 meters of separation.  The green beam at the bottom of the diagram is safe as it meets the 2 meter separation requirement from both the back and bottom balconies.  The 2 meter lateral separation must be measured form the point at which the audience can get closes to the laser beam.

Summary

In order to provide a safe and enjoyable show for the audience proper safety precautions must be observed.  While regulations vary from jurisdiction to jurisdiction, the information above conforms to basic international standards.
Your projector optics should be covered with a metal cover to prevent the emission of stray beams and also to improve the aesthetics of the show by reducing splatter.  the cover should be made of a sufficient thickness of metal to absorb the power of the laser - flammable materials should not be used to prevent fires. The laser equipment should be in a marked restricted zone to prevent unauthorized access.
A vertical separation of 3 meters between the highest point the audience has access to and the laser beams must be observed.  A 2 meter lateral separation between the nearest point to the beam that the audience can access must also be observed.
In all cases, laserists must familiarize themselves with the rules and regulations specific to the jurisdiction in which the show will be performed.