All laser effects have the potential to be dangerous.  As do all motor vehicles.  The most important part of any laser show with regards to safety, whether there is audience participation or not, is the competence and ability of the laser operator.  Simply put, if the operator does not know what they are doing, a dangerous situation may arise.

The most dangerous aspect of laser light is not power, but power density.  A good analogy for this is a sixteen stone man in flat shoes standing on your foot will hurt slightly but produce no lasting damage and 10 seconds after he has stepped off you, your foot will be fine.  However, if a six stone woman in high heels stands on your foot with the heel, it will go straight through your foot leaving a scar and lasting pain.  i.e. a 20 watt laser can be used to scan the audience if the power density at first point of contact is sufficiently low (i.e. a fat beam) but a 1 watt laser may be dangerous at the same distance if it has a sufficiently tight beam. With scanning, the exposure levels are dependent on so many factors that some rather complex calculations and measurements are needed in order to guarantee safety.

Scanning can be explained as moving the laser beam about very fast so that your brain effectively fills in the gaps.  All that is happening is the same thing as when on bonfire night you have a sparkler and use it to make an “O” by spinning it in a circle.  It is just happening much faster and with more precise movements.  The problem with this is that at any given moment in time, the full power of the laser beam is at a given point which could be someone’s eye.  The exposure rate for this point has to be derived if audience scanning is to be used in order to make sure it is safe.

Diffracted effects are different and much easier to calculate as far as safety goes.  By its very nature a diffracted effect changes the laser beam characteristics.  As the beam passes through (or reflects off) the diffracted effect the beam is split in to many beams.  The beam with the most power of these multiple beams is the zero order (or the “hot spot” as it is often called).  As the orders increase (often moving away from the centre of the effect) the power of the beams themselves is reduced significantly (and logarithmically).  The important measurement for a diffracted effect is the highest order (i.e. closest to the centre of the effect) that interacts with the audience so it is important to make sure that that will not exceed the MPE (maximum permissable exposure).  If the MPE is not exceeded then all the subsequent orders will be lower in power and therefore lower in power density.  An important aspect of this type of effect is the physical arrangement of the effect.  If it is a transmission effect (i.e. the beam passes through the effect before entering a public area) the zero order/hot spot MUST finish in a safe place out of audience reach as otherwise, if the effect were to be compromised (drop off, crack, etc) a single beam at an unsafe power density may end up in the public domain.  With reflective effects the same is true of the original beam direction as it may end up going past its original target, but if the beam power density is low enough the hotspot may enter the public area.  As a rule of thumb, the more beams you can see coming from a diffracted effect the safer it is due to the power distribution of the original beam.

In the UK the MPE for a single exposure to a laser beam is 2.5mW/cm2 using a 0.25sec exposure time (aversion response/blink) for the ocular values.  Skin exposure is practically irrelevant to laser show producers as the levels are so much higher than the ocular ones that you could never use something that powerful with regards to an audience anyway.  This means that no part of any effect that enters the public areas can be more powerful than 2.5mW/cm2.

As good practice all alignment of the laser equipment should be done at low power before it is locked off to known positions in order that safety should not be compromised.  Laser equipment should NEVER be left alone while running and should NEVER be operated by untrained personnel unless in the presence of a trained operator.

This is just a brief look at some of the possible effects and is by no means complete for all known effects.  A competent laser operator will have a whole host of tricks up his or her sleeve in order to reduce the power density and create a safe effect while still providing a stunning result.