BLOG: Laser Show Safety Tips
As part of series on Laser Show Safety Tips, expert James Stewart delves into the whys and wherefores of physical masking, and how this often-neglected tool can be used to good effect to increase show safety assurance, and meet HSG95 requirements.
Physical Masking: Why, How and When
An essential tool that should be in every laserist’s toolbox is some physical masking material, because when used properly it’s very effective, doesn’t spoil the show, and provides that comforting assurance that no one can get hurt in the event of scanner system failure or a programming error. The great thing is that it’s one of the rare laser show components that costs very little too..
So why do we need physical masking at all?
Indeed it is true that today’s laser control software systems offer great flexibility in allowing the user to define precise areas where the laser effects will be projected through the use of ‘Projection Zone’ technology. It’s hard to remember how people coped with designing and installing shows without the ease of being able to define areas to project effects upon arrival at the venue.
The problem with zoning laser effects within a scanner output is that it’s great while everything is functioning as intended, with the beams terminating in the defined area, but not so good if, for example, a rogue cue that hasn’t been assigned to the set zone slips through the system. Add to that the simple fact that some software control systems can be incredibly user (or viewer!) unfriendly, outputting strange projections, or overriding settings when certain control combinations are used.
What about software with ‘attenuation maps’?
Software that allows you to define no-go areas or areas where the beam intensity is reduced is helpful, and may be ok to use in some circumstances, but the weakness of such systems are that, at best, the feature only limits output at the software end. The output emission at the projector is still free to do as is wishes, and can do, should something decide to go wayward in the projector.
When you are using a high power laser projector, the risk assessment would almost certainly show that an unplanned beam coming from the projector is an unacceptable risk and needs controlling, and depending upon the beam power density (irradiance) potential at the audience, the control could be physical masking.
This notion is further backed up in HSE’s guidance for laser displays HSG95, where page 20 shows a really useful safety assessment decision chart. In the chart, two tests are considered in the top row of the flow diagram to help determine if the display application will be safe. The first test checks to see if the emissions are inaccessible, (the top row is considering a non-audience scanned show). If the assessment passes the first test, the second test needs to be considered, are emissions inaccessible during reasonably foreseeable fault conditions? For the display application to be considered safe, the assessment needs to pass both tests, reaching the top right box in the chart.
HSG95 defines scanning failure, component faults, problems with the power supply or control system as being regarded as reasonably foreseeable fault conditions.
With electronic masks not yet being able to provide the assurance needed to fail safe, installing a physical mask at the output is the simplest and most effective solution to preventing beams for errant projector/control system behaviour straying into areas that could cause harm.
What type of mask?
Some projectors have a metal plate provision built-in serving a dual purpose of protecting the output window when the laser is being transported, while during operation, able to be lowered to a position along the bottom edge of the scanning effect.
The provision of a built-in physical mask plate is still quite a rarity on many projectors for some unknown reason, so unless holes are drilled in the projector to retrofit a mask attachment, (which is not always practical of course!), it means a more unsightly solution has to be used.
Black-wrap foil, normally used with gaffer tape to hold it in position, or some varieties have an adhesive side, allowing it to be used like tape. The foil can look like a cheap add-on to an otherwise expensive optical instrument, but as long as it is securely affixed it’s a quick and painless solution to keeping unwanted beams away from places they shouldn’t be. Foil also has the advantage that it can be cut to accommodate off-axis or geometrically corrected scanning outputs that may have slants or curves at the projector output window, (even projectors with built in plate masks benefit from some additional foil contouring on occasions). It’s always important to check the foil used is in good condition, has no holes in it, and is able to withstand the power of the laser that it may have to block, without breaking down.
Having established the extent of the projection zone, keep the test pattern on, turn the laser power down so as not to hurt your eyes, then carefully place the mask in position, moving it a close as possible to the outside extent of the zone, taking care to keep your eyes away from the output at all times. The foil should now be securely attached.
The effectiveness of the mask can be checked by adjusting the controller settings to move the zone temporarily away from its desired position, into the mask, where in the display area, it should be seen fade out as intended.
Never commit the cardinal sin of using gaffer tape to make a physical mask (I’ve seen it done a few times!). The tape obviously melts instantly should a beam touch it, rendering it useless, as the beam simply passes through the material.
When should physical masks be used?
Generally when the risk assessment indicates that there could be a risk of injury from direct exposure to high power beams; in practice, this will be most of the time for show applications. So essentially, any time that you want to keep stray beams away from people’s faces, cameras or projectors.
The good thing with physical masks, are that they don’t spoil the look of the show, they only take a few moments to install, and are low cost. As well as giving you piece of mind, their use also creates a good professional and caring impression to your client. So why wouldn’t you?
© 2013 LVR Optical