BLOG: Projector Masking and Masking Fadeout
Masking is an essential technique used to reduce the likelihood of high-power laser beams and effects inadvertently projecting into areas they shouldn’t be, reducing unnecessary risk for people and equipment that could be on the receiving end of wayward beams. Masking is used typically to protect members of the public, performers, and workers, as well as sensitive equipment such as video projectors and cameras.
Modern laser effects control systems and projectors allow users to precisely define the maximum extent of the intended laser effects, meaning that any pre-programmed or live content should only appear in these ‘zones’. But simply setting the zone size and position is not considered sufficient to prevent exposure from a beam that may accidentally occur outside of the defined projection area. Any of the following could cause an errant exposure to occur:
- Laser show / effect content programming error
- Failure to save or use the defined scanning zone
- Software or control system bug or fault
- Scanning system fault, (electronic or mechanical)
- Electronic signal drift or noise
In the event of any of these happening if an effective projection mask is in place, any errant movements beyond the projection zone cause the beam to be automatically blocked by the mask, preventing potentially hazardous beams from travelling into the areas they are not intended to be. Errant beam projection into the audience does happen unfortunately, and for the most part, errant beams are inexcusable when such a straightforward precaution exists to prevent them.
Physical masking refers to a technique that physically blocks the path of the beam, providing a low cost, effective and reliable means of preventing errant beam exposure occurring. Most laser projectors have built into them one or more adjustable metal plates that can positioned so that the edge(s) of the plate(s) align tightly against the outside edge(s) of the intended projection zone.
Built-in masking plates work well for zones that have a straight edge to them. But when using zones that have a different contour such as a curve, blackened foil (e.g. Blackwrap lighting foil) can be an effective choice that offers greater flexibility in being able to be cut or folded to follow the outside extent of the zone. The material used should be firmly secured to the projector and be able to withstand the heat that the beam touching it will create. Under no circumstances should ‘gaffer tape’ be used to create a physical mask.
Due to having a high degree of reliability, physical masking is the preferred approach to preventing errant beams from appearing in areas they shouldn’t.
Software masking refers to a dedicated function in software that is intended to prevent beams from being emitted outside the intended area. In theory it sounds a great idea. Just define where you don’t want the beams to emit, save the parameters, and forget about it. No fiddly physical masks to adjust or having to gain access at height. It’s even possible to define awkwardly shaped no-go areas precisely. All just using a few mouse movements and clicks. Perfect! (?)
Unfortunately, not quite. We are not aware of any mainstream laser controller manufacturer that warrants the use of their software masking as the sole means of offering protection against errant beams. Taking a look at the licence agreement of the laser controller manufacturer should confirm their position on warranting the use of software masking for the purposes of safety.
The issue with running a safety critical software function on a general-purpose mainstream operating system such as Windows® or macOS, is reliability. Software carrying out a safety critical function should be run on safety certified operating system, and be certified itself.
However, the issue of reliability is just one part of the overall challenge, for even if it were possible to implement software masking entirely in software and demonstrate it has a high degree of reliability, unless the software is working as part of a closed feedback loop, that is, it knows exactly how the scanner and laser output is behaving, it is not possible to assure the beam is where it should be. For example, an electronic or mechanical fault or drift occurring in the scanners, or the laser module driver, would go unnoticed by a software-based mask, allowing a potentially hazardous beams into an area it shouldn’t be.
Therefore, in the absence of safety-certified software masks, even though they appear convenient, it’s difficult to recommend their use to protect against errant beam exposure to humans over the more reliable physical mask technique. Justification to use software masking to prevent errant beam exposure to sensitive equipment such as cameras and video projectors is more acceptable though.
An important characteristic to consider when using either physical or software masking is masking fadeout. Neither mask type offers an absolute instantaneous extinguishing of a beam as it travels into the masked area.
In the case of a typical software-based mask, any beam movement commands outside of the intended projection area should be extinguished by the software, outputting a ‘blank’ command, resulting in a no output state from the laser, (if all is working correctly). Just how quickly the laser can go from emitting light to a blanked state depends upon how long the laser will continue to output light after the blanking command is received. To a large extent, this is affected by the type and quality of the laser itself, its driver electronics, and how well calibrated the blanking and colour control signals are. In the case of a typical software-based mask, any beam movement commands outside of the intended projection area should be extinguished by the software, outputting a ‘blank’ command, resulting in a no output state from the laser, (if all is working correctly). Just how quickly the laser can go from emitting light to a blanked state depends upon how long the laser will continue to output light after the blanking command is received. To a large extent, this is affected by the type and quality of the laser itself, its driver electronics, and how well calibrated the blanking and colour control signals are.
While the time for laser output to react to a software mask may result in brief fadeout, it may still be possible to create a hazardous exposure condition in the time it takes to do so.
Physical masking suffers from a different and more obvious fadeout that occurs while the laser beam crosses into the physical mask. Light is still partially visible because the beam is only fully extinguished when the whole of the beam has passed across the edge of the mask and is completely behind it. The result is a visible beam that extend below the intended projection area. The physical mask is unable instantly block the entire width of the beam. The simplified diagram helps to show why this happens.
With a typical good quality lightshow laser beam with a diameter of 4mm, using a mask secured at the output window of the projector, the fadeout length that occurs at various distances from the projector is likely to be as follows:
The diagram shows that the two most significant influencing factors of how much fadeout will occur are the diameter of the laser beam, and how far the physical mask is located away from the scanner mirrors. Larger diameter beams intuitively take longer to fade due to the beam having to move through an increased angle to be fully obscured by the physical mask. The increased angle that the larger diameter beams must traverse translates to an increased fadeout length as shown by the figures below:
One obvious solution is to try and use laser projectors with small beam diameters where possible because they exhibit shorter fadeout characteristics. The longer fadeout is one of the shortcomings of using lower-cost projectors that tend to have wider diameter beams.
Moving the physical mask further away from the front of the laser output window is a better solution in reducing the angle through which the beam disappears, and therefore shortening the fadeout. It is quite practical to have a masking material secured to a projector at ~300mm or so from the output window for a temporary installation. The advantages of doing so are twofold, where firstly the increased distance from the scanners reduces the angle the beam has to move through to be fully extinguished. Secondly, creating a mask at an increased distance increases the resolution available making accommodating awkward shapes easier to achieve.
Table 3 shows how the beam transition fadeout dramatically decreases the further away from the projector the mask is affixed. Laser installations where the user has set the mask at 100mm reduces the 4mm beam diameter fadeout to approximately a third of what it is when masked at the projector window. Creating a mask at 300mm results in a further reduction of fadeout down to approximately one eight, over having masking at the window. From our experience, although using masking at 300mm is not too common to see at the moment with temporary installations, we do regularly see people using masks set at 100mm – 200mm distances. It certainly is practical, and yields a good benefit in both resolution and effectiveness.
Masking - Best Practice
- Physical masking is expected as standard practice in most major venues, events, and venue chains in the UK. It is a straightforward precaution which can be highly effective if installed correctly, and its use does not affect the look of a laser effect.
- The minimum vertical separation distance between overhead laser beams and the audience is supposed to be at least 3m, for a show that is supervised.
- Laser projectors directing beams above audiences should, as a minimum, have physical masking beneath the projection zone.
- Masking should be set so that at least 3m is maintained, and a little extra for fadeout as needed for the mask to function.
- Laser projectors directing beams onto stages should have physical masking in place to prevent errant beams exiting the stage area. This could mean masking ‘above’ and to the sides of the projection zone, depending on the orientation of the laser projector.
- The exact fadeout length created by a particular configuration is difficult to calculate precisely due to the multibeam make up of multicolour laser beams. Therefore, the mask effectiveness should be checked in situ. This can be achieved by moving the zone across the masking material using the control software.
- Always ensure the projector is secure and will not move. Masking attached to the projector will not prevent errant exposure if the whole projector moves.
© November 2022 LVR Optical