Discussion in 'FB series of products' started by aricha, Feb 9, 2017.
Is there a document about the Scan Guard?
What is it guarding, the scanners or audience?
You ask a very good question, and it's one that we really need to clarify in the upcoming FB4 documentation.
Nevertheless, "Scan Guard" is a kind of scan-fail safeguard that is built into FB4. In order to use it, the FB4 generally must be installed directly inside a laser projector, and must have the "position" signal from the X- and Y-scanners connected to the FB4. When that is done, the FB4 can monitor the position signals and get an idea how fast the scanners are going, and also where the scanners are projecting, and then make intelligent decisions as to whether or not to apply blanking to the signal.
So, for example, let's say you are projecting a very slow moving beam. This kind of thing is potential harmful for audiences (if the beam is projected directly into an audience, and if the beam has sufficiently high power, and sufficiently low divergence). So "Scan Guard" can be enabled to prevent this sort of thing from happening. There are parameters that you can adjust which control how fast the beam must be moving, and also parameters which will help FB4 to understand where the audience is located (i.e below a "horizon").
I will STRONGLY CAUTION EVERYBODY that this "Scan Guard" feature SHOULD NOT BE RELIED UPON AS THE SOLE AUDIENCE PROTECTION MEASURE!!! For the absolute best protection, customers should use our PASS system, which is based almost entirely on analog hardware (not software), and which includes multiple redundant circuits to monitor a wide variety of projector behaviors (not just scan speed and position). FB4 "Scan Guard" feature is a good thing to have -- especially as an ADDITIONAL layer of safety.
In your article on Audience Scanning (https://pangolinlegacy.com/resguide09a.htm) you mention a simplified approach to analyzing shows which is based on 2 assumptions:
The actual scanning and beam modulation need to happen at a rate fast enough to keep the pulse-width experienced by the eye around 1millisecond or faster.
The maximum irradiance of a beam measured at the closest point of audience access needs to be somewhere between 5mW/cm2 and 10mW/cm2.
Assuming you don't have a PASS system installed and are using Pangolin Safety Lenses and Beam Attenuation Maps in Beyond to reduce beam power, can you give any guidance for Scan Guard settings (Min Velocity and Dwell Time) that would satisfy the requirement for keeping "the pulse-width experienced by the eye to around 1millisecond or faster"?
Thanks for writing to us, and for paying such close attention to our writings. That article that appeared in The Laserist magazine was written very long ago, but still has great information. However, my general recommendations are as follows:
1. Generate a completely non-moving and unmodulated beam (not terribly easy with FB4, since we *really* try to keep things safe and apply modulation when needed). This may require you to turn on the lasers someway-somehow that does not involve the FB4.
2. Through lensing and also adjusting the maximum laser power (probably operating at below 100% -- i.e. below 5V modulation level), make sure the *irradiance* at the closest point of audience access does not exceed 10mW per square centimeter.
One way of doing this would be to create a 7mm aperture (for example, sheet of metal with a 7mm hole in it), project your non-moving beam toward (and through) that hole, and make sure a regular old laser power meter (located on the other side of that hole) reads no greater than 5mW.
It's a little secret (although sometimes discussed by me and John O'Hagan) that 5mW into a 7mm round aperture is roughly equivalent to 10mW per square centimeter.
(As a review of laser classes: Class 3R is nothing more than "5 times Class 1". Since Class 1 is 1mW, Class 3R is 5mW into the 7mm round aperture.)
The real challenge is finding a laser power meter that will measure in the milliwatts, but Thorlabs actually has a few, including one that can measure white light in the milliwatts.
3. Once you've accomplished points 1 and 2 above, you are *pretty much* home free, and the reason why you are is because you will have created a scenario where the laser projector puts out what is effectively Class 3R light levels. Therefore any scan fail on top of that would improve things even further.
So -- as a general recommendation, adjust FB4 scan guard to the point where it does not "cut up" (i.e. start interfering with) *normal* show content such as sheets, fans, tunnels, etc.
Note that the use of SafetyScan lenses along with FB4 would surely be better than nothing, and we would solute you for doing your part to keep laser shows safe. But it's still not as good as PASS, because PASS plays a more active role in laser show safety, and has an electronic design that includes redundant circuits. So PASS is still the gold standard for touring shows and mission-critical situations.
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