Wild Data Points from Scanner

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Wild Data Points from Scanner

Post by gordonired »

We recently scanned a site for a client, and have found that we have lots of erroneous noise in our scans. The picture I've uploaded is one example.
It is a process building inside of the small clipping box, with corrugated steel and the occasional window. I can't understand why there are so many points, so far outside of the building. I'm now returning to the clusters and manually deleting all of these points with clipping boxes to clean up the scan. Is this something to do with the scanner? Is it a bug? I'm running Scene 5.4 still. On the interior there were shiny surfaces, pipes etc, but these do not typically give us so much grief. Any insight would be welcome, thank you.
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Andrew Torgerson
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Re: Wild Data Points from Scanner

Post by Andrew Torgerson »

Hi @gordonired. If they seem to be in a spherical shape around the outside of the scans it is quite likely you have a dirty mirror. Follow the instructions for cleaning a Focus mirror surface and be very careful not to touch the mirror. I've attached some instructions here, but FARO may have some newer ones. Likely the exact same process though. Hope that helps.
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Re: Wild Data Points from Scanner

Post by gordonired »

That might be it, as far as I know we don't regularly clean our lenses. I'll update the thread if it happens again after it's cleaned. Thanks Andrew.
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Re: Wild Data Points from Scanner

Post by Andrew Torgerson »

No problem. I would also add that it is best not to clean your mirror regularly, only when it needs it.
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Re: Wild Data Points from Scanner

Post by jmiller2103 »

I have seen this when there is fog, you get a dome like pointcloud around the scanner, or it could more likely be the lens
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Re: Wild Data Points from Scanner

Post by CWickersham »

I believe a cause of these points are due to the nature of the measuring laser reflections for phase shift. If something is too close to the scanner (ie. scanning in a dusty/foggy environment, or having a spec of dirt on the mirror, or even being too close to an object), the scanner will give back a value that reads as a value that is the equivalent to it's maximum read distance. This video kind of demonstrates the scanners internal behavior (minus the constructive/destructive light interference, I assume there is a photodiode that converts the light returning to the scanner into an electrical signal.)
[youtube]https://www.youtube.com/watch?v=UA1qG7Fjc2A
[/youtube]


youtu.be/UA1qG7Fjc2A

If you look at the attached image below, you can see that the red sine wave represents the phase of the laser as the light leaves the scanner. After the laser leaves the scanner, it reflects off an object and the phase shifts (purple sine wave) when it returns to the scanner (with a lower amplitude). As the distance increases, the further the phase shifts. As the distance the reflected reaches it's max distance, the phase shifts to a point (y = sin(x+2pi)) where the scanner detects the phase as if it were up close, and vice versa. I think the software would elimate any up close points and assumes that the point is at max distance of the scanner.

If someone with a more experience phase shift background could comment or correct me let me know if I made any wrong assumptions about the scanner's behavior.
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Re: Wild Data Points from Scanner

Post by dcacioppo »

I assume the stray data is not important. You can simply delete it all at once. I see this all the time. The distance filter can be used to delete points outside the distance that you set. For example, if you set the distance at 100 Meters, points outside that distance will be ignored. I am using Scene 6x so I am not 100% sure the distance filter is in Scene 5. If not, you could also polygon select your registered project, right click the selection, and choose delete OUTSIDE selection. Be careful not to accidentally delete your highlighted selection.
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