Detecting Non-Visible Stains through Laser Scanning

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scottgrieve
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Detecting Non-Visible Stains through Laser Scanning

Post by scottgrieve » Sat Jul 13, 2019 3:02 pm

Hello everyone,

I am undertaking a forensic project on detecting stains through the use of laser scanning on different materials. I am finding it really difficult to find information online relating to this.

Can anyone share some knowledge through their experience, of instances you may have came across stains on data that could not be seen on site?

It would be greatly appreciated,

Many Thanks,
Scott

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Re: Detecting Non-Visible Stains through Laser Scanning

Post by caddrafting2013 » Sat Jul 13, 2019 8:38 pm

I’ve scanned some tunnels and by eye you couldn’t see the graffiti and had been removed via councils with remover.

Although in my scans they show up, Similar to stains I would say.

Just do a few tests first


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scottgrieve
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Re: Detecting Non-Visible Stains through Laser Scanning

Post by scottgrieve » Sun Jul 14, 2019 1:11 pm

Hello,

Thanks for that. Is there anything you needed to adjust to see this on the software?

That rings a bell to something similar I have seen on this forum, on instances with old road markings that had been removed could then be seen on scan data. Its really interesting from a forensic point of view!

Kind Regards,
Scott

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Re: Detecting Non-Visible Stains through Laser Scanning

Post by JLarson » Mon Jul 15, 2019 2:30 pm

For the use that you are trying to achieve, there are only two ways that a laser scanner will help: Reflectance and Texture. For reflectance, the scanner sees the difference in the amount of laser light reflected back to the scanner. This pretty much will be matched to the visible light spectrum, but not completely. It would depend on the wavelength of the laser being used and the sensitivity of the sensor on the scanner.

Another way you could detect stains would be in changes in the texture of the surface. I suspect the case of the remove graffiti that this was the case. Chemical remover would probably eat away at the surface a little to remove the paint, thus causing a different surface texture that could possibly be seen by a scanner.

All of this would be completely dependent on the scanner model used. As was stated previously, testing would be the only way to know with your particular equipment.
Jason Larson, PLS
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Re: Detecting Non-Visible Stains through Laser Scanning

Post by Tookie » Thu Jul 18, 2019 11:38 am

As it happens we scanned a graffiti park recently with the RTC360 (infrared laser). So I had a look through the scans..
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scottgrieve
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Re: Detecting Non-Visible Stains through Laser Scanning

Post by scottgrieve » Thu Jul 18, 2019 3:04 pm

JLarson wrote:
Mon Jul 15, 2019 2:30 pm
For the use that you are trying to achieve, there are only two ways that a laser scanner will help: Reflectance and Texture. For reflectance, the scanner sees the difference in the amount of laser light reflected back to the scanner. This pretty much will be matched to the visible light spectrum, but not completely. It would depend on the wavelength of the laser being used and the sensitivity of the sensor on the scanner.

Another way you could detect stains would be in changes in the texture of the surface. I suspect the case of the remove graffiti that this was the case. Chemical remover would probably eat away at the surface a little to remove the paint, thus causing a different surface texture that could possibly be seen by a scanner.

All of this would be completely dependent on the scanner model used. As was stated previously, testing would be the only way to know with your particular equipment.

Hi Jason,

Thank you for your detailed response it is really helpful!

Is it possible to click on an area or single point within software and find out a recorded value for texture or reflectance?

We have done some tests with blood on different materials so far and the blood seems to change the texture slightly. From literature also blood has been seen to absorb light a lot more than most materials. It will be interesting to know the upper and lower limits of detection.

Kind Regards,
Scott

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Re: Detecting Non-Visible Stains through Laser Scanning

Post by scottgrieve » Thu Jul 18, 2019 3:10 pm

Tookie wrote:
Thu Jul 18, 2019 11:38 am
As it happens we scanned a graffiti park recently with the RTC360 (infrared laser). So I had a look through the scans..
Hello again Iain,

Thanks for sending those images they are great.

To see those differences between the red spider background, and they grey spider background. And between the grey lady background and coloured lady background, was there anything in specific on the software you had to do to achieve this?

Kind Regards,
Scott

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Re: Detecting Non-Visible Stains through Laser Scanning

Post by Tookie » Thu Jul 18, 2019 9:16 pm

Hi Scott,

The scanner records the position of each point and the intensity of the return signal. The Grey images are simply a greyscale representation of the strength of the return signal. Lighter colours are more intense returns.
The colouring is applied by overlaying photographs (taken by the scanner). Each point is given the colour of the pixel of the photograph after matching.

It would be interesting to compare the earlier Leica scanners which use I believe visible light lasers (C10 Green and P20 Red). And also to look at the other scanner manufacturers, I wonder how the phase based scanners see this sort of thing.

Iain.

P.S The data is not available at the moment but I may be able to let you have a redacted copy in the future. Drop me an email to remind me.

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