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Scanning small items with a laser

Questions, problems, comments and tips regarding the 3d scanning process.

Scanning small items with a laser

Postby jk15 » Sun Oct 20, 2013 2:16 am

Hi there - I'm new to this forum,

I saw a few samples about people around here scanning coins and other small items successfully and was thinking of trying to reproduce that. I am using a Logitech C910 and an LC532-5-3F (green) which has a 0.4mrad divergence.

However, no matter how much I focused the beam, camera or reduced the exposure, the laser beam was still way too wide to capture any details of the coin (almost 10 pixels wide when the coin is 200 pixels and focused - camera on 1280x720 I think or close by). I tried moving it further/closer away but to no avail, and definitely never had the line be thin enough that it'd capture some of the details that the scans on these forums have been showing (for example: viewtopic.php?f=7&t=3508 even though it uses a blue line laser has a ton of resolution, I remember seeing another one with green line laser that was also quite good.). The same thing happens for all small objects (line too thick to get any detail).

Am I missing something obvious?

Thank you!
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Re: Scanning small items with a laser

Postby WalterMo » Sun Oct 20, 2013 7:52 pm

Scanning of a coin isn't so very simple. You should do the following:
1.Use a sufficiently small calibration corner.

2.Set the resolution of your camera to its highest value. Fill its image area in this way: 1/3 of the width with the coin and 2 x 1/3 with the calibration sheets, respectively left and right of the coin.

3.Coat the surface of the coin to make it matt:
viewtopic.php?f=2&t=1283

4.You can move the laser by hand, better is to tilt it by a very slowly motor. Maybe by a kitchen timer:
viewtopic.php?p=2016#p2016

5.Better is to move the laser line one time very slowly over the coin (and the corner) than multiple times and faster.

6.Use a big intersetion angle, best about 40°:
viewtopic.php?f=2&t=3117&p=17498&hilit

7.Focus camera and laser sharp to the coin, not to the calibration corner walls behind the coin.

8. Disable all automatic functions of the camera, e.g. autofocus.

The green fosusable laser is well suited:
viewtopic.php?p=1393#p1393
A blue laser is not much better, maybe a bit.

Walter

PS. Can you post a photo of your set-up? Have I forgotten something? Maybe if I have seen your set-up. :wink:
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Re: Scanning small items with a laser

Postby jk15 » Sun Oct 20, 2013 9:57 pm

Thanks a lot WalterMo for your quick response. I have attached some screenshots that I quickly put together (still kind of trying things out so sorry if they look terrible).

Here is the view without anything:
first.png


Here is the thinnest line I could get (this one doesn't have a left side but the others did)
fourth.png


Here is the reconstructed coin I could get (ignore the missing half since I am using playdoh to hold the coin vertical)
fifth.png


From what I am seeing the laser line seems thicker than some of the details on the coin (for example it is thicker than one whole letter on the inscriptions on the coin) and I don't know how to improve that any more. Is this because I haven't coated the surface as you recommend or is there some other more obvious fix that can be done before that?

Thank again
JK
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Re: Scanning small items with a laser

Postby jk15 » Sun Oct 20, 2013 9:58 pm

And here is the configuration (sorry, max attachments = 3 couldn't fit it before):
sixth.jpg
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Re: Scanning small items with a laser

Postby Bongobat » Mon Oct 21, 2013 6:19 am

Hi Jk15,

I made a video of how I do it. This uses a motorized scanner but you should be able to do it by hand too if your framerate is high enough ( ~30 fps). You dont have to get the laser in as close as I did but try to get close. This was quick and dirty just to show how to setup, a slower scan would have come out better.

You should place the coin closer to the calibration corner though and more in the center and then level and center your camera on the coin. A smaller calibration printout might also help, For scanning this penny I used one that was scaled 40% of the A4 single printout.

Greg




Right click on image above and select Play
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Re: Scanning small items with a laser

Postby jk15 » Mon Oct 21, 2013 7:24 pm

Thanks a lot Bongobat - OK I understand some of the changes I need to make now.

It looks like your coin is coated in some material though - my question is, how much does this impact quality? Will I be able to make accurate coin scans without it?

Thanks again!
JK
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Re: Scanning small items with a laser

Postby Bongobat » Tue Oct 22, 2013 9:13 pm

jk15 wrote:It looks like your coin is coated in some material though - my question is, how much does this impact quality? Will I be able to make accurate coin scans without it?


Yes, The coin was quickly painted long ago and not cared for since, you can see a gob of paint on the surface and some of the paint flaking off. I just grabbed it because it was available. I think it was coated with a mixture of cheap poster paint, water, and dish soap following the instructions from Mattia that Walter linked to above. The coating helps reduce reflections that can cause noise and distortions, a better quality paint will produce better results. The gob was caused by my carelessness with the aribrush though. I have sprayed coins with common aerosol paint from a can that has come out very good putting on just a very thin coating but don't do that if the item is precious and might be damaged by that type of paint.


I have seen some amazing results from Walter where he scanned a shiny coin without coating it, so yes it is possible. I have never been able to get quite those results myself without the coating though.

Greg
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Re: Scanning small items with a laser

Postby jk15 » Wed Oct 23, 2013 6:34 am

Many thanks Greg - I will try to replicate all your suggestions over the week-end.

Best
John K
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Re: Scanning small items with a laser

Postby Bongobat » Thu Oct 24, 2013 6:08 am

Hi jk15,

I made a video hand scanning a shiny and coated penny that you might find informative. I might make it sticky if it proves useful to others. Anyway here it is: viewtopic.php?f=2&t=5843

Greg
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