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Old 10th June 2013, 05:11 AM   #1
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Default Accurate scaling in a flatbed scanner ?

For a project to reassemble the parts from an old Lenco turntable I thought it might be worthwhile trying a scan of the stamped steel top-plate then convert it to vector (and then to cad). In a test using a flat ruler I discovered that there is considerable distortion that varies with the location on the scanner so that even if the scan of a 10" ruler is printed so the ruler image is 10" long, some segments of the ruler length are shorter than actual and some are longer.
Does anybody know about this technology and how to identify a flatbed scanner that will actually produce correct scaling across the entire image?

Thanks
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Old 10th June 2013, 05:38 AM   #2
Pano is offline Pano  United States
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Are you sure it's the scanner and not the printer? I've found most flatbeds to be accurate. If it's not, then the belt drive is probably at fault.
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Old 10th June 2013, 06:09 AM   #3
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Hi Pano,
No, I'm not sure, but the scanner I have is a very cheap Cannon that runs entirely on USB power and sounds like it just barely has the power to move the sensor across the frame. I think it's because of that I'm looking at the scanner first.
I was told by a salesman at a local store that anything but paper thin flat images won't scan with millimeter to millimeter accuracy unless the scanner has a focussing lens, and that type costs $700+. I don't know enough about the technology to agree or disagree with him.

Maybe it's best is to take the plate to a graphics repro house and ask them to scan it.
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Old 10th June 2013, 01:09 PM   #4
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Maybe. Check the scan itself first if you can in Photoshop or a similar program. That's the first step.
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Old 11th June 2013, 03:30 AM   #5
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Do you mean by opening it and then measuring the image on screen?
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Old 11th June 2013, 04:24 AM   #6
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Well, that's one way to do it. I was thinking more using the rulers in an image program. Do you have anything like that?
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Old 11th June 2013, 04:33 AM   #7
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Maybe try a commercial service? Architectural? Engineering?
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Old 11th June 2013, 05:19 AM   #8
oshifis is offline oshifis  Hungary
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I also use a Canon USB scanner (CanoScan Lide 60). This type has very small depth of field, and suitable for scanning paper images and text only. Also the absolute constant speed of the cart is not guaranteed, neither the same scale in the horizontal and in the vertical direction. Perhaps a more powerful office laser printer/scanner is better (as it should be).
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Old 11th June 2013, 06:43 AM   #9
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Quote:
Originally Posted by Hearinspace View Post
For a project to reassemble the parts from an old Lenco turntable I thought it might be worthwhile trying a scan of the stamped steel top-plate then convert it to vector (and then to cad). In a test using a flat ruler I discovered that there is considerable distortion that varies with the location on the scanner so that even if the scan of a 10" ruler is printed so the ruler image is 10" long, some segments of the ruler length are shorter than actual and some are longer.
Does anybody know about this technology and how to identify a flatbed scanner that will actually produce correct scaling across the entire image?

Thanks
You might have a digital camera or phone with a good camera , make a high res picture with a tripod or some other fixture and you can do anything with the produced Jpeg with various software and it will be linear !

Cheers ,

Rens
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Old 11th June 2013, 07:37 AM   #10
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Originally Posted by doctordata View Post
You might have a digital camera or phone with a good camera , make a high res picture with a tripod or some other fixture and you can do anything with the produced Jpeg with various software and it will be linear !

Cheers ,

Rens
Only by making digital lens correction. All camera lenses distort to some extent
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