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Why do scanners and photocopies scan the material?

Couldn't they just have a digital camera + light behind the glass?

A lens with fixed focus would suffice, since the sheet of paper would always be at the same distance.

There would be no moving parts. Wouldn't than reduce costs? Would such a device be more robust?

I fail to see whether the quality of the produced image would be worse.

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  • \$\begingroup\$ dynamic range and resolution, but I dont really know what that has to do with EE design per se \$\endgroup\$ – PlasmaHH Dec 4 '15 at 19:54
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    \$\begingroup\$ Todays scanners claim to have an optical resolution of 2400dpi and more. A sheet of A4 paper has about 8in x 11in, which results in 507 megapixels. Even for moderate 600dpi, the picture has 30MP. It's hard to get a camera with this resolution and optics which allows to achieve this resolution in reality. \$\endgroup\$ – sweber Dec 4 '15 at 21:12
  • \$\begingroup\$ See the Google Books scanning details for the challenges with a fixed camera method. Since books scan poorly on a flat-bed, they had to use a photographic method. \$\endgroup\$ – user65586 Dec 5 '15 at 2:26
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  1. Consistent even lighting moving with scanner will give even illumination across the full scan. No vignette effect.
  2. Lens distortion - particularly at the edges. Scanner head is a line-scan camera with each pixel vertically aligned with the subject.
  3. Physical depth. Scanner is very shallow.

Try it yourself with a camera and a flat printed page. Look for evenness of tone across the page. Try it with squared paper and see if the lines are straight in your photo.

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    \$\begingroup\$ Good points, though arguably all of them would be easily accounted for in a camera system designed to be a scanner. So they are really just an answer to "why don't we use general purpose cameras as scanners" rather than addressing the real question. \$\endgroup\$ – Samuel Dec 4 '15 at 19:14
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    \$\begingroup\$ FYI, high-speed copiers, such as used in corporate document departments are often camera-based with flash. These are large expensive beasts that will do better than one page per second. The fact that this is only used for high-speed expensive systems supports the assertion that it's easier to do high-quality input with a scanner. \$\endgroup\$ – DoxyLover Dec 4 '15 at 19:22
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    \$\begingroup\$ There's one other reason for a line-scan camera rather than a general-purpose camera: it's cheaper. 600 dpi is the starting point for flatbed scanners. In order to scan an A4 sheet with a general-purpose camera, you need a 35 megapixel sensor; a line-scan camera of the same resolution only needs 5000 sensor elements. At the higher end, you'd need a 560 megapixel sensor to get 2400 dpi -- sensors that size would be insanely expensive even if produced in bulk. \$\endgroup\$ – Mark Dec 4 '15 at 22:00
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Using a fixed focus lens wouldn't be the same. The entire page is not at the same distance when you consider that there is a viewing cone on a camera. The edges are farther from the lens than the center is. That won't only affect focus, but lighting as well. Consider that some inks have different apparent colors depending on angle of view and reflection from lights.

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Actually there exist [book] scanners that are just that, a big sensor that images the whole page with no moving element. But they are expensive compared to getting a similar image quality with a moving-sensor1 scanner. Needs nice optics etc., also big sensor. That one uses a 35Mpixel sensor per color (105Mpixel total). Here's another one like that; this one uses a glass press, so it's more like you envisioned it. Also, a version of that that also turns pages.

1 In fact, you could have it the other away around, i.e. move the item/book as in the Google Books project, but to keep sensor costs down [to line sensor], you need to have relative movement of the sensor vs. material being imaged. Actually if you envision a book scanner that automatically turns pages, you need to have some moving elements anyway... so you might as well design the scanner around the mechanics of that movement.

In fact I've discovered a research project I didn't know about in which they flip a book really fast and just photograph it with regular cameras and then apply sophisticated deskewing/restoration algorithms. So it is possible to have your cake and eat it. I'm not sure of the image quality in this system. There's a paper about it at recent (2014) conference; in fact it got the best paper award there. They scan only at 500 dpi. To get that from the distance where the camera is at, they use ~25Mpixel (6.5Kx4.3K) camera. From this you can see the issues: to minimize optical aberrations you need to have some distance to the camera, which in turn requires increasing its resolution much more than if you scan with a sensor up close.

By the way there exist some more consumer and DIY projects of this kind; something was showcased at CES 2011; I don't know if it made it into a mass market product. Also something DIY.

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