The best resolution for Book scanning?

Question

I am starting a library. I will be scanning the books. What dpi do i need to make the best records for the future and the archivest who be looking in years to come?

Answer 1

The resolution on the screen can be as low as 72 pixels per inch, but texts on the screen still are well readable and look sharp. That's because text on the screen are rendered from mathematically exactly defined vector shape fonts and they are sized and placed optimally to fit into the screen pixel matrix.

It would be a fatal error to think that 72 pixels per inch is high enough resolution to scan books. That's because the scanner cannot know the exact mathematical shapes of the scanned letters, so they cannot be positioned nor sized to fit optimally to the 72 ppi grid. I have found that a lower resolution than 300 ppi scan of say 11 pt text looks inferior on the screen. If one wants to reprint the 11 pt size text from an image of that text, the image should have at least 600 ppi. There's no exact inferior-sharp tresholds. The given numbers are only my opinions based only on my personal habits to see and decide what's sharp. You should make your own decision.

There's possible to make storage space estimations. I inserted some of them already in my comments. If it happens that you do not want any quality degrading compression and you want also paper color and texture or there's colored images you can well expect you need 50 megabytes per an A4 page. That's not much if you have a small library or endless pockets. But 100000 pages may be a reason to think to use some lossy compression to save space. Remember: You need also backups.

I have seen document scanners which output PDFs with JPEG or JPEG 2000 compression. You can well expect these compressions reduce the sizes of the files by 80% or even more.

It's up to you decide how much compression noise you allow to be inserted as the cost of storage space reduction.

Images in the books can generate a surprise which may need special attention book by book. Images are often made of discrete ink dots. Scanning or photographing such regular patterns can generate bad looking interference patterns when the image and the scanner happen to have too close pixel densities. The caused error is called "Moire" effect. One can fade it by changing the pixels per inch number used in scanning, but nothing removes it afterwards. More: https://en.wikipedia.org/wiki/Moiré_pattern

As I wrote in my comment I guess the time will be your major problem. You need a fast way to make the scans (or take photographs of full spreads) and insert the take to the archive in a way which makes possible to check the result and find a certain page easily. In addition you surely expect consistent colors and uniform light with no glosses.

One of us lifted up problem "curved pages". The problem is common. Here in GDSE we have numerous of questions how to fix curved pages in images. People got them by photographing or scanning books which cannot be opened to straight. The only proper way is to avoid them beforehand.

A heavy glass plate doesn't help; many books get broken if one tries to force the opening to 180 degrees. In addition tricky polarization filtering is needed to kill the glosses.

One DIY photographing solution: Open the book only 90 degrees and take a photo of opposite pages simultaneously with 2 pro quality cameras which are aimed and adjusted manually beforehand. Everything must have sturdy mounts. A beforehand adjusted frame presses the page edges straight. There's no glosses if the light comes from a large distant surface and it's directed downwards. Bright light and small aperture keep the image sharp without re-focusing.

My guidance: Develop the scanning or photographing process so that the quality and total time/page needs are acceptable. If you use an ordinary home scanner and its utilities you may need work a minute per page or spread to get the image into your computer. Then there's the making of an usable PDF. Developing a practical way to build PDF books is another problem. Get some local help.