What Resolution?

From Introduction to Scanning & Digital Imaging, by Richard J. Urban, Colorado Digitization Project
http://www.cdpheritage.org/resource/scanning/documents/wrk_scanning_ppt.pdf

Scan originals at a resolution appropriate to their format and use.

Image Resolution - Photographs
  • Master: Minimum 3000-5000 pixels on the long dimension
  • Access: between 150-250 ppi
  • Thumbnail: 72 ppi
  • The resolution depends on: Original Format, Image use, Audience, File size

From A Few Scanning Tips FAQ
http://www.scantips.com/faq2.html#resol

Then the rules are very simple: Scan resolution (pixels per inch) determines image size (pixels). Digital image size is dimensioned in pixels. Dpi means pixels per inch, meaning that if you scan 6 inches of object at 100 dpi, you will create 6 inches x 100 dpi = 600 pixels image size. Or scanning 1 inch at 600 dpi will also create 600 pixels. Or 2 inches at 300 dpi, or 3 inches at 200 dpi, or 12 inches at 50 dpi, or 1/2 inch at 1200 dpi, all of these also create an image with a 600 pixel dimension. Just plug in any appropriate numbers, what you have, and what you want.

So planning depends on knowing what size you have to scan (inches), and knowing what size image you want from it (pixels). We simply select the appropriate scanning resolution which creates the appropriate image size to be appropriate for our goal. More resolution creates a larger image, less resolution creates a smaller image. A larger original (like a full size page) needs much less resolution than a small original (like 35 mm film). Your scanner software should compute and indicate the final image size (pixels) for your current settings (crop size and scan resolution), but of course you need to know what size image you want (pixels)?

Dpi means pixels per inch, which means that if we scan 6x4 inches at 150 dpi, then we create (6 inches x 150 dpi) x (4 inches x 150 dpi) = 900 x 600 pixels. But if we are scanning 3x2 inches, then 300 dpi creates the same 900 x 600 pixels.

Video screens are dimensioned in pixels too. To show the image on a 1024x768 pixel video screen, we need an image that is no larger than the 1024x768 pixel screen size (video screens vary in size, they are not all the same size). The viewing program's window will be even smaller than the full screen. Most images that are appropriately sized for printing will be too huge for the video screen. See section one and following pages about video resolution.

....
So, the answer depends on the size of what you are scanning (inches) and the size of image that you want (pixels) for your purpose. Scanning dpi gives the "pixels per inch" of scanned object which determines the final image size (pixels).

From Handbook for Digital Projects: A Management Tool for Preservation and Access. VI Technical Primer, by Steven Puglia, National Archives and Records Administration - http://www.nedcc.org/digital/vi.htm [Handout]

Cornell recommends 600 ppi for 1-bit scanning or 400 ppi for 8-bit scanning of printed type to achieve preservation quality scanning. Other general recommendations for reproduction are:

Textual records
200 to 600 ppi for 1-bit
200 to 400 ppi for 8-bit grayscale
200 to 300 ppi for 24-bit color
Photographs
3000 to 5000 lines for 8-bit grayscale
3000 to 5000 lines for 24-bit color
Maps/Plans/Oversized
200 to 300 ppi for 8-bit grayscale
200 to 300 ppi for 24-bit color
As computers become faster and memory becomes cheaper, the recommendations for scanning resolution are likely to increase. Today, projects are selecting higher scanning resolution than older digitizing projects.


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Diane Berry for WNYLRC 9/29/05