Scanning resolution, half-tones and moiré patterns

If the image you are working with has a lot of fine detail -- (engravings in old books may display resolution of 300dpi or higher; real objects like leaves or noses have microscopic detail), you may wish to change the default resolution of the scanner from 150 dpi to a higher number.

resolution.gif (15173 bytes)
a.		 resolution2.gif (3966 bytes)
b.

Under "tools" in PrecisionScan, is an option to reset the output resolution (to any of a set of predefined values) -- see (a) above. If you choose to magnify an image (i.e., to scan at higher than 72 dpi), then pay careful attention to the size of the image, as shown at the bottom of the screen -- see (b) above. The software may alert you if it thinks you are scanning at higher resolution that may be needed for the picture. Pay attention, since your images should each be under 800K when scanned -- even in 24 bit RGB!

r67.gif (7394 bytes)
67 dpi		 r127.gif (25084 bytes)
127 dpi		 r150.gif (33659 bytes)
150 dpi		 r183.gif (50922 bytes)
183 dpi
75 dpi
r75.gif (9341 bytes)		 100 dpi
r100.gif (15669 bytes)		 137 dpi
r137.gif (27737 bytes)
 200 dpi
r200.gif (57464 bytes)

Half-toning is often used during monochromatic printing to produce the appearance of grey. At ordinary resolution (75 dpi) the half-tone pattern is barely noticeable. At 200 dpi and 600 dpi (below) the halftoning pattern is quite visible. At intermediate resolutions (127 through 150) we may see moiré patterns introduced through the apparent interference of the periodicity of the printed dots (those used in the image here are printed with a frequency of about 120 dots per inch) with the scanner resolution.

600 dpi
magnify.gif (155725 bytes)
600 dpi

To eliminate such interference patterns, four choices are available; none is necessarily easy to make:

  1. reduce the resolution to a value below that of the halftone frequency.
  2. increase the resolution so that the half-tone dots are very visible
  3. find an intermediate value which is better synchronized (for extra credit do some guided research on what works best for various different patterns)
  4. scan now and smush later -- that is use a photo editor to blur the dots into continuous tones.

Images scanned from the Rocket, newspaper published by department of Communications, Slippery Rock University, February, 2000.