A few scanning tips


Scanning Images in

Scanning images from printed material like magazines has a very special problem (not to mention copyrights, which is a serious consideration too). You will discover a murky herringbone or crosshatched or dotted pattern in your scanned images from printed material, one example is shown below.


Moire patterns

This interference is called a moiré pattern (pronounced more-ay). In a scanned image, Moiré patterns are caused by interference between two sets of fine pattern grids, the scanner samples and the halftone screen in the original image. Any scanner will do this, it's a simple fact of life. Greater scanning resolution can eliminate it (at least 300 dpi, or better 600 dpi).

Any image printed on a printing press (like a book, magazine, newspaper, postcard, calendar, etc.) is printed with halftone screen patterns. The printed image is composed of a pattern of dots. A strong magnifying glass will show them. The halftone dots are printed entirely in black if a B&W image, or there are four screens in each of the three primary colors plus black (CMYK) if a Color image. These fine dots cause optical problems in a scanned image because the scanned image is also composed of fine dots.

The two patterns of dots, the printed magazine's 133 or 150 lpi screened pattern, and the 300 or 600 dpi scanner CCD cells, combine into maximums or minimums every several pixels in the image, depending on the spacing of the dots. It affects the overall light intensity in periodic patterns that become very visible. The pattern is named Moiré.

The scan is possible however.

© 1998 Barrie Smith, Australian Digital Camera magazine.   Contact miffy@ozemail.com.au

A scan of an image printed in the review of the Agfa 1680 camera
in Issue 10 of Australian Digital Camera magazine.

The CCD in this camera is 1280x960 pixels, but the camera can output an interpolated 1600x1200 image. That 1600x1200 pixel image was scaled by the magazine to 300 dpi for creating the 150 lpi screen, which printed at about 5.6 x 4.1 inches in size. The upper right top border shows the edge of another inset photo.

This image was scanned normally from the magazine, except due to Moiré, 300 dpi and the Scanwizard Descreen filter was used. Then it was resampled to 33% size (effectively 100 dpi size). The last operation was sharpening with the Unsharp Mask filter.


Copyright © 1997-2010 by Wayne Fulton - All rights are reserved.

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