Deep Focus Blog Article: How to Minimize Moire Patterns in Scans

How to Minimize Moire Patterns in Scans

by Scarlet

Tue Aug 11, 2009 at 09:11 AM EST

Have you worked with scanned images that appear with an unsightly moiré pattern?

Here is a brief explanation why these patterns occur and a few tips on how you can try to reduce the moiré pattern while scanning and improve image quality after scanning.

Reprint processes have typical line screen frequencies, i.e. magazine prints typically have a line screen of 133 lpi, newspapers 65 to 80 lpi (lpi is the density of the lines of dots used to print a continuous-tone image). The lines are printed at a certain angle, so that the output print does not appear with a moiré or patterning. When scanning from a printed piece, the image already consists of a pattern of lines and the scanner superimposes this image with a grid of photosensitive elements in the scanner. The mismatch of the alignment of the photosensitive elements in the scanner with an angle of a line that already exists in the printed original causes the moiré patterning.

To reduce the moiré pattern use the “descreen” setting of your scanner. This setting uses interpolation during scanning to minimize the duplication of the same angle which causes the moiré effect. If your scanner does not have a descreen setting, try to rotate the original to reduce the moiré patterning. Experiment with the amount of rotation. You will see that the degree of rotation will cause a lighter or stronger moiré pattern depending of the angle of the photosensitive elements and the angle of the lines in your printed original. Try to scan the original at the highest resolution your scanner settings offer to minimize the moiré pattern.

After scanning, use your image editing software to realign the image. See whether your software has a filter named “noise” and select the option “median”. Here you can set the radius of the noise being added to the image. The bet ter the quality of your scan, the lower the radius can be you select to improve image quality. Experiment with this setting to achieve best results. If you still see a slight patterning, try and apply a small amount of “gaussian blur”. Now go into the setting where you can adjust image size and resample the image to the size you want by using the “bicubic” resampling option. After resizing the image, apply an “unsharp mask” filter to make the image look clearer again. Experiment with settings from 50% to 100% and adjust threshold and radius to achieve optimal clarity of the image.

Category: Tips