What is a moiré pattern in printing?
A moiré is an unwanted wavy or checkered pattern that appears when two fine, regular patterns overlap at slightly different angles or spacing, such as halftone screens or the dots of a scanned printed picture.
What it means for your file
In print it has three usual sources: scanning or photographing something already printed, whose halftone dots clash with the new screen; fine stripes and textures in a photo, such as fabric, mesh or screens; and the screens of the different inks set at the wrong angles. Printers set the screen angles, so designers mostly meet moiré in scans and product photos.
The dots that cause it are explained under halftone and LPI.
An example
Scanning a picture from a magazine copies its halftone dots. When that scan is printed, the old dots meet a new halftone screen and a ripple appears across the image. Using the scanner’s descreen setting, or blurring the scan slightly and then sharpening it, softens the old dots before the image is screened again.
A common mistake
Sending a product photo of a finely striped shirt or a mesh grille without checking it at print size. The pattern in the photo can clash with the halftone screen and print as wavy bands that weren’t there before. Ask for a hard proof, and if a moiré appears, soften the texture slightly or change the image size; some printers can also use stochastic (FM) screening, which has no regular grid to clash with.
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