Moire patterns in commercial printing
When using Amplitude Modulated (AM) halftone algorithms, moiré patterns can occur if the dots or matrices containing halftones are not positioned at the correct angle relative to other dots. This typically manifests as an undesirable wavy texture, creating a sense of double vision and color shifting. Developers of halftone structures strive to define dot angles in a way that prevents moiré formation.
This wavy texture can be caused by improper plate placement on the printing press, errors in lithography when determining halftone angles, or any other issue affecting angles. While technological advancements have reduced the likelihood of moiré, it remains a concern that should be carefully monitored during preflight and quality control processes.
In general terms, moiré can be defined as a disturbance in the arrangement of angles within halftones. However, if these angle changes are correctly aligned, they can create a flower-like pattern or rosette texture, indicating high print quality. Wavy textures occur in AM halftones, and one solution to control and eliminate them is to use Frequency Modulated (FM) halftones.
Moiré in Offset and Flexography
One reason for the formation of moiré patterns is the reduction in the angular distance between two different colors. In AM halftones, if the angle between two colors is less than 30 degrees, there is a high probability of moiré. This is particularly likely with the yellow color in relation to the other three colors (CMYK), but due to yellow's brightness, the wavy texture is less noticeable and less of a concern.
In flexographic printing, in addition to the aforementioned factors, the angle of the cells on the anilox roll, which transfers ink, must also be considered. For this reason, halftone angles in flexography differ from offset by 7.5 degrees to avoid interference with the anilox and reduce the likelihood of moiré.
It's important to note that the shape of the halftone dot can also affect moiré reduction. For example, circular halftones, which have the same diameter in width and height, are defined with screen angles of 0 to 90 degrees for the four CMYK colors. This reduces the probability of an angle less than 30 degrees between yellow and other colors. However, elliptical halftones, due to their different diameters, can be easier to control for moiré.
Solutions to Reduce Moiré
1. Use FM Halftones: Instead of AM halftones, consider using FM or stochastic halftones. If the print resolution is between 150 and 200 LPI, a corresponding value of 35 microns can be used to define the dot size in the FM algorithm.
2. Image Quality: Ensure that the images used in the final file are of high quality. Repeatedly resizing images in vector programs can degrade resolution and increase the risk of moiré. After initial design approval, all images should be cropped to their actual size in Photoshop and then reimported into other software.
3. PDF Output: Avoid enabling the Resampling or Compression option when outputting a PDF file. This is because PDF output tends to automatically compress all images to reduce file size and define the format as JPG, which can increase the likelihood of moiré.
4. Color Separation: For fine textures like the surface of a tie or a car body with very thin lines, consider using the Under Color Removal (UCR) color separation model in the color profile definition.
