Halftone Screen Angle Calculator - CMYK Screen Angles and Moire Risk

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Why screen angles exist

Colour printing lays down four halftone dot patterns, one per ink: cyan, magenta, yellow and black. If all four patterns used the same angle, the dots would stack on top of each other and the picture would lose colour. Printers therefore rotate each pattern to a different angle, so the dots form a small rosette pattern that the eye reads as smooth colour. The classic set is cyan at 15 degrees, magenta at 75 degrees, yellow at 0 degrees and black at 45 degrees. Black sits at 45 because the eye notices detail most at that angle, and black carries the detail.

What moire is

Moire is the wavy, shimmering pattern that appears when two regular patterns sit at angles too close together. In print it shows up as visible ripples, especially in flat tints and skin tones, and it can ruin an otherwise good job. The risk grows when two screen angles are less than about 15 degrees apart. Yellow is the exception: it is so light that the eye barely sees its pattern, so yellow can sit at 0 degrees even though it is close to other angles. That is why the classic set works despite yellow sitting near cyan.

How to use this calculator

Enter the four angles you plan to use, one per ink, each between 0 and 90 degrees. The tool finds the smallest separation between the cyan, magenta and black angles, ignoring yellow for the reason above, and rates the moire risk as low, medium or high. A low rating means the set follows standard practice. A medium rating means two angles are closer than ideal and deserve a test proof. A high rating means two angles are within 15 degrees and will very likely show moire.

The standard set and its variants

The most used set is 15, 75, 0, 45 for cyan, magenta, yellow and black. Some shops rotate the whole set by a fixed amount to suit their press, for example 105, 165, 90, 135, which keeps the same separations. What matters is the spacing between angles, not the absolute numbers. Keep cyan and magenta far apart, keep black at the most visible angle, and let yellow sit wherever it is convenient. If you add spot colours, give each one an angle at least 30 degrees away from the nearest process angle.

When custom angles make sense

Custom angles are sometimes needed for special work: printing on textured stock, matching an existing job, or screening a pattern that already contains a strong direction, such as fabric or wood grain. A pattern with a strong direction can beat against a screen angle and create moire even with a standard set, so rotating the screens away from the pattern’s direction can fix it. In these cases, change one angle at a time and proof the result, because every change moves the risk somewhere else.

Proofing for moire

Moire is hard to predict from numbers alone, so proof flat tints at the exact angles and LPI of the real job. A proof at the wrong LPI can hide moire that appears on press, and a proof at the wrong angles proves nothing at all. Look at mid-tones and skin tones first, since moire shows there before it shows in shadows or highlights. If you see ripples, increase the separation between the two closest angles and proof again.

Common mistakes

The most common mistake is setting two inks to the same angle by accident, often after copying settings between jobs. Another is treating yellow like the other inks and distorting the whole set to move it. A third is skipping the proof because the numbers looked fine. Use the standard set unless you have a reason not to, keep separations wide, and proof the real angles before a long run.

Halftone Screen Angle Calculator – CMYK Screen Angles and Moire Risk