Why Screen Tone Moiré Happens: Angle, Line Count, and Reduced Printing

Start with the monochrome overlay rule

Two screens form moiré when their repeated patterns interfere. For monochrome artwork, use the same frequency and the same angle on overlapping tone layers. If the dots coincide and the overlay fails to look darker, move the dot pattern slightly with the tone-pattern movement controls. Do not rotate one layer to 30° or 45° relative to the other as a general remedy. Those differences are sometimes discussed for separate process-color plates, which is a different problem from stacking black screen tones.

For example, both layers can use 60 LPI at 45°. Their absolute angle is 45°, but their angle difference is 0°. Entering the difference into CSP’s Angle field would confuse these two quantities. Check the frequency, angle, dot type and output scale on each layer before changing density.

What the angle formula can explain

For two ideal equal-frequency square grids with a nonzero angle difference θ, a fundamental beat period is approximately d / (2 × sin(θ / 2)), where d is dot spacing. Fold a square-grid difference into 0°–45° first. At 60 LPI, d is about 0.423mm: the estimate is 4.85mm at 5°, 1.62mm at 15°, and 0.55mm at 45°. These are spacing estimates, not pass/fail limits for print.

At exactly 0° or an equivalent 90°, the grids share the same orientation. The formula’s division by zero does not mean a visible infinitely large wave: aligned grids can combine into another regular pattern. Density and positional offset still change the result. A shorter calculated period at 45° does not make that angle a safe monochrome overlay recommendation.

Frequency mismatches and resampling

Matching angles is not enough when the frequencies differ. For aligned 60 and 65 LPI screens, a one-dimensional beat estimate is 25.4 / |65 − 60| = 5.08mm. Use the same frequency for layers that overlap. Different frequencies in separate, non-overlapping areas do not automatically create interference.

Reducing a raster image whose dots are already fixed also reduces dot spacing. A 60 LPI B4 image printed at B5 width becomes about 84.7 LPI. CSP can instead generate its tone effect during export: the Comic export setting and the choice between following output scale and following layer settings change the outcome. Check that choice before applying a reduction formula. Rescreening existing dots can also produce artifacts, so follow the printer’s specification for monochrome versus grayscale data.

A practical check in CSP

1. Select the overlapping tone layers and match Frequency and Angle in Layer Property. For a 60 LPI, 45° example, both layers use those values.
2. Adjust the dot position if coincident dots do not give the intended darkness. Keep the angle the same.
3. Export a copy at the printer’s required dimensions and resolution. Check the tone output settings, rather than resizing an already exported dot image repeatedly.
4. Open the exported image at 100% to inspect dot shape, and inspect a calibrated print-size view or a proof for density. There is no universal 100 LPI safety boundary: paper, density, resolution and printing method all matter.

Use the browser preview to understand interference

The overlay preview intentionally starts with a 15° difference to illustrate interference. Compare that with 0° to understand the effect; do not transfer the demonstration offset into the artwork as a recommended setting. This tool does not reproduce CSP’s dot-position controls or the print shop’s screening process.

A monitor may have only one or two display pixels per dot interval at physical print size. That sampling can create a false moiré or remove detail. If a pattern changes when you zoom, distinguish the display artifact from the exported pixel data. Calibration aligns physical length only; it does not reproduce a 600 dpi press on a lower-density display.

Sources and verification conditions

Sources checked 2026-09-06. Existing screenshots are from CSP 5.0.4; UI labels can vary by version.

• CSP support: Moiré in screen tones (Japanese)
• CSP manual: Exporting files (Japanese)