How to Stop Halftone Moiré on Riso: The Angles That Actually Print

Why does halftone moiré happen on a riso print?

Two overlapping halftone screens sitting too close together in angle — that's where moiré on a riso print comes from. Each ink pass lays down its own regular grid of dots. Overlap two of those grids at a near-identical angle and they interfere, producing a second, unintended pattern: bands or a coarse crosshatch that has nothing to do with the file you sent to the drum.

The spacing between those interference bands follows a fixed relationship. For two screens with dot spacing p rotated by angle a relative to each other, the band spacing works out to D = p / (2 sin(a/2)). Here's the part that trips people up: as a shrinks toward zero, D grows toward infinity. In plain terms, screens that are nearly parallel produce the widest, most visible moiré bands — not the faintest ones, like you'd expect. Widen the angle gap between two ink passes and the moiré band spacing shrinks down to something the eye stops registering as a separate pattern.

That's also why standard halftone practice defines screen angle as a fixed rotation measured clockwise from 9 o'clock, and why every multi-ink separation — riso, offset, screen print — gives each ink its own angle instead of letting two colors share one. Shared or near-shared angles: still the single most common cause of a moiré complaint on a finished print.

One caveat before the angle table. Separating screens by angle reduces the chance of moiré. It doesn't guarantee a clean print on every image. Moiré strength also depends on the frequency content already baked into your source file — a busy, fine-detail photo can show a faint pattern at angles that print perfectly clean on a flatter image. Treat the table below as the starting point, not the whole fix.

What screen angles actually prevent moiré on riso?

Ordered by ink brightness, the darkest and highest-contrast ink gets the steepest angle and the widest gap from its neighbors. The brightest ink absorbs whatever's left over — errors in a light color are the least visible to begin with, so it can afford to.

Two things worth checking against that table before a file goes to the drum:

• Every pair of angles you're actually using needs at least a 15° gap, ideally 30°. A four-ink job that only prints two of its inks on a given piece still needs two angles from the table that are far apart. Don't collapse 0° and 15° onto the same pass just because yellow got skipped that run.
• Single-ink jobs aren't exempt. One color still needs an angle assignment — 45°, per the table — because an unrotated regular grid can read as a visible screen pattern on its own, no second ink required.

What LPI should you use for a riso halftone?

Screen frequency, lines per inch, sets how fine the halftone dot grid is. It interacts with angle choice too: a screen too fine for the drum to resolve shows artifacts no matter how well the angles are separated.

• 38–60 lpi — coarse. Suits flat gradients and posters meant to be seen from a distance; too chunky for detailed photographic work.
• 90–105 lpi — the safe universal default. Performs well across most source types without a test print.
• 120 lpi — better for flat, area-based art than photographic gradients.
• Above 120 lpi — reserved for area images only, and it needs a physical test print before you commit an edition. The riso drum's resolution ceiling — around 600 dpi — can't losslessly resolve a screen that fine, and photographic gradients are the first thing to show banding once you push past it.

Dialing in a new setting rather than reusing a known-good one? Adjust in 5 lpi increments and reprint a small test area. Don't jump straight from 60 to 120 lpi hoping one of them works.

When does a grained screen beat a regular one?

A stochastic, or grained, dither pattern sidesteps the angle problem entirely. There's no periodic grid for a second ink's screen to beat against — moiré is fundamentally a collision between two regular patterns, and a grained pattern isn't regular. It's the same principle behind ordered dithering vs error diffusion on the digital side: a regular matrix, Bayer or a standard AM halftone screen, can produce a beat pattern against anything else regular in the image or the print process. A diffused or grained pattern generally can't.

The tradeoff is real, though. A grained screen gives up the predictable, controllable dot placement that makes a regular AM screen easy to angle and register. And it renders a different tonal range than something like Atkinson, which was built for punchy, graphic contrast rather than moiré-proofing a multi-ink separation. For most riso production work, a well-angled regular screen off the table above is still the more predictable choice. Reach for a grained pattern when the job specifically calls for that texture — not as a default fix for moiré.

A worked failure case

Take one source image, the fixed test subject used across these articles, and run it through two angle pairs.

At 0° and 15°, a 15° gap pulled from the low end of the table, the print shows a visible low-frequency band running diagonally across the midtones — exactly what the D = p / (2 sin(a/2)) relationship predicts for a narrow angle gap. It shows up worst in flat or gradient-heavy regions, where there's no fine image detail around to mask the interference.

Swap in 45° and 75°, a 30° gap, the standard two-ink pair from the table, and the same source prints with no visible banding. The dots interlock into small rosettes the eye reads as continuous tone instead of a repeating pattern. Same file. Same drum. Same ink coverage. The only thing that changed is the angle gap.

Frequently Asked Questions

What screen angle should I use for a two-color riso print?

Use 45 degrees and 75 degrees only, and drop the brightest ink's angle rather than crowding a third value into a two-drum pass. That 30-degree gap is wide enough to keep the two dot grids from beating against each other and producing a visible moire band.

Why does my riso print moire even though I set different angles?

Angle separation is necessary but not sufficient. The same angle set can print clean on one image and show visible moire on another, because moire also depends on the source image's own frequency content, not only the screen angles. Always test-print the actual file, not a placeholder, before running the full edition.

What LPI should I start at for a riso halftone?

90 to 105 lines per inch is the safe default for most source images on a riso drum. Go coarser, 38 to 60 lpi, for flat gradients or posters meant to be seen from a distance, and don't push past 120 lpi without a physical test print — the drum's 600 dpi resolution can't losslessly resolve finer screens, especially on photographic gradients.

The bottom line

Most halftone moiré on riso traces back to two screens sharing an angle that's too close — not a bad file, not a worn drum. Pull your angles from the table above: 45°/75° for two inks, 15°/75°/45° for three, the full 0°/15°/45°/75° set for four. Keep LPI at 90–105 unless you've got a specific reason to go coarser or finer, and always run one test print on the actual file before committing a full edition. Run a source image through kott's halftone module to preview an angle and LPI combination before it ever touches a drum.