Start with the mesh, not the exposure
Most washout problems get blamed on exposure time. Often the real cause is arithmetic that was lost before the screen was ever coated.
A dot has to bridge the woven threads of the mesh to survive. If a 5% dot at your chosen line frequency works out smaller than about two thread pitches, there is nothing for the hardened emulsion to grip and the water takes it away. No exposure time fixes that, because the problem is geometry.
| Mesh | Thread pitch | Smallest dot that holds | Safe line frequency |
|---|---|---|---|
| 110 | 0.23 mm (0.009″) | 0.46 mm (15%) | 24 LPI |
| 156 | 0.16 mm (0.0063″) | 0.28 mm (12%) | 35 LPI |
| 230 | 0.11 mm (0.0043″) | 0.19 mm (10%) | 48 LPI |
Thread pitch is centre-to-centre spacing (25.4 mm ÷ mesh count); the actual opening is smaller because the threads themselves take up roughly 35 to 45% of it. The dot still has to bridge about two thread pitches — roughly three openings — which is where the smallest-dot column comes from.
If your highlights vanish and your numbers sit outside this table, change the numbers before you change anything else.
The four causes, in the order worth checking
1. Line frequency too high for the mesh
Covered above, and by far the most common. Symptom: the lightest tones are missing entirely and evenly across the whole print, on every screen, every time.
2. The film is not opaque
Hold the film up to a bright lamp. If light passes through the black, UV passed through it too, and the emulsion under your dots partially hardened. Symptom: highlights are weak and edges look soft, but not uniformly — it is worse in areas printed with lighter ink coverage. Fix it in the printer driver: glossy photo media type, maximum quality, black ink only if available, colour management off.
3. Underexposure
Undercooked emulsion is soft everywhere, and the smallest structures are the first to go. Symptom: small dots gone, and the rest of the stencil feels slimy or breaks down during a long run. Run a proper exposure test rather than adjusting by feel — a step wedge tells you in one screen what guessing takes five to learn.
4. Overexposure, for the shadow end
Too much light spreads under the film edges and closes the small white gaps between shadow dots. Symptom: the dark end of your gradient prints as a flat solid, so 85% and 100% look identical.
Grey pixels in the film: the silent version of this problem
A film should be 1-bit: pure black or pure white, nothing between. If the file contains greys — because it was exported as a normal greyscale image, or because the dots were antialiased to look smooth on screen — those greys expose partially. The dot edge hardens halfway, which is exactly the condition that makes a dot fragile.
This is why smoothing a halftone "to make it look nicer" on screen actively damages it on press. Hard edges are the point.
Why good software clips your tonal range
Given the physics above, the sensible thing to do is stop pretending that 3% and 97% are printable. Clipping means the software rounds the unusable extremes to nothing and solid, so the tones that do print carry the image.
PrintFilmReady clips automatically per mesh preset, and says so in the interface rather than silently. A gradient will end at the clip point instead of fading to nothing — that is the honest representation of what your screen can hold.
One more source of trouble: the tone curve
Simple halftone filters have a habit of printing dark. Ask for 50% coverage and you get a dot that covers around 78% of the cell, because the naive round-dot maths does not grow area linearly. Everything muddies and your midtones vanish into the shadows.
The fix is tone linearisation: a lookup that maps requested coverage onto actual printed dot area. In PrintFilmReady this is measured accurate to 0.1 percentage points across the range, so 50% on screen is 50% of ink on the film. It is invisible when it works, and unmistakable when it is missing.
Want to try it on your own artwork?
PrintFilmReady separates your file into inks and screens each one into a print-ready film, in the browser. Free while in beta.
Diagnostic shortcut
- Highlights missing everywhere, consistently → frequency too high for the mesh.
- Highlights weak and edges soft → film not dense enough.
- Small dots gone and stencil breaking down → underexposed.
- Shadows printing as flat solid → overexposed, or no shadow clipping.
- Everything too dark overall → no tone linearisation in whatever produced the film.
Common questions
Why do my halftone highlight dots disappear during washout?
Usually because the dot is smaller than the thread pitch it needs to bridge, so it has no threads to hold onto. Check that your line frequency suits your mesh: roughly mesh count divided by 4.5. Other causes are a film that is not opaque enough and underexposure.
Why do the dark areas of my halftone print as solid?
The white gaps between shadow dots are too small for the mesh to keep open, so ink bridges across them. Overexposure makes it worse. Clipping the shadow end at around 88 to 90 percent avoids promising tones the screen cannot hold.
Should halftone dots be antialiased or smoothed?
No. Films must be 1-bit, pure black and white. Grey edge pixels expose the emulsion only partially, which is what makes dots fragile during washout.
What is tone linearisation in halftone screening?
A correction that maps requested coverage onto actual printed dot area. Without it, a naive round-dot screen prints 50 percent coverage as roughly 78 percent dot area, making everything too dark.
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