Darkroom guide · film positive halftone maker
How to make halftone film positives that expose clean
A film positive is a promise: wherever it is black, UV light does not reach the emulsion. Halftones break that promise at the edges first. This guide covers the file, the printer, the registration and the test exposure.
The file
Why grey edges wash out
Most halftone exports are anti-aliased: each dot has a ring of grey pixels so it looks smooth on screen.
On film, a grey pixel is a partial blocker. Part of the UV gets through, the emulsion under the dot edge half-hardens, and the dot shrinks or closes during washout. At 600 DPI one pixel is 42 μm, so a one-pixel grey fringe around a 146 μm highlight dot on 156 mesh can eat most of its 50 μm margin over the 96 μm wash-out limit.
A strict 1-bit file has only two values: opaque black and fully clear. The Pro PNG films are indexed 1-bit images with a transparent index, so a count of grey pixels is always 0, and the PDF places the plate as a 1-bit image mask at 600 DPI.
The page
How the Pro PDF page is laid out
One page per plate, at the real print size plus margins for marks and a test strip.
- Page size
- Print width + 2 in, print height + 2.5 in. An 11 × 13.75 in plate sits on a 13 × 16.25 in page.
- Crosshairs
- Six double-ring crosshairs: four corners plus the top and bottom midpoints, in the same place on every page.
- Header
- Plate name, mesh (for example 156T), LPI, angle, dot shape, emulsion, design name and “Print at 100% / Actual size — do not scale”.
- Strip
- A 10-step exposure strip under the plate at the plate's own LPI and angle.
The printer
Printing film on an inkjet
The file is only as good as the black the printer lays down.
| Check | Setting | Why |
|---|---|---|
| Printer | Pigment inkjet with a dense black, or a dedicated film RIP printer | Dye black lets UV through |
| Film | Waterproof inkjet film, coated side up | Uncoated film beads the ink and spreads dots |
| Ink density | Highest quality, black-only or heaviest black setting | The black must block UV at the dot edge |
| Scale | 100% / actual size, fit-to-page off | A 98% print shifts registration by 0.2 in on a 10 in film |
| Emulsion side | Printed side against the screen's emulsion (right-reading through the mesh) | No gap for light to creep under the dots |
| Registration | Stack films on the crosshairs before taping to the screens | Every plate shares the same mark positions |
Measure one inch on the printed ruler before you burn anything. If it is not exactly one inch, the driver is scaling, and every plate on that print will be off by the same ratio.
The exposure
Reading the 10-step strip
The strip shows tints from 10% to 100% at your LPI over positive and negative fine lines.
Expose a test screen in steps: cover the strip with a card and pull it back one patch at a time, so patch 1 gets the shortest exposure and patch 10 the longest. After washout, the best step is the shortest exposure where the negative lines are still crisp and the 10% dots are all there.
If the 10% dots are missing on every step, the dots are too small for the mesh: raise the highlight clip or lower the LPI in the halftone generator. If the negative lines fill in at every step, the film black is too thin. The mesh to LPI calculator shows the dot size each mesh can hold.
Troubleshooting
Common film faults and their fixes
Most bad screens trace back to one of these, and each has a quick check.
- Highlights vanish
- Dots under 1.5 × thread wash out. Raise the highlight clip from 5% to 8%, or drop the LPI by 5, and re-check the preflight.
- Shadows plug
- Too much dot gain on press. Raise the compensation from 15% toward 20% so the film dots in the 70–90% range shrink.
- Films do not line up
- Two plates printed at different scales. Re-print both at 100% and compare the inch rulers before taping.
- Moiré on the shirt
- Rows of dots beating against the mesh. Use 22.5° for single-color elliptical dots and keep CMYK at 15/75/0/45°.
Keep the settings TXT with each film set; it records mesh, LPI, angle, dot shape, dot gain and clips, so the next burn starts from what worked.
Before you burn
Pre-exposure checklist
Five things that cost nothing to check and a screen to get wrong.
- Preflight is green. WASH-OUT SAFE and 0 risk zones in the studio.
- Ruler measures true. One printed inch equals one inch.
- Black is opaque. Hold the film to a window; no light through solids.
- Films register. All plates line up on the crosshairs on a light table.
- Strip read. You know your exposure time for this mesh and emulsion.