Halftones for screen printing: LPI, angle and mesh
Starting settings for screen printing halftones, each from a named source: lines per inch for your mesh count, dot shape, angle, and how to make the film.
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A screen stencil is either open or blocked, so a screen print can only show a fade, a shadow or a photograph as halftone dots: big dots where the ink should look heavy, small ones where it should look light. Three choices decide whether those dots survive the mesh: how many lines of dots per inch (LPI), the angle of the dot grid, and the mesh count of the screen.
Suppliers and press makers publish starting points, and this page gathers them with their sources. They are starting points only. Emulsion, exposure, ink, squeegee and fabric all move the result, so the last section is a test to run on your own set-up.
The numbers at a glance
| Setting | Starting point | Who says so |
|---|---|---|
| LPI from mesh count | Mesh count ÷ 5. With a well-controlled darkroom, ÷ 4 | ScreenPrinting.com |
| LPI in general | 45 or 55 | ScreenPrinting.com |
| LPI in general | 45 for most images, up to 65 for photorealistic ones | Anatol |
| LPI for four-colour process | 20 to 40 lines per cm, which is 51 to 102 LPI | ISO 12647-5:2001 |
| Mesh for halftones | 230 to 280, thin thread | Anatol |
| Mesh for the finest halftones | 305 | ScreenPrinting.com |
| Angle | 22.5° | ScreenPrinting.com |
| Angle | 25° | Anatol, and ScreenPrinting.com’s Photoshop guide |
| Dot shape | Elliptical | Anatol |
| Dot shape | Round is “the most popular” | ScreenPrinting.com |
| Smallest and largest dot | At least 6%, no more than 97%; many printers stop in the upper 80s | Anatol |
| Dot gain allowance | “Screen printers typically choose 30% dot gain” | ScreenPrinting.com |
| Film output | 600 DPI “is pretty normal in screen printing” | ScreenPrinting.com |
The sources: ScreenPrinting.com’s crash course in halftones, its Photoshop halftone guide, its mesh count guide and its post on dot gain; the press maker Anatol’s guide to printing halftones; and ISO 12647-5:2001, the process standard for screen printing, read in the identical text published by the Bureau of Indian Standards as IS 15963 (Part 5).
Lines per inch and mesh count
The dots have to hold on the mesh. ScreenPrinting.com’s mesh guide puts it this way: “Fine halftone dots need very high mesh counts in order to hold and expose properly.” Its rule ties the two together: “Say you want to use a screen with a 160 mesh count. Take that number and divide it by 5. So for a 160 mesh count screen, the ideal LPI would be 32.” Dividing by 4 gives 40 LPI on the same screen, for shops with “a dialed-in darkroom”: “The dot size will be smaller and it will be harder to keep all that detail.”
| Mesh count | LPI at ÷ 5 | LPI at ÷ 4 |
|---|---|---|
| 110 | 22 | 27.5 |
| 156 | 31.2 | 39 |
| 200 | 40 | 50 |
| 230 | 46 | 57.5 |
| 280 | 56 | 70 |
| 305 | 61 | 76.25 |
Read the other way, the common 45 and 55 LPI want a mesh of about 230 and 280 at the safer ratio, which agrees with the range Anatol gives.
It helps to see how small the dots are. One halftone cell is 25.4 ÷ LPI millimetres wide, and a round dot covering a share c of its cell has a diameter of the cell width times the square root of 4c ÷ π.
| LPI | Cell width | 6% dot | 10% dot |
|---|---|---|---|
| 45 | 0.56 mm | 0.16 mm | 0.20 mm |
| 55 | 0.46 mm | 0.13 mm | 0.16 mm |
| 65 | 0.39 mm | 0.11 mm | 0.14 mm |
Taking mesh count as threads along one inch, the threads of a 230 mesh are 25.4 ÷ 230 = 0.11 mm apart. (That reading is an assumption here: ScreenPrinting.com’s guide says only that a 110 mesh has “110 threads crossing per square inch”.) On that reading, at 65 LPI a 6% dot is about as wide as one thread spacing. Highlights are the first thing to go, and Anatol’s floor of 6% reflects that: “anything smaller becomes difficult to print accurately”.
Angle
The angle is about the mesh. The threads are a grid, the dots are a grid, and two grids laid over each other can produce moiré, a pattern that is in neither of them. CMYK halftone shows how it arises between two grids of dots.
ScreenPrinting.com explains the choice: dots “at right angles on the screen (0°, 90°, 180°, and 270°) will line up with the threads of the mesh”, and at 45° they “hit the knuckles of the mesh diagonally”, so “a good rule of thumb is to use 22.5° angles”. Anatol’s version: “a good starting point for those just beginning to print halftones is 25 degrees”.
The ISO standard makes the same point with a different number. With the mesh at 0° and 90° to the frame, “no colour should align with mesh warp or weft, or diagonal. In order to achieve this, one colour should be rotated by 7,5° with respect to the mesh.” For four-colour work it keeps cyan, magenta and black 30° apart, with yellow 15° from another colour, and its example figure for dots without a long axis, such as round ones, shows black at 37.5°, magenta at 67.5° and yellow at 82.5°.
If a pattern shows on the screen after exposure, before any ink, only the dots and the mesh are involved: try another of the angles above, or another mesh count.
Dot shape and the ends of the scale
Round and elliptical dots are both in use, as the table shows. The difference is in how the dots join as they grow. By geometry, round dots on a square grid touch all four neighbours at once, at 78.5% coverage. Elliptical dots join in two stages, and ISO 12647-5 sets limits for them: “the first link up shall occur no lower than at 35 % tone value and the second linkup no higher than at 65 % tone value.”
At the ends of the scale, dots fail. Anatol’s advice is to keep tones between 6% and 97%, and it notes that “many screen printers will cut the maximum percentile in the upper 80s, as dots that are any larger can merge to create a solid image due to dot gain”. ScreenPrinting.com’s dot gain post suggests testing whether your darkroom “can expose details of a 10% dot” and adds that “most shops won’t be able to get a 2% or 3% dot”. The ISO standard notes that the range of tones that prints is narrower with a coarser mesh, and “narrows considerably” as the screen ruling goes from 20 to 40 lines per centimetre.
Dot gain is the reason prints come out darker than the film. ScreenPrinting.com says it “occurs whenever a squeegee is pulled across a screen, causing pressure”, and Anatol that ink spreads when it meets the fabric, “creating a larger dot than the one on the image or on the stencil”. The answer is in the artwork: ScreenPrinting.com’s dot gain post adjusts the image with Photoshop’s Curves before the halftone is made; its example changes a 90% value to 75%.
Making the film in Photoshop
Screening software (a RIP) does this at print time: in ScreenPrinting.com’s words, “All halftone conversions will be done automatically in RIP software when you print the films”. Without a RIP, Photoshop’s Bitmap mode makes the dots. The steps below are the ones in ScreenPrinting.com’s Photoshop guide, dated December 2022.
- Do one ink colour at a time. Each screen needs its own film.
- Choose Image > Mode > Grayscale.
- Adjust the tones now with Curves. The guide warns that once the design is converted “you won’t be able to make any changes to the image”. Allow for dot gain, and check that the lightest and darkest tones are inside the range your screens can hold.
- Choose Image > Mode > Bitmap, and agree to flatten the layers.
- Enter the output resolution, for example 600.
- Choose Halftone Screen as the method, and click OK.
- Enter the frequency in lines per inch, the dot shape and the angle, and click OK.
- Print the film.
At 600 pixels per inch and 45 LPI each cell is 13 pixels wide. At 65 LPI it is 9. The guide’s rule is that “the higher resolution you use for the conversion, the smoother the dot shape will be”, so if the dots look ragged, the output resolution is too low for the frequency.
Vector dots for bold halftones
Fine photographic halftones are a job for a RIP or Bitmap mode. Big, deliberate dots are different: a fade behind a logo, a comic-style shadow, a radial burst. Those can be drawn as vector circles, which stay sharp at any size and can be set in a spot colour like any other shape.
The free halftone patterns are built for this. Every dot is a closed outline drawn from a circle, none touch, and each pattern’s page lists the smallest dot at seven sizes so that you can scale the pattern until the smallest dot is one your mesh will hold. Vector halftone covers making your own. The dot gradient and the radial fade are the usual starting places.
Test before a job
Numbers from someone else’s shop are only a first guess. One film settles it for yours.
- Make a strip of flat tints from 5% to 95% in steps of 5, each a few centimetres wide.
- Screen it at the LPI and angle you plan to use, and again one step coarser and one step finer.
- Expose it on the mesh you plan to use, wash out, and hold the screen to the light. Note the lightest tint whose dots are all there and the darkest tint whose holes are all open.
- Print it on the real fabric with the real ink. Note where the print stops looking different from the patch next to it.
- Keep your artwork inside the range that survived, and pick the finest LPI that printed cleanly.
When something goes wrong
| What you see | Likely cause | What to try |
|---|---|---|
| Bands or checks across a tint | Moiré between dots and mesh | Change the angle to 22.5° or 25°, or change the mesh count |
| Highlights missing | Dots too small for the stencil or mesh | Raise the lightest tone, lower the LPI, or use a higher mesh |
| Shadows printing solid | Dot gain closing the holes | Cap the darkest tone lower and lighten the mid-tones |
| Ragged dots on the film | Output resolution too low | Raise it, or lower the LPI |