It’s something that printers are always trying to avoid, whether the printer in question is an inkjet machine or a more traditional screenprinter. Eliminating it altogether is a bit of a dark art, but with a bit of background knowledge, not impossible.
Firstly, what do we mean by “moiré”?
The Wikipedia definition, although accurate, can be pretty meaningless when considering it in the context of printing:
Moiré patterns are large-scale interference patterns that can be produced when an opaque ruled pattern with transparent gaps is overlaid on another similar patternWikipedia
… so basically, it is a pattern that occurs when two or more layers of similar patterns are overlaid.
One example of this would be if you’ve ever tried to film a TV screen or monitor with a video camera. You will tend to see pattern on the resulting footage which is caused by the grid of light sensors in your camera clashing with the pixels or lines on the TV screen.
Four colour printing
In the case of CMYK film separations for example, it could occur between the halftone dot patterns of any (or all) of the ink layers when printed on top of each other. In this case, the effects of the moiré clash can be vastly reduced by altering the angles that each halftone dot pattern is produced at.
Decades of experimentation with halftone angles by printers around the world, has found that ideally the dot angles should be at least 30° apart. Which is why we often run a CMYK colour set with the angles run at 15° for the Cyan, 45° for the black, and 75° for Magenta.
The Yellow however, is a problem, because if we add 30° to the Magenta angle we will end up with 105° – which is actually the same angle as the Cyan (remember, because the halftone dots are formed according to a strict grid, after 90° the angles are repeated). So a compromise has to be reached, meaning that the Yellow halftone is generated at an angle of 90°, which is 15° away from the Cyan and Magenta halftones. Believe it or not, this does actually create a moiré clash, but because the yellow ink is so light, it is generally regarded to be acceptable.
Of course, screenprinters have an additional layer to deal with when considering unwanted moiré that is not present with any other form of printing, even if only one colour is being printed – and that is the weave pattern in the mesh of their screens. The mesh can cause an angle clash with the 90° Yellow film, but also a frequency clash with any of the other colours. By that I mean the dots in the halftone creating a pattern as their alignment coincides with the strands of the mesh used on the screen.
We try and alleviate the moiré caused by the clash between halftone and mesh by suggesting an optimum halftone ruling for each mesh used on a screen. For example, a T-shirt printer using a relatively coarse mesh of 65 strands per cm would be recommended a halftone of 37 lpi (lines per inch*), while a fine art printer using a fine mesh of 150 strands per cm would need a much finer halftone of 75 lpi.
*we use the term “lines per inch (lpi)” when referring to halftone frequency to avoid confusion with the term “dots per inch (dpi)” which is more commonly used when referring to image resolution, even though most halftones are in fact dots.
Since digital output has become the norm (as opposed to the traditional analogue methods of creating halftone using screens and cameras), there is another type of moiré that we often have to consider, and that is the clash between the halftone dot and the resolution of the output device.
If you’ve ever tried to create your own halftone dot in an app like Photoshop, and then tried to print it out on your desktop inkjet or laser printer, you will no doubt been disappointed with the moiré pattern that occurs. This is because even high quality printers operate at a maximum resolution of around 720 dpi, which frankly is not high enough to output a halftone any greater than 35 lpi without unsightly patterns appearing across the image.
It is for this reason that we still use a high resolution imagesetter. Operating at over 4000 dpi, it exposes the photographic film by directing a finely focused dot of laser light, which means it can produce halftone dots of up to 300 lpi, with each dot being both perfectly formed and dense enough to prevent any light from passing through – both characteristics are essential when producing a quality stencil for screenprinting.
When we output films, we never try and create the halftone in Photoshop. We print them as either greyscale or CMYK files directly to the RIP on our imagesetter. It is the RIP that converts the printed document into the required separations and applies the halftone dots at the requested frequency and angles.