The dot gain of the imagesetters and CTPs can produce a moiré phenomenon, commonly called patterning on individual films/plates. This moiré is caused by the interference between the halftone raster (e.g. 170 lpi) and the device raster (e.g. 2400 dpi). When this moiré occurs, it is visible on individual colours (especially visible on films, in the dark areas, when viewed on a light table). Typically the cyan or magenta screens have lines or other patterns in the 80-95% shades.
To examine how this phenomenon occurs, we will consider the case of a typical imagesetter with a dot size of 20 microns at 2400 dpi. This dot size is twice as big as it should be (mathematically) for that resolution, and causes the dot distortion shown in the illustration below.
The first picture shows how the RIP makes halftone dots of 5 white pixels each (so mathematically their area is the same). Once this pattern is imaged, using the oversized laser (which may not be perfectly focused), the sharp white dots become blurred, and the black runs into the white. Depending on the actual shape (pixel structure) of the dot, the effect of this penetration is different – compact dots have less blurred areas (see the second picture). Imagesetter films will transform the blurred areas into black: the result is sharp dots of different sizes (see the third picture) – though initially the dots had the same size.
Unfortunately, for statistical reasons, the dots tend to be grouped in groups of bigger and smaller dots, and this causes the moiré effect. The dark areas are especially vulnerable, but on poor quality screen sets it can also appear in mid-tones.
As this problem does not occur on high quality print engines (such as drum imagesetters, where the dot is perfectly focused and is the right size), some RIPs, designed for such print engines, can afford to ignore this problem. However, on capstan devices, a good RIP is necessary – one that provides pattern free screens even with high dot gain.
StudioRIP combines two technologies in order to address this problem:
- Within a given tolerance, it searches for different line screens and slightly different angles that will be ‘immune’ to this phenomenon, having the dots of different sizes evenly dispersed (rather than grouped in groups of smaller and bigger dots).
- It will slightly change the sizes of the dots stochastically, suppressing any ordered patterns.
As a result of the two technologies, StudioRIP halftones are always smooth and pattern free.
