Screen E-Book (.PDF) - MacDermid Autotype

Moire, causes and cures118what this means is that as soon as you goeven slightly away from the perfect rosetteconfiguration, you end up with some lowerfrequency sub-patterns in your rosette.And the human eye can pick these out. Ifyou try to simulate them, the amplitude isfound to be relatively small. But the humaneye can spot even low amplitude variationsprovided they are at low frequencies,which is where you find yourself as soon asyou deviate from the perfect rosette.And the sad fact is that we are almostguaranteed to be away from that perfectrosette. If you ask your imagesetter at1200dpi to produce 100 lpi screens at 0,30 and 60° the one thing you don’t get iswhat you asked for. Instead (if the imagesetteris using standard PostScript® settings)you get 0°/100 lpi, 30.964°/102.9 lpi,59.036°/102.899 lpi. So before you’ve evenstarted, you have settings that are awayfrom the perfect rosette. Now introducesome errors in placing your positives onyour screen (or in the registration of yourprinted sheets), and add some changesin mesh tension and you are even furtherfrom the perfect rosette.So you are effectively guaranteed to havethis sort of moiré. In a way, this is goodnews. Most of the time, most printersdon’t have severe unstable rosette moiré,so what we normally do is normally goodenough. Remember, the amplitude of thismoiré is relatively small and you can oftenget away with it.But you can’t get away with it if you aretoo sloppy in your choice of screen sets(and the error from the imperfections getsworse the higher you go in line ruling) or ifyou don’t register your prints accurately orif you are sloppy in controlling mesh tensions.Figure 10 This rosette is definitelyunstable. Your customersprobably wouldn’t like it.