line screen indicator
You don't actually even need a line screen indicator, to do what you want to do. All you need is some old film separations of the screen rulings you want to test for. If you have a filmsetter you can make up some screen tints at the various screen rulings to do the job.
When you place a film separation over the printed job, and then rotate it, you'll see a moiré pattern that gets larger (perhaps dissappears when the film registers with printed dot pattern exactly) and then gets smaller again. It will do this for every colour, so you'll see some colour shift depending on which angle it is most closely matching. If you cannot get the pattern to "moiré-out" then just try another screen ruling. The one that gives the largest pattern will be the one closest to the screen ruling printed.
Paper stretch/shrink/distortion will be one reason why even the actual separation printed may not "moiré-out" perfectly. Most imagesetter/RIP combinations also don't output all colours on exactly the same ruling; you'll probably find cyan and magenta the best to test for, as black and yellow could be finer or coarser than the nominal ruling (cyan and magenta are also easier to see).
That's an old photolithographer's tip.
regards,
Barry
