I'm often asked about what AM/XM halftone screens are equivalent to a certain FM screen - i.e. "What AM/XM screen is equivalent to a 20 micron FM screen?"
There are two ways that this halftone screening equivalency is usually measured.
One is equivalency of detail rendering - the ability of the screening to render image detail. The other is lithographic equivalency - how they perform on press lithographically. Note that in both cases, because the respective screening technology is so different, equivalency can only be an approximation.
Equivalency of detail rendering
Since halftone dots form the printed image - more dots per linear inch translates into more detail that can be rendered.
With an AM screen the detail rendering ability is specified in lpi (or lpc) - i.e. halftone dots per inch (e.g. 175 lpi or 60 lpc).
Since an FM screen has no "lines per inch" determining the equivalent detail rendering equivalency is usually done by drawing a line through the FM screen and counting how many dots are intersected (crossed) in a distance of one inch.
The above example shows an FM screen enlarged. The distance measured is 1/16th of an inch. In that 1/16th of an inch approximately 36 dots are intersected. So, in one inch about 576 dots would be intersected (16 x 36). Put another way, there are 576 dots per linear inch - 576 lpi - to render detail, i.e. this FM screen is equivalent to a 576 lpi AM/XM screen.
Lithographic equivalency is a bit more complicated to figure out. It is usually measured by counting the number of edges (transitions) in a square inch.
Halftone screens with a similar number of edge transitions will have similar lithographic properties.
AM/XM equivalents of some popular FM screens.
Keep in mind, these are approximations only, however they do give a good indication as to screening performance.