AM and FM gamuts compared

This short video (click on the "play" arrow) shows a 20 micron FM (Kodak Staccato) gamut (translucent) over top of a 175 lpi AM screen gamut. The characterization data used to build these two profiles came from press sheets run to GRACoL 7. The FM screen had a curve applied to the plate to align its tones to the AM screen.

Here is a view from the top looking down:
And here is a view from below looking up:What's important to note is that the FM gamut volume is larger (more chroma) than the AM screened gamut. The difference will be seen primarily in one and two color screen tint builds in the 10% to 90% tone areas. Since it is not usual for raster images to contain areas of only two colors, the difference in gamuts may not be always visible in image reproduction. The difference in chroma at specific tone values will also affect the hue of screened Pantone spot colors.
The increase in gamut is the result of the smaller dots of the FM screen covering more of the paper compared with an AM/XM screen at the same reproduced tone value, rather than how the dots are arranged. Hence, if the frequency of the AM screen is increased to about 350 lpi, its gamut will increase and will closely match that of the 20 micron FM screen.

A higher resolution of the gamut comparison video is available - contact me at pritchardgordon@gmail (dot) com for a link to download it.
Technically speaking, FM - or more properly microdot screening - does not actually increase the gamut (as demonstrated in part 1). Instead, it is more accurate to say that FM reduces the potential gamut less than the larger dots of an AM screen does. The function of ink in printing is to filter light, when that happens you see color according to what part of the spectrum is filtered by the ink.
So, how is it that FM screening increases the gamut (as demonstrated in part 1)?
Some light passes through the film of ink and is filtered by it as it is reflected of of the substrate. Some light scatters in the substrate under the dot of ink causing what is called optical dot gain - a colored shadow around the dot of ink. Some light passes between the dots and comes back through the film of ink. While some light is simply reflected off of the surface of the dot rather than actually passing through it.
However, some of the light is not filtered by the ink. Instead it goes between the dots of ink and is simply reflected off the surface of the substrate. This unfiltered light ends up mixing with the light that's been filtered by the ink and contaminating/greying it.
Because an FM screen distributes more dots of ink per tone area, and because the small dots have a greater perimeter to area ratio (more optical gain) the result is that more light is filtered by the ink. There is much less empty space between dots for light to reflect without being filtered.
For comparison here is a 15% and 40% tone in AM:
and here are the same 15% and 40% tones in FM (Kodak Staccato):
You can easily see that there is more ink coverage of paper with the FM screen hence more opportunity for light to be filtered. As a result, less white paper contamination and therefore less of a loss of gamut.

There is another factor at work which helps FM dots retain more of the possible gamut of the ink.
As ink thickness increases its becomes a less efficient filter of light. Instead light tends to reflect of the surface of the dot. FM screen dots have a more uniform film of ink that is thinner than that of an AM dot at the same tone value.
Here is a photomicrograph of a Magenta AM dot on the left and FM dot on the right both representing the same measured tone value on press:
Below is the photomicrograph transformed in 3D imagery that plots density to height:
Note the thickness of the film of ink for the AM dot. Note also the different densities across the surface of each AM dot. Those micro areas of increased density are effectively areas of reduced ink filtering ability.
Lastly, FM screens, because of their thinner ink films, dry slightly faster than the larger thicker ink AM dots. Hence overprint trapping in screened areas is slightly more efficient.
All of those factors contribute to FM screening delivering a wider gamut than AM screening.