Correct me if I am missing something here,
You said that FM screen does not increase the color gamut, but on your blog you mentioned that 20 micron FM screen provides 10-15% greater gamut.
And 1 more question...
Why the dot gain has no effect on color gamut?
Correct - FM screening (typically 20-25 micron) has a larger gamut than AM screening (typically 175-200 lpi). When AM screening gets to about 325+lpi the dot size and distribution approaches 20 micron FM and so at that lpi the AM screening gamut becomes similar to 20 micron FM.
You see color because light is filtered by passing through a film of ink.
When you look at a printed area you are looking at the combination of light that is filtered by the ink and light that is just reflected off the sheet that has not been filtered by the ink.
If light does not pass through the ink but just bounces off the white paper it effectively contaminates the color - it greys it.
If the ink fully covered the sheet (as in continuous tone lithography) then all the light would be filtered by the ink and you would have the maximum gamut.
Because, for a given tone value, more light is filtered by an FM screen the gamut reducing effect of light bouncing off blank paper is less than with an AM screen. So, the potential gamut is reduced less by an FM screen than with and AM screen.
So, because FM screening reduces gamut less than an AM screen the effect is that the FM screen gamut is larger than the AM screen.
Dot gain is also called Tone Value Increase. Tones are degrees, %, of light and dark not degrees of Chroma/Saturation as far as gamut is concerned. A 100% patch of, for example, Cyan represents the maximum chroma/saturation in 2D space. Where FM screens provide extra gamut is in the tones between about 20% and 80%. So, for example, a 40% 20 micron FM Cyan patch will have a higher chroma than a Cyan 40% 175 lpi screen even though they measure as the same density/tone value on the press sheet. The hue (angle) will also be different. The hue of a Cyan 40%
20 micron FM patch cannot be matched with a 175 lpi screen. However, the hue of the 40% Cyan 175 lpi screen can be matched by a 20 micron FM screen by contaminating the FM patch with Black and Magenta or Yellow.
The
extra gamut in the tones between about 20% and 80% for an FM screen is seen in 3D gamut plots and can be seen in presswork. When I was at Creo/Kodak we distributed press sheet samples to demonstrate this effect.
You can see a video of the gamut differences here: http://the-print-guide.blogspot.com.au/2009/03/am-and-fm-gamuts-compared.html
I hope this makes some sense.
