Back up the truck a bit
Vendors can be a bit....shall we say, fuzzy....when it comes to specifications.
A starting point to keep in mind - on a 2400 dpi CtP device, at 240 lpi, a 1% dot is one pixel. On a 2400 dpi device one pixel is 10.6 microns in size. (For a 2540 dpi CtP it's a 10 micron pixel). Also, a CtP device can only image complete pixels. I.e. the smallest mark a 2400 dpi device can make is a pixel that's 10.6 microns. In order to make halftone dots, the device will group (cluster) pixels of 10.6 microns.
Using your example, mathematically, a 1% dot at 200 lpi requires a halftone dot of about 14.5 microns.*
However, your CtP device cannot image a dot of 14.5 microns since it only has a 10.6 micron pixel to work with. So, to create a 1% dot, the screening engine will alternate between single 10.6 pixel dots and groups of 2 pixel dots i.e.10.6 x 21.2 sized dots so that the average of dot sizes for that tone area works out to 14.5 microns.
If you increase the lpi e.g. to 385 lpi in your example, the smallest dot will still be a 10.6 micron dot - which is too large for a 1% tone. You'd need a pixel that's about 6 microns. So, since it can't make smaller pixels the screening engine simply removes a percentage of 10.6 micron pixels from the tone area. Removing pixels from an area makes it lighter, and thus allows the representation of a 1% tone even though the pixels are too large.
So, if the vendor says that the plates say certified for 10 micron (one pixel at 2540 dpi or one 10.6 sized pixel at 2400 dpi) that's not quite the same as saying 1%-99% 200lpi conventional AM since the 1% tone at 200 lpi will be a mix of single and grouped pixels. In my experience, when a vendor says 1%-99% at 200lpi conventional AM they actually mean that it cannot image single pixels, only groups of pixels. I.e. it means they cannot reliably image a dot smaller than 21 microns (a group of four 10.6 pixels). If they could reliably image single pixels they'd say: "1%-99% 240 lpi conventional AM".
Yes, because you are using positive plates you may be overexposing and losing the highlight dots, or your CtP is just not capable, or your plates don't actually have the resolution, or your plate processing is causing the loss.
When someone says they're printing 20 micron FM, they are printing dot sizes that range from 20 microns (1% dot at 120 lpi made up of 4 pixels) to a dot that's about 30 microns in the midtones.
Presses are not the limitation to printing fine screens. Presses can and do print screens like 10 micron FM. For example Canadian Stamps and those of over 30 European, African, Asian and Caribbean countries have been print 10 micron FM for at least the past 10 years:
Also, most of the Yellow Pages Directories and newspaper inserts in North America are printed FM.
So, yes, absolutely people doing this successfully in commercial sheetfed type work? (Over 70% of the print entries for the Benny Awards use FM screening).
Yes, it's correct that you may not be able to do it based on platesetter and plates and press - but if you can reliably print 175 lpi then you should be able to print 20 micron FM.
Why do you want to? Here are a few reasons:
1 - No screen angle moiré
2 - No subject moiré
3 - No rosettes
4 - Photographic/contone look
5 - Greater tone and color stability as SIDs naturally vary during press run
6 - Larger color gamut
7 - Faster drying
8 - Reduced ink usage
9 - Tonal and color stability when misregistration occurs
10 - Halftone dot structure stability when misregistration occurs
11 - Competitive differentiator
(More info here:
Quality In Print: FM/Stochastic Screening - Part 1 of 5 - The benefits )
If you want to go to a finer screen than 175 lpi then I would "bite the bullet" and go straight to FM. Just like making any other change in the print process - how you go about making the change will determine how successful you are. Most of the problems I've encountered are people-related not technical. But that's a whole other thread
best, gordon p
*The formula for calculating dot size in microns is here:
Quality In Print: How to calculate halftone dot sizes in microns