Hybrid AM Screening/XM Screening - Addendum, part 1

I've been asked to provide some guidance on how to go about isolating which of the problem areas (plate, plate imaging, processing, and press condition) might be the cause of the resolution limitation that creates the need for a Hybrid AM/XM screening workaround.
This is actually an important topic with broad reaching implications, especially if you are considering a CtP purchase – no matter what halftone screening you use.

Background - Resolution vs Addressability

As one important feature, vendors describe their output device's (CtP, inkjet, etc.) capability in terms of "dots per inch" (dpi) output resolution. For example, the Fuji Luxel V-8 is listed as having "Eight multiple resolutions supported from 1,200 to 3,657dpi" while the Heidelberg Suprasetter family is listed having a "resolution 2,400 or 2,540 dpi". Unfortunately dpi does not define resolution. Instead it defines "addressability." In other words, dpi tells you how many locations a spot of energy can be focussed on – not the actual size of the spot of energy (or splat of ink).

Resolution vs Addressability

A CtP device uses a beam of energy to expose the plate:The exposing beam of energy is guided by a grid - much like the grid of a city map. However, instead of locating streets using X/Y coordinates, the grid locates the target pixel location/address for the the laser exposing energy:In the above example, the addressability grid has 2,400 locations per inch ("2,400 dpi"). Therefore each location is 1/2400th of an inch, or 10.6 microns in size – the same as a 1% dot at 240 lpi.
The energy beam, following the grid, is then swept across the media to expose/image it.
This graphic shows the media being exposed at 2,400 dpi by six different CtP devices:Note that they are all 2,400 dpi - that is that they all can hit the target location with their beam of energy - however the exposing spots of energy are all different sizes, in this example ranging from about 2 microns on the left to about 30 microns on the right.

So, what's the big issue about using/needing a Hybrid AM/XM workaround?

For metal plate CtP, if the CtP device is unable to image a single pixel (1% dot at 240 lpi/10.6 micron at 2,400 dpi) the argument can be made that it cannot image the rest of the halftone screen tone range consistently. This is because the halftone dots themselves are made up of individual 10.6 micron spots/pixels.Left - Coarse AM screen. Center - High lpi AM screen. Right - FM screen

On large dots, or coarse AM screens below about 133 lpi, inconsistent dot edges due to an inability to reliably image 10.6 micron pixels will have little effect on the final presswork – the loss is within the "noise" of the system. However, as halftone dots become smaller and made up with fewer pixels, as with finer screen rulings (above about 175 lpi or FM screening), the impact in dot consistency, and therefore presswork, is much greater – one pixel lost when only 4 pixels make up the dot is a significant loss. With FM screens which may use only single pixels to make a tone, or draw "worms" as in the rightmost graphic above - the loss of a pixel or two can make a significant tone shift or contribute to a grainy appearance in flat screen tint areas.
Since the industry trend is towards finer, not coarser halftone screens, the ability to reliably image 10.6 micron pixels, in turn it is argued, becomes more important when making an investment in CtP equipment.