Concentric screening and color gamut
Chroma in press work derives primarily from the ratio of light being filtered by ink carried on halftone dots vs light reflected off the paper that hasn't been filtered by the ink. Light that is unfiltered by the ink effectively contaminates the color reducing the potential gamut of the inks. If one compares Concentric halftone dots with conventional AM/XM halftone dots at the same lpi - e.g. 175 lpi. what is clear is the difference in ink coverage area through which light can be filtered.
Note that dividing the dot into rings actually lessens the area of ink and increases the area of unprinted paper. Effectively it increases the contamination of color by light reflected off of the unprinted substrate which can actually reduce, rather than increase, the potential gamut.
In the below plot, the CIEL*a*b* values of the same tone values for 175 lpi AM/XM/Paragon screening (in green) is compared to 175 lpi Concentric (in red). If the Concentric had a larger gamut the red dots would be significantly above the green dots indicating a greater chroma. Instead they track at, or are below, the chroma for 175 lpi AM/XM/Esko Paragon screening.
What this means is that, as far as I can determine, Concentric screening offers no additional gamut, and possibly less of a gamut, when it is compared with AM/XM screens at the same lpi.
Concentric screening and image quality
Since it is still an AM screen there is still the opportunity for screening and subject moiré - although the finer the screen (AM/XM or Concentric) the less likely that will be a problem. Because it's still an AM halftone screen it has rosettes - just like any other AM/XM screen - formed by the screen angles.
From a print buyer point of view there will likely be no visible difference between a 200-300 lpi conventional AM/XM screen and Concentric screening - even if viewed under a loupe.
Concentric screening and ink reduction
The two primary causes of the reduction in ink usage with high lpi screens are the thinner ink films and the need for tone reproduction curves for plate imaging to bring the press tone response in line with the standard 175 lpi AM/XM screening. Ink reduction with the use of Concentric screening should be similar to the ink reduction enjoyed by high lpi conventional AM/XM as well as FM screens.
Concentric screening and on press color stability
Greater color stability when solid ink densities naturally vary during the press run is a characteristic of high frequency screening (i.e. smaller dots) whether AM/XM, FM or Concentric. The actual ink film thickness of Concentric vs conventional AM/XM screening at the same lpi is actually very similar. Projecting dot density to height in 3D one can see this quite clearly (Concentric is left of the black line - AM/XM is right of the black line.)Of course, if the Concentric screening is run at a very high lpi it will acquire a stability that is similar to conventional screens (AM/XM and FM) that are run to the same high frequency.
Concentric screening and imaging system resolution
Concentric screening is effectively an AM screen ruling multiplier. What this means is that the resolution of the imaging system needs to be able to image the minimum specified ring width. Put another way, if the ring thickness called for is 10 microns wide then the imaging system (plate and press) must be capable of consistently imaging a 10 micron pixel/dot even though the actual final halftone dot size may be almost five times wider (e.g. 48 microns wide a 50% dot at 250 lpi).
As a result, using Concentric screeing can push the effective screen frequency so high that process stability and imaging may be compromised and it can be difficult to support their use on plate let alone find a way to implement them in the press room. The problem is that some concentric screen settings can drive rulings way over what plate imaging can support - on the order of 1-2 pixel widths for the rings, which is understandably problematic. For example, a 200 lpi screen with 2 pixel ringwidths = 600 lpi which is finer than, for example, a 10 micron FM screen.
Coarser ringwidths are easier to support but at that point it is probably more effective to use an AM screen of equivalent lpi.
For printers contemplating the adoption of Concentric screening
Since Concentric screening is a conventional AM screen using a unique halftone dot design, I suggest that when you are evaluating this type of screening that you "compare apples to apples". That means that you should compare the on press performance of Concentric against a conventional AM/XM screen imaged at the same lpi. Use a combination of subjective (pretty pictures) as well as objective measurable targets (single and two color step wedge gradients and IT8 profiling targets).
N.B. The data that I used as the basis for this post is derived from published promotional samples printed by Esko. I have contacted Esko as well as members of public prepress/press forums asking for press profiles and/or printed test samples of Concentric vs conventional AM/XM screens run at the same lpi under the same press conditions. Despite the product being in the market for over four years I have been unable to acquire such a basic color profile or press samples. If you have that data I would appreciate hearing from you by email ( pritchardgordon @ gmail (dot) com ).