Halftones and grey levels - part 2 of 2

(scroll down in the blog for Part One which will help you make sense of this part :-))

In 1984 the screening technology described in part one was the state of art for halftone screening with Postscript devices.
The only way to recoup the lack of tones as one went to higher lpis was to increase the device dpi. I.e. go from 2400 dpi to 3200 dpi or higher. The penalty was slower imaging times and increased process control required in the film workflows of the day.
However, the formula is only true for the tone represented by a single, isolated, halftone dot based on an individual halftone dot cell - something that never occurs in real production environments. So, around 1989 a new approach began to be adopted. The approach is based on the fact that we don't care about individual halftone dots. What is important is the tone represented in an area. For example, let's say that we want to see a 17% tone patch value in the presswork. However, if we cannot represent that area with individual 17% dots – because of that classic formula limitation – we can still create the 17% value by alternating 16% dots and 18% dots (this is called "dithering"). The eye (and instruments) integrate the alternating 16% and 18% dots and the result is the average value - in this example 17% – our desired tone value.
Another way to look at it is: if we constrain our halftone cell to a pixel matrix of 16 x 16 pixels then we will always have 257 levels of grey in an area irrespective of how the dots within the cell are organized. However, if we build a tone area based on multiple halftone cells – a "supercell" we can get around the grey level limitations the formula would suggest.
As one example, the highest lpi on a 2400 dpi device that I'm aware of was 1694 lpi on a poster printed with plates imaged on a Creo CtP device in 2000 by Metropolitan Fine Printers in Vancouver Canada. It won a "They said it can't be done" award at GrapExpo in Chicago.
Supercell screening gets around the grey level limitations of the classic formula by looking at a tone area (the important criteria) rather than an individual dot. As a result, since about 1995 all AM screens from all vendors adopted variants of supercell screening technology:
Agfa - ABS - Agfa Balanced Screening
Heidelberg - HDS - High Definition Screening, and later IS screening
Harlequin - HPS - Harlequin Precision Screening
Creo/Kodak - Creosettes/Maxtone
Fuji - just since 2004 CoRes screening
etc.
As a result, 2400 dpi has become the defacto standard for imaging resolution in the commercial print industry. Higher resolutions, as far as halftone screening and grey levels is concerned, provides no additional value while imposing a penalty on imaging time.

Where the various vendors distinguish themselves with their individual implementation of supercell screening is how they deal with issues such as rosette drift - the gradual shift from clear centered rosette to dot centered rosette - over the width of the plate, single channel moiré, miniscus effects as dots first touch, e.g. at the 50% point, and other nuances of halftone screening.

Once you've passed the 200 lpi frequency - the human eye can no longer resolve the halftone structure at normal viewing distances. Beyond 200 lpi the argument can be made that there is no need to be constrained to the AM halftone structure. You might as well use an FM type screen. The lithographic issues will be the same since the imaging and press issues result from the size of halftone dots - not how they are organized.