Could you please explain the advantages/disadvantages of traditional, stochastic & hybrid screening

Part 3 - Answered by Mike Rundle, Worldwide Program Manager - Thermal Plates, Kodak Polychrome Graphics

Note from the editor: We believe there are many experts qualified to answer this question. Kodak Polychrome Graphics' answer is the third of three answers that we will publish over a three week period.

Could you please explain the advantages and disadvantages of traditional screening, stochastic screening, and hybrid screening?

Executive Summary
Stochastic screening, also known as FM screening, received a great deal of attention in the early and mid 1990s as an alternative to conventional halftone screening techniques. The argument for it was that stochastic screening could produce an image with a higher apparent resolution and greater detail than conventional halftoning.

Difficulties in transferring the (relatively small) stochastic dots from imageset film to plates caused stochastic screening to be all but abandoned after 1996 or thereabouts. With the widespread adoption of computer-to-plate (CTP) technology, which eliminates the film transfer problems, there has come to be a recent resurgence of interest in stochastic screening.

In this white paper we look at the different approaches to stochastic screening, evaluate the various claims being made on its behalf, and provide some suggestions to printers and print buyers about it.


Stochastic strengths and weaknesses
One of the primary benefits advanced for stochastic screening is that the use of many very small dots can create a fine-grained appearance more like a continuous-tone photograph than a conventional halftone. This is to some extent true, particularly in comparison with halftone screening at 150 lpi or less. A printer using conventional 150 lpi halftone screens who converts to 20µ stochastic screening will encounter a significant overall improvement in apparent image resolution.

At a traditional screen ruling of 150 lpi, the 1% dot is about 19µ in diameter. For this reason, 20µ stochastic screening is often compared to 150 lpi center-weighted halftone screening—with the implication that to achieve results comparable or superior to 200-300 lpi halftone screens, one must use a significantly smaller stochastic dot. This, in fact, is the rationale underlying the current enthusiasm for 10µ-14µ stochastic dot sizes.

In reality, 20µ stochastic is not directly comparable to 150 lpi halftone screening. The variable nature of stochastic dot positioning will necessarily place more dots in a given area than traditional screening; more dots equals more data, which in turn equals more detail. The dot density of 20µ to 25µ stochastic is nearly equivalent to the dot density created with traditional screening grids at 175 lpi to 200 lpi. It is therefore more valid to compare the image detail–that is, the perceived quality–of a 20µ or even 25µ stochastic screen with that of the same image reproduced at 175 lpi or 200 lpi.

However, the apparent superiority of a stochastic-screening image over a conventional halftone image begins to diminish sharply at the equivalent of 175 lpi or thereabouts, and there is virtually no difference in apparent resolution at conventional halftone rulings of 200 lpi and above.

In addition to providing (under some circumstances) a more detailed image, stochastic screening effectively eliminates most of the common causes of moiré, especially moiré between the different colors of a separation. Moiré arises from a conflict in overlapping patterns, and because the dots in a stochastic separation are placed more or less randomly, there are no patterns to overlap. There can, however, occasionally be moiré caused by a conflict of the patterns of the scan pixels themselves and details in the artwork. This source of moiré cannot be avoided by stochastic screening; it has to be eliminated by going to a higher scan resolution or rotating the image relative to the scan.

Notice the stipulation “more or less randomly.” Because all imagers place dots within their addressable grids of spots, completely random positioning is impossible to achieve. The resulting “more or less random” positioning can itself introduce artifacts into the image, particularly in the case of first-order stochastic. Highlights often appear excessively grainy, light skin tones can look gritty, and wood tones can appear flat. To minimize these problems, most stochastic solutions employ second-order stochastic or hybrid screening or a combination.

Press control issues
Whichever form of stochastic printing is employed, the reproduction of at least part of the image will rely on the reproduction of a relatively large number of small dots of more or less the same size. This has significant implications for the kind of control that will be required on press. For example, a 20µ stochastic dot, as noted, is the equivalent of a conventional 1% dot at 150 lpi. This means that if one is printing with 20µ stochastic screening, a significant part of the image (all, in the case of first-order stochastic) is composed of the equivalent of 1% highlight dots—and nothing else. Depending on press conditions, sometimes 1% dots print, and sometimes they do not. In a conventional halftone, the loss of the 1% dot means a slight falloff in highlight detail. With stochastic screening, the loss of the 20µ dot means the loss of an entire portion of the image.

This is the primary reason that KPG does not recommend so-called “micro-dot” stochastic screening. A 14µ spot is equivalent to a 1% dot at 200 lpi. A 10µ spot is equivalent to a 1% dot at 275 to 300 lpi. If you are considering printing with 14µ or 10µ stochastic, it would be a good idea to run a job with 275 lpi or 300 lpi conventional screening and analyze the press sheet. If reproduction of the 1% dots at these rulings is erratic—which for most printers it is going to be—you can expect severe problems when trying to use stochastic techniques at the same dot sizes.

Printers experimenting with stochastic report other press control issues as well. Overall ink film weight is lower with stochastic, which increases blanket piling, significantly in web applications. This in turn can shorten plate life. In general, stochastic screening is unforgiving on press; traditional screening is more tolerant of press deviations.

Dot gain
No less crucial than the issue of dot loss is dot gain. Again and again, printers who have experimented with stochastic techniques stress that dot gain calibration is critical to getting maximum benefit and predictable results from the technique. A 21µ stochastic dot has 20% more dot gain over conventional 150 lpi halftone at the 50% dot area level as computed by the Murray-Davies dot gain formula. At dot sizes under 20µ, say 10µ to 15µ, this figure can approach 50%.

Because stochastic screens behave differently on press than traditional screens, problems controlling color on mixed-screen forms can arise. Especially troublesome are color-fidelity issues when stochastic and traditional images run inline on press. In fact, Heidelberg only recommends mixing stochastic and conventional screening within a job, not within a form—which has unpleasant implications for printers using hybrid stochastic solutions.

Stochastic screening is a tool that provides significant benefits in some situations. It is not a panacea, nor is it (as it is sometimes thought to be) a way of producing very-high-resolution printing without having to deal with the control issues associated with 200+ lpi conventional screening. It requires, in fact, more control. For printers interested in trying stochastic screening, or who have customers requesting it, KPG in general recommends not going below a 20µ-25µ stochastic dot. Screening with 10µ-15µ dots is possible and some printers are having success with it, but it is difficult and requires extremely tight control of the entire process. If a printer has work that requires the capture of an extraordinary amount of detail, a more forgiving alternative that might be considered is the use of conventional screening at 300 lpi or thereabouts.
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