Choosing the Right Screen Ruling

This article covers screen ruling selections for a variety of print processes for the printing industry and pre-press professionals.
Today’s advanced screening technologies present the print buyer and
prepress manager with a seemingly endless variety of screen algorithms
from which to choose. How then does one choose the right screen ruling
to deliver the best results on press? The answer: it depends. The
following outlines the options available and their best fit.

Factors Influencing the Status Quo

Traditional rosette-based amplitude modulated (AM) screening had been
virtually unchanged from its inception in the late 1800′s, until the
advent of super-cell AM screens in the early 1990′s. Soon these modern
AM screens, such as Agfa’s Balanced Screening (:ABS), became the
standard.

In 1993, two events turned conventional wisdom upside-down. Stochastic
or frequency modulated (FM) screens, such as :CristalRaster, were
introduced. And the first platesetters from companies such as Gerber,
Optronics and Creo, (now Esko-Graphics, ECRM and Eastman Kodak
respectively) became commercially available.

Traditional Screening

AM screens vary the size of the dot on an established grid or
line-screen ruling to change the tonal value. The finer the grid, the
higher the frequency or number of dots and the closer  the rows of
dots are to each other. Varying conditions of the prepress process and
the types of presses being used limit the screen ruling. The printing
process, therefore, determines the choice of a traditional AM screen
ruling; it is not solely a decision of preference. 

Stochastic Screening

Stochastic screening enabled new levels of detail. Previously, and at
an imager resolution of 2400, the finest AM screen ruling possible that
could deliver a continuous 1-99% tonal range, was 240 lpi. Considering
the standard screen ruling for magazine production is based on 133 lpi
(still today’s SWOP standard), the ability to deliver screening beyond
133 lpi or 240 lpi seemed revolutionary.

While mezzotinting  and stippling (precursors to  stochastic
screening in etching and engraving) were popular as far back as the
American and French Revolutions, the concept of modulating tones by
controlling the frequency or number of dots (FM), rather than varying
the size of the dots (AM) was indeed revolutionary. Stochastic
screening in a PostScript workflow was the first method to faithfully
reproduce a broad tonal scale at high fidelity with line-screen
equivalents of 300, 350 and 400 lpi.

The trick in today’s advanced screening algorithms is to control the
highlight and shadow detail in an FM fashion, utilising no smaller dot
than the process can easily hold. Often one hears how a magazine
manufacturer has settled upon a minimum-sized dot of 28 microns, which
equates to a 2% dot at 133 lpi.

SWOP standards were defined around best practices in magazine
production. Such conditions dictated that in order to print a 2-98%
tonal range, the finest dot the process could hold was a 2% dot at 133
lpi, or 28 microns. And yet, with the inherent variables of a
film-based workflow, consistently holding a 28-micron dot was a
challenge, let alone 14 or 21.

The Arrival of CtP

As PostScript-based advanced screening algorithms were challenging the
practical limitations of traditional film-based workflows, another
revolutionary technology arrived: computer-to-plate (CtP). CtP devices
were designed to reduce the steps and variables in delivering dots to
press.

At first, CtP delivered several production benefits to the printer.
However, it was the simple removal of variables (no film imager or
chemistry fluctuations, no exposure-frame draw-down issues, no
exposure-frame lamp and timing variables, etc.) that proved to be the
factor that enabled the marriage of two revolutionary technologies: FM
screening and CtP.

FM Screening and CtP

With the marriage of FM screening and CtP, prepress systems could now
push the envelope of what these two technologies could deliver. What
before had been a nearly impossible task – to hold a 1% dot at 240 lpi
on plate – was now feasible. And because the addressability of standard
2400-dpi devices maxed-out at that 1% dot (10.6 microns), FM screening
seemed a natural fit.

Regardless of the first order FM distribution (random), or second order
FM (variable dot size placed into mid-tone swirls or worms), FM
screening was too grainy, especially in the mid-tones where the
frequency caused dot clumping, and it was still difficult to manage on
press.

1% pseudo ‘AM’ tone at 300 lpi and 2400 dpi

The Inherent Benefit of AM Screens and Disadvantage of FM Screens

FM delivers finer detail than AM screens. However, this is the benefit
of the small dot or high-frequency and not the random distribution.
Until they max-out at 240 lpi, AM screens deliver smoother flat tints
than FM, and are more forgiving on-press than FM screens. It is also
easier to control grey balance with AM screens.

While some argue that using ink density to control mid-tones should be
the exception and not common practice, they do agree that FM does not
respond to density adjustments. So, the challenge to the industry was
to come up with a screening algorithm that combined the best of FM
(higher fidelity, and more consistent highlight and shadow details)
with the best of the AM world (smoother flat tints, greater operating
latitude on press).

XM Screening: The Best of Both Worlds

The problem with hybrid screens however, was the visible crossover
where FM and AM meet. The challenge was to combine the two technologies
seamlessly, without noticeable intersections. XM or cross modulation
screening provided the solution.

Agfa’s :Sublima XM technology applies a common sense approach to
advanced screening: match the screening to the pressroom environment,
rather than changing the pressroom to match the screening requirements.
XM screens take into consideration the type of paper typically used
(coated, uncoated, recycled, newsprint, etc.), the printing
architecture (sheet-fed, heat- or cold-set web, flexography), and other
variables (such as typical ink tack, blanket release etc.). XM
screening works within the established parameters and uses the smallest
optimised and printable dot for the application.

As you can see in the example below, the smaller and higher-frequency
XM dots to the right are still placed along an established AM grid, but
no smaller dot is used than can be easily held within this press
condition (in this case – 28 microns for heat-set web).

So, What Screen Ruling Should I Use?

The question is not really what screen ruling, but rather, “What is
minimum sized dot I can easily print?” This smallest dot size varies
based on press architecture and typical press en
vironment. The higher
the line ruling, the higher the risk of dropping the highlight detail
on press yielding blotchy or posterised effects. So, by establishing
the smallest sized dot that can be easily held, the next task is to
ensure a full tonal range.

XM screens deliver a full tonal range by using AM screens in the vast
mid-tones, and then converting to an FM (but not randomly distributed)
placement of the dots in the highlights and shadows. XM and FM
algorithms deliver 1-99% tonal ranges by placing (or leaving) fewer
dots of that optimised and minimally-defined dot. So just what size is
that dot?

The Magic Number for HeatSet Webs: 28 microns

The smallest sized dot depends upon the application. Magazine printers
have optimised their operations around a 28-micron size minimum dot,
which equates to a 2% dot at 133 lpi. However, at 175 lpi, that 2%
equates to a 21 micron sized dot, a size that might work well for
sheet-fed presses, but presents a challenge for typical heat-set web
environments. Therefore, XM screening algorithms tend to find that a 28
micron dot works well (2×3 pixel for a 2400 dpi device, or 2×2 pixels
for an 1800 dpi device). So, instead of a traditional standard of 150
lpi, with XM screening, heatset web printers find that they can nearly
double the resolution – up to 240 or 250 lpi, with no extra effort on
press.

The Magic Number for ColdSet Webs: 35 microns

The issue for newspapers is not the imager quality, the quality of the
plates or even the ink. The newsprint substrate is the single aspect
that defines the screening parameters. By using a minimum dot size that
ranges between 35 and 40 microns, newspaper publishers are realising
the benefit of advanced screening technologies, without having to
change the pressroom. From what used to be a maximum standard of 100
lpi, newspapers are now attaining 180 lpi. And they accomplish this
without reducing dot size, but by simply using XM screening.

The Magic Number for Sheet-fed Presses: 21 microns (but it depends)

The sheet-fed environment in general is quite standardised, and
products such as :Sublima have been carefully formulated to ensure the
customer makes the right screening choice.

With :Sublima, Agfa engineers have assembled compensated screen sets
with pre-established minimum and maximum dots and frequencies based on
a variety of imager and plate characteristics, in combination with a
variety of press environments.

Provided a shop can consistently hold a 2% dot at 175 lpi, then that
dot size equates to a 21 micron dot size. Therefore, XM screening
algorithms based on 21 microns are quite popular in the optimum
sheet-fed environment. However, should the printer use a recycled
stock, then a bit larger minimum dot – 28 microns – should be the
default. Again it depends. With an XM technology, standard 21-micron
screen rulings exist at 210, 240, 280 and 340 lpi.

Regardless of technology or environment, one aspect rings common:
optimised process control is a must, and today’s CtP technologies help
to stabilise the environment.

When should one use 240 lpi versus 340 lpi?

Considering that a given combination of stock and ink sets can easily
deliver a 21-micron dot to the press sheet, then why not always use the
finest 340-lpi screen? A finer line screen cannot uncover what is not
there. However, it does allow you to get more detail from larger image
files that do have more information. With today’s rasterising speeds,
processing is not an issue but image archival and retrieval overhead
may be.

The finer the screen ruling, the more XM behaves like FM. FM can
deliver fine detail, but FM dots also resist on-press colour
adjustments.

At a normal viewing distance, it is difficult to tell the difference
between 240 and 340 lpi with the naked eye. Yet ink density and
reflectance can generate a greater measurable colour gamut or
brilliance with finer screens. And upon closer examination, one can see
differences in detail between 240, 280 and 340 lpi screen rulings.

With XM screening, when using 21-micron based 240 or 340 lpi screen
rulings, the dot size in the highlights and shadows are the same: 21
microns. At 240 lpi, the tonal range between 1% and 4% is built from
the same-sized 21-micron dot. With 340 lpi, the tonal range between 1%
and 8% is built from frequencies of those same-sized dots. Due to the
increased line ruling, and based on the AM aspect, 340 lpi mid-tone
dots are naturally smaller than the 240 lpi mid-tone dots.

These smaller 340 lpi dots in the mid-tones, and their lower ink
density yield a narrower on-press operating latitude than their 240 lpi
counterparts, and yet both allow for more on-press management than
traditional FM dots. Therefore, 240 lpi XM is more forgiving on press
than 340 lpi XM, and 340 XM is more forgiving on press than FM. But
whether the XM screen is 210, 240, 280 or 340 lpi, no smaller dot is
ever needed than that 2% dot at 175 lpi within conventional AM
screening.

Both 240 and 340 lpi XM screens are designed to work well within the
capabilities of the standard sheet-fed press environment. True, there
are indeed subtle and at times valuable differences in the rendering of
the finest image detail and the brilliance of the hues with finer
frequencies, but the practical difference between the two is that on
press, the finer the screen, the narrower the press latitude.

The Choice is Yours

Today’s advanced screening technologies prove to be a perfect fit for
today’s CtP technologies. Since XM screening algorithms combine the
best of both the AM and FM worlds, the matter of best fit depends on
stock characteristics and how much flexibility one desires on press.

It has taken over 250 years for imaging and screening technology to be
optimised to match the performance characteristics of the printing
process. With today’s XM screening, print buyers and printers can
choose not from a position of inherent system limitations, but rather,
from an optimised screening palette based on personal preference and
ease-of-use. The choice is yours.

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