The eternal conflict - ink/water balance - the tale of the tones

An AM/XM halftone screen has a builty-in conflicting ink/water balance requirement on press. The highlight dot and quarter tone range from 1-35% requires minimal water and maximum ink in order to prevent those dots from being washed away. The three-quarter tone range from 65-99% requires the opposite - a larger volume of water in order to prevent the shadow dots from filling in and disappearing. On the other hand, the mid-tone range from 35-65% is more of a balance between ink and water.

Halftone dots and the tones range they represent are affected differently by the condition of the ink on press - assuming of course, that the plate, press, and chemistry are set up correctly. Unfortunately, if the press operator attempts to fix tone reproduction in some areas, that built-in difference in ink/water requirement can exaggerate the inherent conflict and cause problems in other parts of the tone range.

1 - 1-35% This tone range is primarily affected by the body/viscosity of the ink. If the body is too soft the highlight area will print too full which may cause the press operator to decrease solid ink density in order to reduce the dot size. Alternatively the fountain solution may over-emulsify this tone range causing poor ink transfer and loss of highlight detail. If the ink body is too heavy the dot may print too sharp causing the press operator to increase the density or blanket pressure.

2 - 35-65% This tone range is primarily affected by the strength (pigment load) of the ink. If the ink is too weak the press operator will increase solid ink density which will cause increased dot gain and result in presswork that appears too full. If the ink is too strong the midtones may print too light. Also, the strength of the ink also impacts how well the inks trap, which in turn affects the color gamut the press should be able to achieve. Varying the strength and stiffness of the ink to achieve good tone reproduction in presswork is a method press operators, who don't have good communication with prepress, often employ. It's almost always better to use tone reproduction curves applied in plate imaging than to modify inks.

3 - 65-95% This tone range is most strongly affected by mechanically induced dot gain or chemistry issues i.e. (poor ink water balance). If the tone range from 1-65% is evenly balanced then excessive gain in the shadow tones is usually caused by running excessive water, too much blanket pressure, and/or mechanical slur.

Slur

Slur is often confused with doubling as their initial appearance is very similar. However slur is invariably an elongation of the dots in the sheet travel direction. The usual cause of slur is either over or under cylinder packing. Loose blankets, too much plate-to-blanket pressure, too much ink on coated paper, and ink rollers set too hard will also cause slur.

Doubling

Doubling is often confused with slur as both exhibit an elongation of halftone dots. However, slur is usually an elongation in the direction of sheet travel through the press while doubling can be in any direction. Doubling (and slur) often manifest as a problem with the range of tones available in the presswork being compressed and loss of detail, particularly in the shadow areas (a.k.a. muddy halftones). Doubling can be caused by many of the same factors as slur. When the cylinders rotate the halftone dots are not placed in exactly the same position with every revolution. As a result the dots print up as double or multiple images. Doubling between units occurs when a blanket picks up a previously printed ink film. This is known as backtrapping. Examine the dots, or line art graphics, under a loupe to confirm whether the problem is doubling or slur.

On-press stability and consistency

A printing press is a device for laying down a film of ink onto a substrate. Each component on each press unit, from ink ductor rollers to ink train oscillation moves or rotates with a repetitive frequency and this shows up as solid ink density variation. When the natural solid ink density variations through the press run are graphed, the result is sometimes referred to as the "heartbeat" of the press because of its resemblance to the rhythm of a human heartbeat.
The heartbeat of a press - the solid ink density variations measured over through the press run. Each of the 3 sections contains 100 press sheets from the beginning, middle, and end of the press run. Each "heartbeat" in each section is the solid ink density variation between 10 sheets. So, each of the three sections contains the SID measurements of 100 sheets.
The human heartbeat.
Every press design has its own "heartbeat" profile that represents its normal condition. What one looks for are abnormalities in the heartbeat profile itself as well as in comparison between different press units. Plotting the heartbeat of the press, just as it does with humans, can help diagnose problems in the system that may need correcting.

Solid ink densities varying naturally through the press run cause dot gains - tone reproduction - to also vary which in turn causes color shifts in the presswork. Below, courtesy of data provided by Alwan Print Standardizer, is a movie showing dot gain variation through the press run.
Click on the expand icon (the four arrows) to enlarge the video for greater clarity. Then click on the play arrow.
It's fair to say that a printing press in proper working condition is stable - but not consistent. And that fact applies equally to the presswork itself. Therefore, since variation is an integral characteristic of the printing process, the important thing is to establish, and communicate between buyer and print provider, what the target for presswork color should be, how it will be measured, and what range of variation is acceptable based on the needs of the specific job at hand.

Printing Standards and Specifications

Printing standards and their associated specifications bring an independent, authoritative, and concrete basis for file preparation, proofing, presswork, and output evaluation. They reduce proofing cycles and enable faster approval processes. They also help synchronize expectations between print buyer and print provider.

I'll begin with a few definitions.

A Standard according to the International Organization for Standardization (ISO) defines a standard as:
"A document established by consensus and approved by a recognized body that provides for common and repeated use, rules, guidelines or characteristics for activities or their results, aimed at the achievement of the optimum degree of order in a given context."

A Specification is not a standard. Rather, it is a detailed description of the criteria for a piece of work. Specifications for printing can include characterization-data, ICC-profiles, PDF preflight criteria, calibration targets for the print process, etc.

A Method is usually defined as a way, technique, or process for doing something. It is a recipe.

To use a cooking analogy - a standard represents the desired final outcome, for example a cake.The specifications describe the ingredients needed to make the cake. The method describes the steps required to make the cake. There can be many different methods to achieve the desired standard based on the specifications.

The most important standards for print production are:


• ISO 12647
 which describes color for different types of presswork (sheetfed, heatset web, coldset web, etc.). Of all of the standards within the Graphic Arts, what is of most importance to most printers and print buyers are contained within the ISO 12647 family.
ISO 12647 is broken down as follows

ISO 12647-1 Parameters & Measurement methods
ISO 12647-2 Offset Lithographic processes
ISO 12647-3 Coldset Offset Lithography on Newsprint
ISO 12647-4 Publication Gravure
ISO 12647-5 Screen Printing
ISO 12647-6 Flexo Printing
ISO 12647-7 Proofing process from digital data


• ISO 2846-1
 which describes ink color and transparency.


• ISO 3664 defines lighting conditions for viewing color copies and their reproduction with either incident or transmitted light.


• ISO 15930-X 
(PDF/X) for data exchange in print production.


• ISO 15076 for the ICC color profile format.


• ISO 12640 for the data format from which ICC profiles are calculated.


The most important specifications for standardized print production are:


PSO Process Standard Offset printing developed by FOGRA / bvdm / ECI

This implementation of ISO 12647-2 is included with most digital proofing solutions used in Europe and is pre-configured with the UGRA FOGRA Mediawedge for verification.
Grey balance target values in PSO are done by comparing K- and CMY-patches with similar grey side by side. PSO does not contain procedures and tolerances for judging grey balance by measurement.
PSO also includes the possibility of a certification for pre-press and printing.


GRACoL/SWOP managed by IDEAlliance


These standards take their basis from ISO 12647-2 but place a higher importance on grey balance during calibration and the press run compared with the PSO. Most proofing solutions for the North American market include the needed setup to produce proofs that represent the GRACoL/SWOP target.


System Brunner


System Brunner is a standardization method for print production. It places a very strong emphasis on grey balance (in combination with TVI and Solids) for controlling and certifying the press run. System Brunner is included with some printing press manufacturer's press control solutions.
It can also be combined with PSO, SWOP, or GRACoL.

There are various ISO 12647 standards according to the main types of printing methods. Here I will try and cover the essentials of just one of these - ISO 12647-2 - as an example of the type of specifications that guide printers to achieving the standard. Note that specifications do change over time, so, although the information I present is correct (as far as I know) the actual current published standards should be your guide.ISO 12647-2 specifies a number of process parameters and their values to be applied when preparing color separations for four-color offset printing by one of the following methods: heat-set web, sheet-fed or continuous forms process printing, or proofing for one of these processes; or offset proofing for half-tone gravure. Note that the specifications are based on plates imaged in a film, not CtP, workflow.

ISO 12647-2 is
▪ directly applicable to proofing and printing processes that use color separation films as input
▪ directly applicable to proofing and printing from printing formes produced by filmless methods as long as direct analogies to film production systems are maintained
▪ applicable to proofing and printing with more than four process colors as long as direct analogies to four-color printing are maintained, such as for data and screening, for print substrates and printing parameters
▪ applicable by analogy to line screens and non-periodic (i.e. FM) screens.

ISO 12647-2 Type 1 for offset lithographic processes on gloss-coated paper specifies:
Ink: ISO 2846-1
Substrate: L* 95 a* 0 b* -2
Primary Colors (black backed): K: L* 16, a* 0, b* 0 , C: L* 54 a* -36, b* -49, M: L* 46, a* 72, b* -5, Y: L* 87, a* -6, b* 90
Secondary Colors (black backed): R (M+Y): L* 46, a* 67, b* 47 , G (C+Y): L* 49 a* -66, b* 24, B (C+M): L* 24, a* 16, b* -45, C+M+Y: L* 22, a* 0, b* 0
Solid ink densities: (informative information)*
Halftone screen: 133 lpi, 150 lpi, 175 lpi, 20 micron FM
Dot Gain/TVI: 12 -16% or 18 -22%
Grey Balance: 25%-19%-19%, 50%-40%-40%, 75%-64%- 64%
Neutral definition: substrate or equivalent tone of black
ISO profile: ISOcoated_v2_eci.icc
Characterization data: Fogra39L.txt

GRACoL 7 The General Requirements and Applications for Commercial Offset Lithography publication that is a common reference in North America and is based on ISO 12647-2 specifies:
Ink: ISO 2846-1
Substrate: ISO 12647-2
Primary Colors: ISO 12647-2*
Secondary Colors: ISO 12647-2*
Solid ink densities: Not specified
Halftone screen: 175 lpi AM round dot
Dot gain/TVI: Not specified - replaced by Neutral Print Density Curve values: @ 25% Grey: CMY .25/K .22, @ 50% Grey CMY .54/K .50, @ 75% Grey: CMY .90/K .90
Grey Balance (required): 50%-40%-40%
Neutral definition: a* 0 b* -2
ICC profile: GRACoL2006_Coated1v2.icc
Characterization data: GRACoL2006_Coated1

Some peculiarities of ISO 12647-2 and GRACoL 7

Where GRACol 7 differs from ISO 12647-2 (as per the implementation guidelines of PSO (Print Standard Offset-print)) is the method used for adjusting the mid-tones. The PSO advises adjusting the inking until the TVIs of CMY come close to their aims, while GRACol 7 would have the neutral densities of the CMY and the K grey patches brought close to the prescribed aim values regardless of the individual TVIs that result. In practice this means that each color will have a different TVI curve to achieve the neutral grey. GRACol 7 allows for deviation of primaries in order to obtain grey balance.

ISO 12647-2 includes the following chart of dot gain/TVI curves:There are a few peculiarities with this chart. First is that there is no definition in ISO 12647 of what printing condition the letters "A" through "H" represent.** ISO 12647 does not clearly state whether these dot gain curves are intended to be tone reproduction targets or aim points or simply what you get when you use linear film to make printing plates. It appears that, because the specifications state that "direct analogies to film production systems are maintained" the intent is to use these curves as the reproduction targets. To, me, the idea of having different tone reproduction curve targets for presswork for different processes or different halftone line screens is counterproductive to standardization.

Resources:

Data set/s for profiling, separation, and proofing
ECI www.eci.org Profiles based on FOGRA data sets
FOGRA www.fogra.org Data sets that closely comply with ISO 12647
GRACoL http://www.idealliance.org
IFRA www.wan-ifra.org Profiles based on ISO 12647-3 (Newsprint)
SNAP www.naa.org SNAP profile
SWOP http://www.idealliance.org

Part 1 on this topic can be viewed by clicking HERE

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*Standards documents include two broad classes of information: "normative" and "informative"
Normative elements are defined as "elements that describe the scope of the document, and which set out provisions". Provisions include requirements that convey criteria to be fulfilled if compliance with the document is to be claimed and from which no deviation is permitted.

Informative elements include supplemental information such as additional guidance, supplemental recommendations, tutorials, commentary as well as background, history, development, and relationship with other elements. Informative data is not a requirement for compliance with the standard.

**As near as I can guess (so I may be wrong), here are the printing conditions represented by the letters "A" through "H" in the ISO dot gain chart. The curves are organized from the bottom up. They sometimes do double duty.

So:

A is the curve for CMY for Coated positive plate (ISO Coated v2 and FOGRA 27, FOGRA 39 etc)
B is the curve for K for same above condition

B is also curve for CMY for Web (paper type 3)
C is the curve for K for uncoated

C is also the curve for CMY for paper type 4/5 (uncoated and uncoated yellowish)
D is curve for K for same above condition

F is the CMYK curve for 20 micron non-periodic/FM screening

E, G and H are unknown (by me anyway). I believe that "H" has been dropped from the latest ISO 12647 document so these may no longer be in force.

To linearize your CtP plates or not?

A bit of background
Back in the old film to plate days the standard prepress procedure was to linearize film output. That means a specific tone request in the original file results in halftone dot in the film equal to the file tone request. So, for example, a 50% in the file became a 50% tone in the film. Linear film was the agreed standard interchange file format between prepress tradeshops, publishers and printers. At that time, the final tone on the plate was not measured. Instead, the resulting tone in the presswork was measured and deemed to be in specification, or not, relative to the supplied linear film. I.e. At 133 lpi, a 50% tone in the film resulting in a final tone of about 71% in the presswork would be considered in specification. Interestingly, although the film was linear, the resulting plates were not linear due to the dynamics of exposure in the vacuum frame.

The arrival of CtP in the late 1990s eliminated film as the intermediary. As a result, measuring tone values on the plate became a process control metric. However, CtP plates seldom have a linear response to laser exposure and if a tone reproduction curve is applied to them to make them linear - the resulting presswork is usually too "sharp" - i.e. not achieving enough dot gain.

At the same time that CtP was rapidly being adopted, printers also began to use finer halftone screens, including FM screens, which had very different dot gain characteristics compared to the old published standards. Printers began to leverage the flexibility that CtP provided in being able to apply different tone reproduction curves to their CtP plates to achieve the tone reproduction on press that they required.

So the question for the printer becomes: should prepress first apply a curve to linearize the plate and then, if needed, apply another curve on top of the first to achieve the desired final press tone response?

I was shocked
So, just to confirm that the method that I have been using for the past 13 years was indeed the standard method used in the industry, I posed the question to an internet printer's forum: "Do you linearize your plates before applying a press curve (a two curve workflow - e.g. one to linearize the plate followed by another one to compensate for dot gain) or do you only apply a press curve to the uncalibrated plate (a one curve workflow - e.g. one to compensate for dot gain)?"

The response shocked me - a whopping 70% said they first linearized the plate with a curve and then applied a press curve while only 30% responded that they simply applied a press curve to the uncalibrated (natural state) plate.

70% using one curve on top of another? That makes no sense to me at all.

In a film to plate workflow, linear film is exposed to the plate in a vacuum frame. The function of the plate exposure is to reproduce the halftone dots in the film as consistently as possible across the surface of the plate, and perhaps more importantly, to create a robust halftone dot on the plate that will maintain its integrity on press. However, although the film may be linear, the resulting plates are not linear due to the dynamics of exposure in the vacuum frame. In North America using negative film there is typically a 2%-5% dot gain on plate at 50% (i.e. 50% in the film creates about a 54% on the plate) while in Europe and Asia where positive film was used there is typically be a 2%-5% tone loss at 50%.

In a CtP workflow, as with a film to plate workflow, the important thing is to set laser exposure and processing (or lack thereof) to the manufacturer's specifications so that the result is a robust halftone dot on the plate that maintains its integrity on press. However, as with a film workflow, the resulting plates are typically not linear due to the dynamics of laser exposure, individual plate characteristics, and processing.

In this example, the thick line that dips below the 0 line is the natural uncalibrated plate curve after the engineer has done their work setting up exposure and processing for the most robust dot possible.With this particular positive thermal plate the uncalibrated plate curve results in a negative value through the tones. The bottom numbers in the graphic are the requested tone values in the file - 5%, 10%, 20%.... 90%, 100%. The "0" line represents linearity. I.e. if the plate was linear then that 0 line would be straight and be the "plate curve". But, in this case, a 50% request has resulted in about a 47% on plate. This is fairly typical - a well and properly exposed CtP plate does not have a linear response (i.e. a straight line). Also note that it is typically not a classic Bell curve - there is no symmetry. Different CtP/plate combinations will each have their own characteristic natural curves.

So, from a CtP vendor engineer's perspective, it does not matter whether the result of their setup is a linear plate or not since a tone reproduction curve can always be applied to achieve whatever tones are required on plate - including linearizing the plate. What's important is that the exposed dot is robust and that the plate imaging is consistent across the plate and repeatable from plate to plate.Put another way - the key criteria is that when properly set up the plate will have a characteristic non-linear tone response. And that's fine - as long as the plate responds the same - i.e. delivers the same non-linear tone response – every time because without that consistency it is not possible to build any tone reproduction curves at all.

Some definitions

These definitions are not "official" however they are useful to keeping the issues and discussions clear.

A "plate curve" is a tone reproduction curve that is applied in the workflow to a plate in order to have it render tone values that are different from those it delivers when the laser exposure and processing (or lack thereof) have been set to the manufacturer's specifications. So, applying a linearizing curve that makes an inherently non-linear plate linear is an example of the use of a plate curve.

A "press curve" is a tone reproduction curve that is applied in the workflow to a plate in order to have it render tone values that are required to deliver a specific tone response on press. The assumption is that the laser exposure and processing (or lack thereof) have been set to the manufacturer's specifications.

By this definition, if only a linearizing curve is applied because a linear plate is needed to deliver the correct tone response on press then that linearizing curve is a press curve.

A plate curve in this sense is not related to tone reproduction on press. It is effectively a calibration curve. It brings the plate to a known condition. However, in a CtP environment, the manufacturer's setup of laser exposure intensity, processing chemistry, and processing time effectively calibrates the plate plate to a known condition. It might not be linear but it is known. There is no need to recalibrate by applying a plate curve to what is already calibrated.

Another way to look at the question

Let's suppose that a linear plate provided the tone response on press that we need. Would it make sense to then use two curves - one to linearize the plate (a plate curve) and a second curve (a press curve) to linearize the linearized plate? I doubt it. Makes more sense to just apply the one linearizing curve - based on the uncalibrated natural condition of the plate.

So, if that logic makes sense, why wouldn't it make equal sense if we needed a non-linear press curve? Just apply the one non-linear press curve based on the uncalibrated natural non-linear condition of the plate.

As long as the plate's tone response is consistent then it can be the basis on which to build press curves. However, if the plate is inconsistent in its tone response then the use of linearizing plate curves as well as the use of press curves will fail. You cannot use curves, plate or press, on a device that is inconsistent.

What the "authorities" have said*Some quotes on this topic from the Idealliance G7 guides:

6.2 Origin of NPDC curves
To determine the 'natural' NPDC curves of commercial CtP-based printing, G7 research analyzed numerous press runs made with ISO-standard ink and paper, and a variety of plate types imaged on “un-calibrated” CtP systems (no RIP curves applied, not even to “linearize” the plate).

5.4 Set up the RIP
Set up the plate making RIP exactly as you would for a normal job, but clear out any values in the current calibration table, or begin with a new, empty table. The first press run is best made with ‘un-calibrated’ plates – i.e. no calibration values in the RIP.
IMPORTANT: Do NOT linearize the plate-setter so that measured dot values on plate exactly match original file percentages. Contrary to common belief, this may reduce accuracy of subsequent steps.

a. PRINTING IDEALIZED TARGETS VALUES - Achieving calibration condition with raw or linear plates, not requiring a curve, is an ideal situation.

*A note about authorities. I had trepidations about including these points from G7 because I do not believe that people should blindly do what some authority says they should do. It is not enough to say "Do it this way because I say it should be done this way." If the authority cannot explain exactly why one way is wrong and another right then it is just an opinion and without evidence to back it up it is not a credible opinion. I included these quotes only because they may carry credibility for some readers of this post.

Scenarios“We’ve always done it this way!” or “This way works just fine!” Even when we have the time to think about how or why we do things a certain way, our thoughts are often clouded by that kind of thinking. However, it can make it easier to understand the merits of a one curve workflow compared with a two curve workflow if one breaks down the sequence of steps required to get a plate into the press room. Given the same final result, the fewer the steps - the better the workflow since it provides fewer opportunities for error.

Here are some examples of workflow scenarios to see what happens with a one curve workflow vs a two curve workflow:

One CtP & one plate shop - to achieve the same final result on press:
One curve workflow: one press curve = one curve total.
Two curve workflow: one linearization plate curve plus one press curve = two curves total.

One CtP & one plate shop using three different curves to optimize for three different papers. To achieve the same final result on press:
One curve workflow: one press curve per paper type = three curves total.
Two curve workflow: one linearization plate curve plus one press curve per paper type = four curves total.

One CtP & two plate shop - to achieve the same final result on press:
One curve workflow: one press curve per plate type = two curves total.
Two curve workflow: two linearization plate curves plus one press curve = three curves total.

One CtP & two press shop - to achieve the same final result on two presses:
One curve workflow: one press curve per press = two curves total.
Two curve workflow: one linearization plate curve plus two press curves = three curves total.

One CtP & one plate shop - what happens if a new batch of plates do not perform as the previous batch did:
One curve workflow: modify one press curve so that the plate tones are the same as the previous plate batch = one modified curve total.
Two curve workflow: modify one linearization plate curve plus apply the standard press curve so that the final plate tones are the same as the previous plate batch = two curves total.

One CtP & one plate shop - what happens if the press curve needs to be tweaked/adjusted:
One curve workflow: modify one press curve to achieve the required tone reproduction on press = one modified curve total.
Two curve workflow: one linearization plate curve plus modify one press curve to achieve the required tone reproduction on press = two curves total.

One CtP & one plate shop - what happens if the CtP device is replaced:
One curve workflow: measure the new plate output and modify one press curve to achieve the same tone reproduction/dots on plate as with previous CtP = one modified curve total.
Two curve workflow: measure the new plate output and modify the linearization plate curve to linearize the plate then apply the existing press curve = one modified curve for two curves total.

Looked at this way, the linearization plate curve, in the vast majority of cases, is redundant. It serves no useful purpose except to add complexity and another point of failure.

The principle of dot gain compensation plate curves

In a film workflow the industry standard was to create film output that was linear. This meant that a 25% tone request in the original Postscript file would create a 25% dot on the film, a 50% request would create a 50% dot, and so one for all requested tone values. However, in a CtP workflow controlling tonality in the print reproduction process, allows you achieve the presswork quality you want without adjusting the press. It also provides the flexibility to tailor the print characteristic to meet different customer expectations.

Dot gain, or tone value increase (TVI), is a normal part of the print reproduction process. Controlling tones using calibration means that you can manipulate the exact size of the dots on the printing plates so that tone saturation and gray balance are controlled on the press sheet.

Tonal calibration can account for:
• type of plate or film used
• type of paper stock used for printing
• type of dot shape used
• type of screening used—for example, FM/Stochastic or AM/XM conventional, and frequency (lines per inch (lpi))

(Note: Adjusting CtP laser exposure is not tonal calibration and will affect the run length and performance of the plate.)

You cannot use tonal calibration as a substitute for stable operating conditions. Operating conditions must be controlled as a separate process. In fact, without a stable operating environment, you cannot achieve accurate tonal calibration let alone reliable press output.

What Is Tonality?

Printers are used to being concerned with dot gain/TVI. Indeed dot gain values are often included in printing specifications. However, for the purposes of calibration - tonality or dot area, rather than dot gain, is the key metric. It does not matter what dot gain you have. What matters is whether you achieve the required final tone values or dot areas at each originally requested tone.On the left is the desired "correct" tone reproduction and on the right is incorrect tone reproduction.
Tonality in printing is the progression of tints from blank paper to solid ink for each requested tone value in a printing job. It is measured with a densitometer, and reported as either dot gain/TVI or dot area:Dot area and dot gain - two ways of charting the same data.
The target print characteristic tone curve

Building dot gain compensation plate curves always begins with a target print characteristic, i.e. what you want to achieve on press. This is called the target curve - the current tone reproduction that you wish to achieve. It could be your current press work, a proof, or it could be an industry supplied set of tone values. You measure the target sample and enter the dot area (tonal value) for the tints achieved on the target curve graph. If the target is a press sheet, for example, your current 150 lpi AM/XM presswork, the graph will represent your current tone print characteristic:Target print characteristic tone curve - what we want our presswork to look like.
If you change your screening, for example going to FM screening, higher solid ink densities, or higher lpi AM/XM screening, etc. then, if nothing else changes, the tonal response on press will change due to the difference in dot gain:New print characteristic tone curve caused by a change in screening method being used - what the presswork now looks like after changing the halftone screening.
The goal of implementing dot gain compensation plate curves is to make the new press work mimic the original target press tone response. In the above example, the boy's face should appear the same as the original image despite the dot gain caused by changing the halftone screening.

Creating the dot gain compensation plate curve

Building a dot gain compensation plate curve starts with comparing the current target tone response with the tone response of the new presswork. In this case run to the same solid ink densities, on the same paper and press - only the screening has been changed:On the left is the current target tone curve and on the right is the new tone response resulting from the change in screening.
The graphs are then examined by looking at the original requested Postscript tone and the target response (left chart) and comparing it with the new tone response (right chart):In the current target tone curve a 50% tone request resulted in a 68% tone in the presswork. That same target 68% was delivered in the new presswork from a requested tone value of 30%.
Put another way, we are looking for what requested tone value in our new presswork delivered the same final tone value in the target presswork. In this example a 30% tone request in the new presswork delivered the same tone value as a 50% request in the old while a 50% request in the new gave the same tone as a 70% request in the old.

Here's another way to visualize it:Target 150 lpi compared with FM tone response.
Remapping the tones is simply doing this:Find the tone in the new presswork that delivers the required tone response in the old target presswork.

The comparison between target curve and new current curve is made for each 10% change in tone.

The idea is then to map these values so that a tone request in the original file gets changed to a new value that produces the same final tone as the same tone request did in the old target presswork. The result is a lookup table for tone swapping.

In this example:

The requested 10% tone is remapped to request for a 4% tone
The requested 20% tone is remapped to request for a 10% tone
The requested 30% tone is remapped to request for a 18% tone
The requested 40% tone is remapped to request for a 24% tone
The requested 50% tone is remapped to request for a 30% tone
The requested 60% tone is remapped to request for a 40% tone
The requested 70% tone is remapped to request for a 50% tone
The requested 80% tone is remapped to request for a 65% tone
The requested 90% tone is remapped to request for a 80% tone

The lookup table creates the dot gain compensation plate curve.
The lookup table is applied in the workflow to remap the requested tones to the actual tones on plate that will deliver the desired final tones in the presswork. The result is tonal alignment of the presswork despite differences in dot gain.On the left is the original target 150 lpi tone response. On the right is the "normalized" tone response of the FM screen.
Some points to keep in mind

1 - It does not matter if the plates are initially run "uncalibrated" or linear for the target presswork.
2 - A dot gain compensation plate curve is not usually applied to the tone range from 0%-5% and 95% to 100%.
3 - One dot gain compensation plate curve is usually applied to all process colors.
4 - There may be a need to apply a specific dot gain compensation plate curve to one of the process colors to maintain gray balance.
5 - Dot gain compensation plate curves cannot compensate for differences in gamut between FM/Stochastic screens and conventional AM/XM screens.

All about Dot Gain/TVI (Tone Value Increase)

When a tone value is requested, for example, in a page layout application:it becomes represented by a halftone dot pattern generated in prepress by the workflow RIP (Raster Image Processor)
which is then imaged onto a printing plate
which is then inked
and, in the case of offset printing, transferred under pressure to the blanket
from which, again under pressure, the inked dots are transferred to the substrate – paper in this example:
As the halftone dots move through each stage of the process they are altered slightly. "Dot gain" is the term that is used to describe the difference between the requested tone value in the original application file and the resulting apparent final tone value on the substrate as measured with a densitometer. In a film-to-plate workflow, the requested tone value is usually considered to be the tone measured on the film, rather than the tone requested in the application.
Dot gain is inherently neither good nor bad, it is simply a characteristic of a process that uses pressure to transfer an ink to a substrate.
By convention, total apparent dot gain is an incremental, or add-on, increase in apparent dot area – not a multiplier. For example, 18% dot gain means that the tone value, e.g. 50% on the plate has resulted in a 68% tone value in the final presswork (50% + 18%) rather than 59% (50% x 1.18).
Traditionally, dot gain is used as a process control metric, i.e. for a given tone request there is an expected, target, apparent dot gain value – e.g. for a requested 50% tone the expectation is the presswork will gain 18% resulting in the 50% request measuring 68% on the press sheet. If the target dot gain is not achieved then the print process is examined to determine the cause for failure.

There are two primary components of dot gain; mechanical (the physical spreading of the ink under pressure), and optical (the effect of light scatter within the substrate around the perimeter of the dot) - effectively the shadow of the dot within the substrate.The physical and optical dot gain combined are loosely referred to as "dot gain" and, although a reasonably appropriate term, it can be misunderstood and misinterpreted. "Tone value increase," or simply TVI, is a more appropriate term that better describes the over-all effect and is a term gaining in popularity. From a measurement point of view dot gain and TVI are identical.
Because of the optical gain component, it is not possible to directly measure dot gain/TVI. The method most commonly used involves using a densitometer to measure and compare a patch of solid ink (100%) to a specified tone patch (e.g. 50%) with the application of a formula to calculate the total apparent dot gain/TVI.Because of the complexity of dealing with the non-linear mechanical and optical components of dot gain, many formulas have been proposed to calculate total dot gain/TVI, including; the Demichel/Neugebauer equations, the Murray-Davies equation, the Yule-Nielsen equation, the Clapper-Yule equation, the Huntsman model, etc. Currently, the most popular equation, although faulty, none the less is the one that is built into most densitometers - the Murray-Davies equation. Since different formulas give different results, the important thing is to make sure that when discussing dot gain/TVI values, the formula that was used is also communicated.

Historically dot gain/TVI target values for presswork were provided for the 50% tone only. These values, for a 175 lpi halftone screen were: C 20%, M 20%, Y 18%, K 22% at their appropriate solid ink densities. Unfortunately, defining a single tone value as the target for dot gain/TVI does not indicate the actual appearance of the actual presswork.
For example, here is the theoretical print characteristic plotted using only the published 50% dot gain/TVI target values:
However, measuring and plotting dot gain/TVI at several points through the tone scale defines the "print characteristic" and provides a much more effective appearance-based target for the presswork. Here, based on an actual press run using a 175 lpi AM screen, is the print characteristic targeting the same published 50% dot gain/TVI target values:
In order to achieve better alignment between proofs and presswork as well as presswork from different locations, industry standards and specifications are increasingly adopting appearance-based targets (i.e. a specified print characteristic) for press work. The goal is not to achieve a specific dot gain/TVI at a single tone, but to target the tonal print characteristic irrespective of what dot gains/TVI values are needed to achieve it.

Making the print characteristic the target also enables different screening technologies, such as FM/Stochastic screens that have a different inherent print characteristic, e.g.:to make use of dot gain/TVI curves applied in plate making to align their presswork appearance to the industry target/standard/specification.

Dot gain/TVI can provide more than just the print characteristic, it can also reveal issues with ink transfer. In this example, plotting the dot gains through the tone scale:shows that although the 50% gains are within specification and produce correct grey balance, there are issues with ink transfer in other parts of the tone scale, and hence this print condition should not be characterized/profiled until the on-press issues are resolved. One of the characteristics of proper ink transfer in CMYK presswork is when all four dot gain/TVI curves have a similar contour, are clustered together, and are smooth as in this example:
Having a documented standard for dot gain/TVI also helps print production to forensically determine whether a color shift (as in the right side of this image):is caused by incorrect curves applied to plates, too high SIDs, slur, doubling, pressure/squeeze, etc.

Some of the factors that cause dot variations (dot gain/TVI):

Film: mounting, exposure time, vacuum in plate exposure, development time, development chemical condition
CtP: laser exposure integrity, development time, development chemical condition
Printing plate: material wear and tear, dampening solution quantity, pH value, water hardness, temperature, incorrect tone curves
Inking: ink film thickness, consistency, temperature
Printing plate/blanket: packing, type of blanket
Printing substrate: coated, uncoated, surface texture
Presswork: ink transfer, pressure/squeeze, slur, doubling, offsetting, over/under emulsification


Below are the historical dot gain/TVI value (and SID) targets for different classes of presswork. These values are based on densitometers set to Status "T" black backing, measured dry.

Sheetfed offset:
Grade 1 & 2 premium gloss coated @ 175 lpi:
C: 1.40/20%, M: 1.50/20, Y: 1.05/18, K: 1.70/22
Grade 1 & 2 premium matte coated @ 175 lpi:
C: 1.30/22%, M: 1.40/22, Y: 1.00/20, K: 1.60/24

Web offset (SWOP):
Grade 3 & 5 coated @ 133 lpi:
C: 1.40/20%, M: 1.50/20, Y: 1.05/18, K: 1.70/22

Newsprint (SNAP):
Coldset @ 85 lpi:
C: .90/30%, M: .90/30, Y: .85/28, K: 1.05/32
Heatset @ 100 lpi:
C: 1.08/32%, M: 1.15/32, Y: .95/30, K: 1.20/35