How to extend the life of your inkjet proof

A great deal of effort is put into the making of a contract quality inkjet proof. However, if it is not properly taken care of, its useful life can be shortened and its integrity as a color reference compromised.

How the inkjet media and proofs are stored and handled, as well as their exposure to ambient office or light booth lighting, affect the color of the proof and its color integrity over time.

Here are a few simple things you can do to help maintain, or extend, the color integrity of your inkjet proofs.

• Depending on the surrounding ambient temperature and relative humidity, it can be several minutes, or even hours before an inkjet proof is dry enough to handle. So, before handling a proof, make sure that the ink is actually dry.

• Handle the proof by its edges to avoid leaving fingerprint smudges on the imaged area. Fingerprints may also leave residual oils on the media which may repel ink when the proof is imaged. After the proof is imaged fingerprint oils may dissolve water-based proofing inks leaving unsightly marks on the proof. Try to always use lightweight cotton gloves when handling proofs.

• Avoid bending the corners of the proofing media.

• If you will be stacking proofs, always place a light-weight slip sheet between the proofs.

• Store proofs in cool, dark, low humidity shelves or envelopes away from light or dust as inkjet proofs will fade with exposure to ambient office or light booth lighting.

• Keep all unimaged media in their boxes when not in use and store in a cool dry place away from light and dust.

• Always allow the proofing media to acclimate for 48 hours in the proofing area before printing.

• For optimal results, if available, follow the recommended environmental conditions provided by the proofing media manufacturer.

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.

Standardize your print process

A standardized print manufacturing process is critical for achieving effective press to proof color alignment, short press makereadies, and minimal wastage of time and materials.


The five basic steps are:

1. Choose the appropriate standard and/or specification target for the presswork.
Management's responsibility (with input from prepress, press room, and sales) is to establish what the targets and tolerances (dot gains, standards, specifications etc.) are for the presswork - because those are marketing/business decisions. Then management must provide the tools to allow prepress and pressroom to achieve those targets. That target then drives the complete print production process and determines the selection of all of the components (proofs, press calibration, inks, paper, etc.) needed to hit the target. Worldwide the most adopted standard for printing is ISO 12647-x. The various ISO 12647 standards provide specifications for data, proofs, inks, paper and press calibration. In North America GRACoL and SWOP specifications are largely based on ISO 12647-2 and ISO 12647-3 respectively.


2. Clearly communicate with print customers about requirements for data/file submission.
This includes items such as whether files should adhere to industry specifications such as SNAP or face rejection. It may include items such as whether native application files are acceptable or only PDFs, what resolution and color mode that images need to be, what minimum text size is acceptable, etc., etc.

3. Implementation of standardized proofing.
In-house proofing must be set up to the same standard and/or specification target for the presswork (i.e. ISO 12647-x, SWOP, etc.). If print customers will be providing proofs for the pressroom then the printer has to communicate that customer-submitted proofs must adhere to the appropriate standard. All proofs must include a color control strip so that they can be verified for conformance to the target standard/specification. There should be virtually no difference in appearance between a customer-supplied proof and one done by the printer in-house. It should also be made clear that the printer's in-house proof is the final proof.

4. Optimize prepress.
The responsibility of prepress is to align proofing to the target established by management as well as to maintain proofing within the tolerances established by management. Prepress must also output plates that enable the press operators to align their presswork to the proofs with the press performing in a repeatable, stable condition. Prepress should have well defined procedures for ensuring that platemaking and the resulting plates are consistent and within tolerance.

5. Optimize press performance.
The responsibility of the press operator(s) is to manage the press in such a way that the the films of the appropriate inks (as defined by the chosen specifications) are laid down in a manner that meets the targets and tolerances (hue, trapping, etc.) established by management and that the halftone dots on the plate are reproduced with fidelity on the various substrates (slur, doubling, etc.). Also, the press operator needs to make sure that all press-related consumables (fountain solution, inks, etc.) are within the tolerances needed to achieve the management defined targets for pressroom output.

Conducting The Press Check

Printing is a completely different imaging process than proofing. As a result, it is not always possible for the presswork to perfectly "match" the proof. Unfortunately, there is no objective, practical, way of defining what is an acceptable variation from the proof that still constitutes an acceptable pressrun. The role of the press check is to enable the customer, or their representative, to directly communicate their presswork concerns and acceptance, with the press operator, so that a successful pressrun can be achieved.

The customer should be able to go to the press and speak to the press operator directly. Working through an intermediary like the pressroom manager only slows down the process and causes communication errors especially when if they are managing multiple press checks at the same time.

Press check basics for the printer

• The customer should not be press side during initial make ready. Ideally you would have a comfortable holding area/lounge where customers can wait until the press operator has a sheet ready for inspection.

• An initial make ready sheet marked clearly "For Content Only" could be given to the customer so that they can check the sheet for content issues like substituted fonts, low resolution images, missing graphics, etc. This reduces time wasted at the press when doing the color approval. Also, if content errors are discovered then the job can be halted soon enough to avoid excessive make ready wastage.

• The press operator should be told ahead of time, by the CSR or sales representative about issues, concerns, and critical success goals the print buyer has for the job. For example, if a particular blue in a product image is important to the customer - the press operator should know that fact. Having this knowledge shows the customer that the shop is personally involved in the success of both the press run and the customer. It also helps establish better communications, if any color adjustments need to be made.

• When the press operator is satisfied with the color then a press sheet should be pulled and filed as "first press operator OK." Press operator OK'd color sheets can be used later by the printshop to identify and evaluate print manufacturing issues from prepress through the pressroom.

• When the customer arrives at the press, the press operator should introduce themselves.

• When the customer is at the press the press operator should stand aside and allow the customer "breathing room" to examine the press sheet against the proof.

• Tools such as Pantone swatchbooks, ink draw downs, loupes, note paper and pens for customer use should be readily available.

• The press operator should try and use correct terms and use them consistently. For example, do not use the term "blue" if you mean "cyan."

• Encourage the customer to explain what concerns they have rather than tell you how to fix them. Their role is to identify the problem - your role is to know whether, and how the problem can be fixed.

• If you make a press adjustment to fix an issue, tell the customer what you will be doing and how you feel it will fix the problem. This helps confirm to the customer that you correctly understood their issue and it also helps educate them which, in turn, will make future press checks go quicker.

• If there are any issues that you have with the press to proof match - let the customer know that right away rather than have them discover it themselves. Doing so tells the customer that you are not trying to hide anything from them. It also helps establish a mutually respectful environment.

• Be aware of time - every minute the press is idle the company is losing money. Remember, you can get press sheet sign-off with notes to cover some issues (e.g. "OK for color - but must remove all circled spots and hickies). Don't rush the customer, but don't let them dally at the press either.

• Some shops will have a "light room" where customers can take the sheet to view it under "standard" office lighting. This helps mitigate metamerism and substrate fluorescence issues.

• Make sure that the OK press sheet is signed and dated by the customer, the sales representative/CSR, and the press operator. Any continuing issues to be dealt with later should be noted on the sheet.

• Have a cardboard tube ready so that OK'd press sheets can be taken away by the customer.

• Thank the customer for attending the press check.

Press check basics for the print buyer
The role of the print buyer at the press check is to directly communicate their presswork concerns and acceptance with the press operator in the presence of the sales person to help ensure that any color issues are dealt with according to the customer's requirements.

• Once you get the call from the printer, gather up any material related to the job, samples, proofs, spot color draw downs, paper samples, mock-ups, folding dummies, etc. Also, make sure to bring loupes, color swatchbooks, scissors/x-acto knife, pens and notebooks.

• It's a good idea to get an imposition proof from the printer so that you can check three things; are there any potential inline color issues, are pages on the press form imposed the same way as they were on the imposition proof, and finally when the sheet is backed up on press do the pages back up correctly.

• Arrive at the printshop on time. Identify yourself at reception and explain why you are there. Then wait for the sales rep or CSR to be escorted to the waiting lounge or press floor.

• If you are part of a group attending the press check, identify which single individual will be the lead. That is the one person who consolidates the opinions of the folks attending the press approval and therefore the one who speaks for the group to the press operator.

• While waiting to go out to the press floor, ask for an initial make ready sheet that can be checked for content issues like substituted fonts, low resolution images, missing graphics, etc. This will reduce time wasted at the press when doing the color approval.

• At the press, introduce yourself and your team to the press operator. Wait to be invited anywhere near the press console.

• Do not touch or use any equipment at the press unless you specifically ask permission first.

• When you are offered the sheet for examination, ask the press operator if they are happy with the sheet and if they have any concerns/issues with it.

• Engage your sales representative for input and guidance with any thoughts/concerns you have. Tap into their experience.

• Recognize that time equals cost so be focused on the task at hand. If you are working with a team, assign checking roles to each. For example, someone checking registration, another checking for hickeys/specs, low resolution photos, swapped or dropped fonts, etc.

• If you are alone, have a written, organized, step by step yes/no, pass/fail procedure to checking the press sheet . Typically the process goes like this:
1) Is it printed on the correct paper? If a specific paper grain direction was required ensure that it is running in the correct direction.
2) Is it in register?
3) Over all, does the press work color align with the proof? Are there any obvious color issues?

4) When critical color alignment is required, cut the press sheet through the important color and overlay that section of the press sheet on the proof. Colors that may appear correct when compared side by side may appear different when directly overlaid.If you cannot clearly see where the press sheet ends and the proof begins you know you have a critical match.

5) Use your reference material to confirm the correctness of special/spot/brand colors.
• If you have any color concerns/issues, try to describe them clearly and unambiguously. Then describe just as clearly and unambiguously what you want to see. Do not tell them how to fix the issues. Your role is to identify any problems - it is the press operator's role is to know whether, and how any problems can be fixed. You can ask whether a solution you thought of might solve the problem. For example, you could say: "I think this area is too red. Would reducing the Magenta a touch fix it?" Phrasing a suggestion as a question can also help your press operator better understand your meaning according to how you describe the problem.

• Try and use correct and unambiguous terms and use them consistently. For example, do not use the term "blue" if you mean "cyan." Try to avoid terms like: "This area is too hot" or "Can you punch it up a notch?"

• If a press adjustment is made to fix an issue, ask the press operator what they are doing and how they feel it will fix the problem. This helps you to better understand the print production process, and its limitations, better.

• Most press operators will try very hard to achieve what you’re looking for, however, once they've made their press moves and you are still not satisfied it will be up to the sales rep to authorize trying anything else. If it's a really serious issue, the sales rep may stop the press and pull the job.

• Keep in mind that the start/stop/start/stop press cycling during a press check means that the press is not yet running in a stable fashion. Once the press OK is complete and the press is running at optimal speed, some small color issues will clear up by themselves.

• Be aware of time. Respect the printshop's need to maintain their production schedules. Remember, you don’t need to remain until the sheet is absolutely perfect. Just mark it as “OK with changes as noted.” (e.g. "OK for color - but must remove all circled spots and hickies).

• Some shops will have a "light room" where you can take the sheet to view it under "standard" office lighting. This helps mitigate metamerism and substrate fluorescence issues.

• Make sure that the OK press sheet is signed and dated by the person in your team who has authority to take responsibility. Any continuing issues that are to be dealt with later should be so noted on the sheet.

• Ask for a few copies of the OK'd sheet to take away with you for your records.

• Thank the press operator and crew for their performance during the press check. They really appreciate it and will remember you in a positive light during your next press check.

How CtP plate readers read plates

Dedicated plate readers take a different approach to measuring dot area than densitometers. What they do it use an internal CCD camera to take a continuous tone photograph of the plate area at high resolution and magnification:

Depending on the instrument's software and display technology, this original image may or may not be shown to the user.

In order to calculate the dot area, virtually all plate readers use a thresholding algorithm to determine what is the non-printing plate and what is the printing dot.* Put another way, the software decides that a pixel of X tone level and lighter in the captured image is the plate while levels darker in the captured image than X tone level are identified as ink carrying dots.

However, because the halftone dots in the photo will have a slight softness to them, the result is a slight ambiguity as to where the transition from non-image plate to ink carrying dot occurs. Change the threshold and you change the size of the dot the instrument "sees" and hence the tone the instrument reports.

For example at a threshold value of 139 (from a range of 0-255), the software sees this as the printing dots:
and reports a dot area value of 50%

If the threshold is a tone value of 175 the software sees this as the printing dots:
and now reports a dot area value of 55%

So which is best, or more accurate, for measuring halftone dots on plate? Well, in a GATF study of products for measuring the dot area on CTP printing plates (RTR 27: Plate Reading Technologies and Their Performance on CTP Plates) a spectrodensitometer gave very similar overall results compared to the best CCD-based solution for plates.

In their defense, dedicated plate readers can provide more information, such as halftone frequency and screen angle, than a densitometer when measuring plates.

* The extremely sophisticated and expensive Jandel planimeter requires the user to trace where they believe the dot edges are in order to differentiate between plate and dot areas.

Using a densitometer to measure plates

Despite what you may have heard from vendors - yes, you can use a densitometer to read offset plates. This can be especially helpful for the smaller printshops who may not have the resources to purchase dedicated plate readers.

Densitometer basics

Color reflection densitometers are designed to accurately measure Black, Cyan, Magenta, and Yellow colorants on substrates like paper. However lithographic plates usually don't have C, M, Y, K images on a white substrate, instead the plate material is usually brushed aluminum with a grey color. The color of the image itself will vary according to the plate type and vendor.

You need to be aware of these reflectance and color properties since they effect how you use your densitometer when evaluating your plates.

Contrast

Because of plate color properties, the contrast between image and and non-image areas of the plate is typically less than half that of ink on paper, as a result it may not be possible to calculate dot area for some densitometer/plate combinations.

Aperture size

The densitometer aperture (measuring window) should be the largest possible for your brand of instrument - as long as it's not larger than the image area you are measuring. This helps average out the reading and minimizes the effect of random plate patterns.

Densitometer Status

The instrument should be set to "Status T" unpolarized in part because this setting has a wider response to the various plate coating colors.

Consistency is more important than absolute accuracy

Conventional and digitally imaged plates typically use the same aluminum base material. However the different types of mechanical or chemical graining that prepares them for lithography results in a different surface texture which in turn scatters light differently. In order to achieve consistent dot values. To help achieve consistent dot value measurements, you should try to maintain a consistent alignment of the densitometer to the plate. Perhaps using the base of the densitometer aligned to the edge of the plate. Also, zero the densitometer and read the 100% area on the same spot each time and take readings from the center of each target patch.

If you know that a patch on the plate represents a 50% tone (i.e. it is a checkerboard) but your densitometer reports the patch being, for example, a 54% tone while the plate prints correctly on press then 54% becomes the target for that patch on the plate. The idea is that you are controlling your plate imaging process by monitoring and minimizing variation. Your priority is to maintain consistency in the measured dot areas.

Alternatively you could change the dot gain calculation "N" factor. Densitometers use one of two formulas for measuring dot area; the Murray-Davies or the Yule-Nielson equation. They are the same equation however the Yule-Nielson equation utilizes an "n"-factor to "factor" out the optical dot gain (the Murray-Davies equation is equivalent to the Yule-Nielson with n set to 1.00). The n-factor is experimentally determined by adjusting it until the densitometer reads the ‘desired value’ at a known dot percentage. Typically a 50% tone is used because it is easy to spot. You'd print a number of tone values close to 50% (e.g.45% and 55%) then use a loupe to find the tone patch that ‘looks like' a 50% tone - a checkerboard. Next adjust the n-factor on the densitometer until that patch reads 50%. The n-factor is typically applied when a densitometer is used to measure printing plates since printing plates are assumed not to have any optical dot gain.

Grey Balance Unbalanced – An inconvenient truth

Warning: This post may be considered by some, perhaps many, as heresy, sacrilegious, and blasphemous – read it at your own peril.
Achieving grey balance in presswork is touted as the holy grail in today's print production world. It's become the paramount metric - recently defined in unambiguous CIE L*a*b* values - for the G7™ calibration method to align press and proof color as well as for achieving presswork that conforms to GRACoL® 7. Interestingly, despite its promotion by industry pundits, as far as I can determine, there has never been a formal, objective, study of the relationship of grey balance targets measured in press sheet color bars and how they relate to the live image content of the press sheet.

While grey balance certainly has value - particularly in the initial setting up of a print production system - the grey balance targets in color bars, IMHO, have virtually no value as a print reproduction metric in day to day offset printing.

The industry-wide assumption is that the three-color grey balance patch of the color bar comprised of 50C 39M 39Y, performs and shifts identically to the four-color grey balance of a live image comprised of approximately 26C 18M 18Y 32K (at the same tonal density).

Put another way – on press, the assumption is:
I don't think so.

For one thing, the tone values 50C, 39M, 39Y are where dot gains/TVIs are at their greatest and where on-press tonality (and therefore color) is much less stable than at the 26C 18M 18Y tone values – smaller dots are proven to be tonally more stable than larger dots (which is why dot gain is measured at 50% where dots are at their largest).

Also, the 32K in the separation provides a great deal of grey balance color stability in the imagery which doesn't exist in the color bar grey balance patch.

Bottom line:
The grey balance target in the color bar is overly sensitive to minor solid ink density variations in presswork and has no tonal relationship with the screen tint values that are its supposed equivalent in CMYK separations. This is even more true in workflows that use ink optimization heavy GCR separation techniques) on incoming files. As such, the grey balance targets in color bars cannot reflect color shifts that may be happening in live presswork areas.

Put another way – on press, the reality is that:
There are other issues with grey balance being used as a process control metric in production presswork besides the mismatch of screen tint values compared with greys in the live image areas.

The inline ink usage issue

Unlike grey balance in scanning and proofing - grey balance on press is affected by ink usage/coverage. For example, in this press form:The grey patch targets in line with the blueberry image will have a distinct blue/purple cast due to the lack of yellow in the image as well as the need to push cyan and magenta densities to deliver a rich blue in the live image area. On the other hand, the grey patch targets in line with the tomato will have a redish cast due to the lack of cyan in the image and the need to push magenta and yellow densities to deliver a rich red in the live image area.

Bottom line:
The live image areas can match the proof while grey balance on the press sheet do not.

The human visual system issues

C, M, Y grey balance targets are typically located beside a corresponding 50% patch of black in the color bar. The assumption being that the press operator, by comparing the two, can make a quick visual assessment of grey balance in the presswork. There are some significant problems with this assumption.

First of all is that the visual perception of grey is affected by several factors:

1) Unlike instruments, we perceive color in the context of surrounding colors. If you look at the grey bar in this graphic:you'll see that it appears to change its tonality depending on whether it's against the red or green background even though the grey is in fact exactly the same across its width. You may also see a slight shift in color. This optical illusion is called "simultaneous contrast."

Bottom line:
Even if the grey patch in a color bar appears perfectly neural compared with the 50%K patch - the grey within the live image area may not appear neutral due to the effect of surrounding colors.

2) Our perception of color, especially neutral grey, not only varies from individual to individual, but is also affected by age and gender. Older men (e.g. typical press operator) will not see a 3/C grey vs a 50% K grey the same as younger women (e.g. typical print buyer). You can test this yourself with a chart like this one:On the left is the reference 50%K. On the right are a variety of 3/C greys. Try and pick out which one is identical to the reference grey. In fact, if you were to print out the patches, cut them into squares, and asked co-workers to match the 3/C patch to the 50%K patch you'll most likely see the pattern of men/women, younger/older, choosing different 3/C patches that they see as perfectly matching the 50%K one.

Bottom line:
The visual match assessment of 3/C grey and 50% K is inconsistent between people particularly buyers and suppliers.

Lack of instrument agreement

According to a report by Greg Radencic, PIA/GATF, on Spectrophotometer Device Agreement presented at a recent Splash technical color conference, out of seven spectrophotometers tested, four had a DeltaE error over 1.5 when measuring grey balance on 80lpb gloss coated paper. One instrument had a Delta E error of 1.75, another 3, another 4 and the last had a whopping Delta E error of 7 when measuring grey balance.
The seven instruments were factory new and calibrated.

Bottom line:
The measured match assessment of 3/C grey and 50% K is inconsistent/unreliable between different types of spectrophotometers. Even different instruments of the same model may disagree.

The metameric issues

One final issue is that a chromatic 3/C grey (50C, 39M, 39Y) and an achromatic 50% black form a metameric pair. What this means is that they may match color under one lighting condition, however, under a different lighting the 3/C grey will shift hue because it is spectrally different than the 50% black. To illustrate:
This mismatch due to lighting conditions can be exacerbated between inkjet proofs where the 50% black patch may be made up of 4/C and presswork where the 50% K is only made up of black ink.

Bottom line:
Metameric issues can arbitrarily and continuously cause 3/C grey patches to mismatch against the 50%K reference patch as viewing conditions change.

The issues of Optical Brightening Agents in paper and ink

As ICC color managed workflows become more prevalent in the graphic arts, so do the difficulties caused by OBAs (Optical Brightening Agents) that are encountered in the pressroom, prepress, and their customers. OBAs are used to increase the apparent brightness and whiteness of papers and their use is becoming more prevalent in paper manufacturing. They increase brightness and whiteness by absorbing energy in the ultra violet and emitting (fluoresce) the energy in the blue area of the visible spectrum. Because, to the eye, blue/white looks "whiter" than yellow/white OBAs are not really whiteners, but bluing agents. OBAs are also used in ink to expand gamut or brighten 4/C image printed on poor substrates - e.g. newsprint.
When fluorescence is present, the light coming from the sample is the combination of the light that is reflected and the light that is fluoresced. ISO 12647-2 printing paper grades specifies low OBA content, however, there is no specification as to the amount of OBA content. And, although they typically use light sources with little or no UV radiance, there is no specification describing the UV content of light sources in measurement instruments such as spectrophotometers. The addition of fluorescence to either the inks or the substrate greatly increases the level of uncertainty in instrument readings of the optical properties of printed images. This, in turn, may lead to a significant lack of reproducibility between two imaging centers that attempt to apply color management principles to their individual measurements of the same image printed on various substrates.

While it is not practical for printers to quantitively measure the OBA content of the materials that they use, it is quite an easy matter to qualitatively see the OBA content. All it takes is an inexpensive (less than $15 USD) "black light" such as the one illustrated below (do not bother with incandescent black lights):For example, with the black light it is easy to see that the paper used for the Pantone Goe system swatch book (on the left in the image below) contains more OBAs than the conventional Pantone spot color swatchbook on the right. Also, it's clear that the uncoated paper section in the Pantone spot color swatchbook contains more OBAs than the coated section.The bottom line – the significance of which will be more apparent in the next parts of the blog on this topic – is that, although you may not be able to do anything about it, just being able to be aware of OBA content can help solve issues related to their use.

OBAs are sometimes use as additives in ink. In the example below it has been used in the yellow ink in a process set – while the paper itself contains little if any OBAs.Using OBAs in the yellow ink is a common strategy with newspaper printers as a way to add brightness to imagery and compensate for the poor whiteness of newspaper stock. The fact that the OBAs fade and cause a color shift over time is typically not a concern in that market. It is important to be aware of the OBA content of your process ink set particularly if the print specifier is concerned with the longevity of their printed materials - especially if they will be exposed to sunlight.
OBAs are also used in so called Hi-Fi inks, notably those used in the Pantone Hexachrome process, to add vibrancy and expand the gamut beyond conventional four color process. However the OBAs can make the inks more problematic in the pressroom as well as result in presswork that does not have a long shelf life (due to fading and color shift).

The inks that are typically used in four color process printing block, to varying degrees, the fluorescence in papers containing OBAs. Black and magenta block the greatest amount, yellow a lesser amount, and cyan ink least of all. What this means is that when an image is printed using a halftone screen, lighter/pastel tones allow more more of the brightening and color shift of OBAs (towards blue) than the shadows. Color is effectively skewed towards the blue from shadows to highlights – but only when the paper being printed on has a high OBA content.In daily presswork this disconnect usually appears in midtones and pastels – sometimes the color matches the proof in those areas and at other times the color doesn't – depending on the OBA content of the paper being run. FM screenining will lessen this effect. The effect on an AM screen is emphasized in the image below to illustrate the issue.
The use of OBAs in paper has a significant impact on the reliability of proofing and alignment of presswork to the proof. This is an issue where the use of a black light really "shines". When the proofing paper contains OBAs the hue of pastel colors can shift depending on the amount of UV being emitted by the viewing light source as illustrated below:Although they cannot control the light under which their customers evaluate proofs, many printshops will use UV blocking filters to cover the D50 bulbs in their viewing booths. The notion is that, most of the time, the proof will be looked at by the customer under lighting with little UV content. The UV block filter helps the press operator to "ignore" the presence of OBAs in the proof/presswork.
Another strategy is for the printshop to try and align the OBA content of their proofing paper and press sheets. A black light can provide a qualitative measure of the OBA content of the media as illustrated here using a vendor's swatchbook of their proofing media:Selecting pairs of press and proofing papers according to their OBA content helps in the alignment of presswork and proofs and thereby enhances the printer's ability to set expectations correctly with their customers.

The "Golden Reference"

Once in a while, even in the best-run printshops, production can go sideways, go south, foul up, mess up, screw up, and go wrong. When that happens a well run printshop turns to their "Golden Reference" as one tool to diagnose the problem and determine a solution. A Golden Reference typically it consists of a file, press sheets, proofs, plates, and documentation. It preserves a record, or "snapshot," of the prepress and print conditions when everything was working correctly. As such, when a problem occurs, the Golden Reference file can be rerun through the workflow and even on press if required. Comparing how the Golden Reference performs at each stage of the current process to the original can help isolate where the differences are and therefore reveal what may be causing the live job to go wrong. The Golden Reference is also useful for validating consumables - such as ink and plates – which may change from batch to batch. It is also useful as the standard by which new processes – such as a change in halftone screening, or a new ink type – can be compared and introduced.
Ideally, the Golden Reference test form should combine objective as well as subjective elements. Sometimes it's easier for evaluation if they are kept separate on the press form as in the sample above. Here are some suggestions for test elements you may want to include (click on image to enlarge).
Remember to make sure that, as much as possible, inline ink usage conflicts and/or influences should be avoided. Also remember that a test form like this should be run "to the numbers" - no proof should be anywhere near the press that runs it. The press operator should be instructed to achieve, as much as possible, the correct solid ink densities across the sheet and not try to make the images look "nice."
A - Standard color bar including targets for CMYK solid ink density, RGB trap, dot gain
B - Background 25C, 18M, 18Y gray with 25 K checkerboard. The primary function is to even out ink usage on press. Can also be used to check across the plate imaging consistency.
C - Solid bars of CMYK in line with the press sheet direction of travel to test solid ink density evenness around the cylinder.
D - Your standard profiling target.
E - CMYK step wedges used for building dot gain curves and ink performance curves.
F - Industry standard target.
G, H, I - Gray balance ring-around targets used to determine whether gray balance is being achieved at the required tone value combinations.
J - Gray balance targets.
K - Industry standard images.
L - Your standard images separated according to your specifications.
M - Image resolution test. The same image is sampled at 100, 200, 300, 400, 600 dpi to show minimum image resolution required for quality reproduction at your halftone line screen frequency. Also useful when changing screening type and/or frequency (lpi).
N - Max black/total area(ink) coverage target. Used to determine maximum TAC.

Aligning proof and presswork

The process of aligning proof and presswork generally follows the basic steps outlined below. This procedure applies primarily to Strategy One as outlined in my January 18, 2009 blog posting. However, it can certainly be modified to for Strategies Two and Three.

01) Identify the target for your presswork. This can be an internal standard or industry specification (e.g. ISO, GRACoL7, SWOP, etc.)

02) Get consensus and buy-in from the stakeholders (prepress, pressroom, management). Change is always disruptive, however, if everyone affected is on board you will have a much better chance at success.

03) Determine who's going to be the lead implementer (i.e. internal resource or outside consultant will be used). Be honest in your evaluation of internal competence. Recognize that if you do it yourself, even if you fail, you will get a greater understanding of the process and therefore be in a better position to evaluate an outside resource should that end up being needed.

04) Open a docket. The process will consume company resources which need to be accounted for. Also the project will need to be treated seriously and given the proper time and involvement. Dockets help to ensure that happens.

05) Contact your ink vendor to determine if your current inks can achieve your target. If not, identify with your supplier, an ink series that has proven capable.

06) Contact your proof/workflow vendor to determine how to set up your proofer to output proofs that align to the industry specification you have decided on.

07) Find a paper for your press that aligns to the brightness defined in the specification. Try to find one that has a similar UV optical brightner content to your proofing media. You can use a $10 black light to do a qualitative comparison. Make sure the bulb is fluorescent not incandescent. Proof and press paper should glow, or not glow, by about the same amount. If there is a big difference with your press/proofer paper in terms of optical brightners used you will have problems.

08) Confirm that instruments used in prepress and pressroom are in proper working condition, calibrated, set to the same standard, and agree with one another.

09) Confirm that your CtP device imaging is consistent, across the plate and plate to plate. Consistency is more important than accuracy.

10) Test the press. This involves running a test form, preferably without images or proof, to determine the mechanical/chemical soundness of the press. If the press not stable that will need to be addressed first.

11) Assuming that #10 was a success (with your inks and papers) and that you were able to hit the required CIEL*a*b* values at the appropriate solid ink densities - then build plate curves to align the tone response of your presswork to your proof.

12) Go on press to confirm alignment between proof and press sheet. For this test form you can add subjective reference images to your objective measurable targets.

13) Assuming that #12 was a success. Catalog and document plates, proofs, and presswork. This will become your "golden reference." If anything goes wrong in future production, you would run this golden reference to determine what has changed.

14) You may, optionally, characterize your presswork and use the resulting profile to fine tune your proof to your press that is now conforming to the industry specification.

Press and proof alignment strategies

Strategies for answering the critical question: should your press color align to your proof or should the proof color align to the press?

• Strategy One: Targeting an Industry Defined Specification - the press aligns to the proof

If you do not control 100% of the digital files entering your pressroom it would be best to target an industry-defined specification as a way of bringing some degree of order to the various inputs that you receive. This strategy also allows prepress tradeshops and production graphic designers to prepare image scans and digital artwork appropriately even when they don't know which printer will be doing the final presswork.
This strategy is most appropriate in a distributed printing environment where prepress and scans from a variety of sources must conform to a common print characteristic. Examples are magazine and newspaper advertising or brochures that will be printed at a variety of locations.
Aligning the press to the proof is embodied in the traditional, analog film-based, laminate proofing systems with the most popular implementation for offset printing being SWOP. It is also the process embodied in the GRACoL 7/G7 specification.

• Strategy Two: Proofing to a Shop-Specific Presswork Target - the proof aligns to the press

With this strategy, the presswork color is the target and the proof is aligned to it. This strategy is appropriate in a non-distributed printing environment where prepress and scans are completely controlled by the printer or where the printer has a close relationship with the print specifier. Examples include brochures, collateral materials, annual reports, corporate brochures, art reproduction, and so on. This strategy is often used by printers wishing to differentiate their presswork from their competition. In this case print purchasers are primarily concerned that their particular project looks as good as possible in print. The presswork is unique — either to the individual print shop or even to a specific press in the shop.

• Strategy Three: - Align to Your Customer

Just as one size does not fit all - one print characteristic may not suit all. In today's highly competitive print environment, it can make competitive sense for the printer to leverage the flexibility of digital proofing and plating systems to offer multiple print characteristics to their customers. I.e. both strategy One and Two could be offered as options.
This initially involves greater complexity in workflow, however, once set up can provide the printer with tremendous flexibility in differentiating themselves and better meeting their customer's requirements in print.