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.

The Color Bar

Color bars (a.k.a. color control bars, color control strips, or proofing bars) are essentially test targets that are used to measure print and/or proof attributes. Normally, but not always, it is printed in the trim area of the press sheet.Typical placement of a color bar on an offset press sheet - at the trailing edge (back end of the sheet).
However, it can take many different forms - sometimes hard to recognize - but always serving the same purpose.

Sometimes the "color bar" is incorporated within the graphic design of the publication. In this case the color makeup of the title (Cyan) and section headers (Blue, in this example, - Magenta overprinting Cyan).
Sometimes it is hidden in the spine (in this case the grey line running from top to bottom on the front edge of the photo).
While it is certainly possible to measure the color of the actual live image area, the technology is expensive and, as result, few printers are fortunate enough to have it at their disposal. Also, measuring the live image area doesn't provide as much useful information as a color bar can. Color bars therefore act as proxies, or substitutes, for the live image area as well as provide additional data.

The logic behind color bars

1) Unlike the live image area of the press sheet, color bars are consistent job to job. Therefore they are more efficient at providing a benchmark and can be used to track trends in variation over time.

2) Color bars can be tailored to meet the needs and measurement capabilities of individual print shops.

3) Color bars may be used to measure all aspects of the "print characteristic" - solid ink density, overprinting (ink trapping), dot gain, grey balance, as well as issues such as slur and dot doubling.

4) Color bars can reveal issues with ink hue, blanket condition, impression cylinder pressure, etc.

5) They can be used forensically to help understand why a specific job did not meet expectations.

6) They are efficient since, unlike the live image area, they are a constant made up of well defined elements that continue from proof to press sheet.

Solid ink density
A printing press is essentially a complex machine for laying down a specific film thickness of a specific color of ink onto a substrate. The ink is metered out in zones across the width of the press sheet according to how much ink coverage is required for each color in each zone.Therefore, for most press operators, the minimum requirement for a color bar is that it contains solid patches of the inks that will be printing since solid ink density is the only thing on press that an operator can adjust while the press is running.Those solid patches are then repeated over the width of the press sheet so that each ink zone is represented by at least one complete set of patches - containing one patch for each color being printed.
Information provided by only using solid ink density targets in the color bar
In this example, cyan is misregistered while the black printer is over emulsified (fountain solution breaking down the ink).
1) Provides a solid ink density value, measured using a densitometer, to determine if the press sheet is conforming to published industry, or shop specific targets.

2) Is an indirect, but practical, method of determining optimum ink film thickness and hence the balance of maximum color gamut without introducing image degrading inking issues such as slinging/misting.

3) The balance of the primary solid densities determines the hue of the overprints - i.e. the SID of magenta and SID of yellow determine the hue of the resulting red.

4) Indicate misregistration which can then be examined in the live image area.

5) Reveal defects such as slinging/misting/tailing, over emulsification, slur, doubling.

6) If records are kept, the hue of the ink currently on press compared with the hue of ink used in the past to determine if there is any contamination, change in color due to ink batch differences, etc.

Forensic targets on color bars are image elements that are typically not measured by the press operator unless there is a problem in aligning presswork to the proof. If that happens then these targets may provide useful information as to the cause of the problem.

Two-color overprint ink trapping targets
Ink "trap" is an objective indication of the ability, or inability, of a printed ink to accept the next ink printed compared with how well the substrate accepted that ink. Poor ink trapping results in presswork color shifts in reds (magenta plus yellow), greens (cyan plus yellow), and blues (cyan plus magenta) as well as a loss in total color gamut.
The two-color overprint solids allows for the objective measurement and evaluation of ink trap efficiency as well as the overprint hue error and greyness.
Typical trap values for three print conditions running a CMY ink sequence with Black first or last down:
Offset sheetfed: R=70, G=80, B=75
Heatset web offset (publications): R=70, G=87, B=72
Coldset web offset (newspaper): R=50, G=89, B=50

Slur and doubling targets
Slurring and doubling are print defects that occur when halftone dots and type blur as a result of a slight second contact or movement between press cylinders or the paper and blanket. (More about slur HERE and doubling HERE)
There are many different styles of slur and doubling detection targets. Here are two of the most popular:Of course, every halftone dot or letter character on the printed sheet will reveal slur and doubling, however the targets in the color bar signal the defect easier and quicker.
Grey balance targets
Grey balance targets are made up of a patch of three screened process colors that are balanced so as to appear as neutral grey under standard printing conditions. They are typically printed adjacent to a black screen tint of a similar value to allow for a quick visual, or measured, evaluation of how grey balance has shifted.Grey balance targets can be useful since variation in any of the three process colors because of dot gain, slur, doubling, density, trapping, and registration will be reflected by a shift in hue away from neutrality. The 3/C patch will take on a bluish, reddish, or greenish color cast.The idea behind this target is that any grey balance color shift away from neutrality suggests a possible color shift in the live image area. However, in production printing the grey balance target may not be a reliable indicator of presswork issues.

Other targets
Other targets that may be included in the color bar are:

Dot gainThese targets are intended to capture dot gain variation information. The dot gain targets may consist of just two patches for each process color to measure the dot gain a one location on the tone scale, or, with the addition of more patches, to measure the dot gain at the quarter, mid, and three-quarter tone values. Dot gain can be useful because issues like slur, doubling, or incorrect solid ink density, will be reflected by a variation in the measured dot gain.

Brown balance targets
Brown balance patches are similar to grey balance patches in function except that they are made up equal percentages of cyan, magenta, and yellow. Unlike grey balance patches which allow the press operator to make a subjective visual assessment of hue shift, brown balance patches can only be evaluated objectively with instruments.

ProprietaryProprietary targets such as that used by System Brunner are typically used to drive on-press closed loop color control systems.

Spot colorIf a spot or brand color is being used then it will warrant at least a solid patch in the color bar so that its solid ink density can be measured. Space permitting, the solid patch will be adjacent to a screened back patch so that dot gain information can be measured.

For process control, color bars should be included on every proof and press form of every job. If that is not possible because there is no room on the sheet (as often happens in newspaper work) then there are several options;

1) Run color bars on occasion by including it in the live image area.With the publisher's permission if required.

2) With the print buyer's permission, incorporate color bars test elements into the graphic/editorial design of the printed piece (see the USA Today example in Part 1).

Color bars are not a requirement for quality printing, however, they are key to making proofing and printing more efficient and effective while reducing overall production costs.

Presswork should be run "to the numbers" i.e. the solid ink density aim points, at which time the presswork should align to the signed-off proof. At that point the press operator should be free to make any needed ink key adjustments to refine the match. The color bar then becomes a record of initial match and needed adjustments. That information can be used in statistical process control to spot any trends, or issues, revealed by the kind of ink key moves that are made over time.

Color bars can be placed anywhere that they fit on the press form, including the lead and trailing edge as well as across the center of the form. In fact, placing it in the center of the form parallel to the inking rollers is ideal, since there is less likelihood of seeing the variation that occurs at the lead and trailing edges. Color bars can even be placed in the gutter inline with the direction of the sheet through the press, although doing so is not optimal since it provides information from only one ink key zone.

Ideally the color bar should use the same halftone screening as the live image area and have had the same press curve applied.

Fashion in the pressroom

In well run printshops the owners will usually provide prepress and pressroom workers with clothing, in part to present a clean unified look to visitors and customers. However, there are some outfits that should never be seen near the press.

Here are two prime examples of pressroom-fashion faux pas:

From North America:
From Japan:
Apart from their difference in weight, what is their sartorial sin? In a word, color. The color of what is worn gets reflected into the press sheet color. It can be a difficult problem to be aware of since our eye/brains instantly auto-white balance - so white paper will still look white. Unfortunately, the red or blue outfits in the above examples will distort the hues of the color in the presswork and may lead to incorrect color adjustments.

However, the below fashionistas are more in keeping with the discipline of the press room:Grey shirts, grey pants, and optionally, grey hair. Grey balance on press and grey balance in the closet.

This rule also applies to print buyers/specifiers when conducting a press check. To evaluate color effectively wear the neutrals, black, white, grey.

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.