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.

Tolerancing color in presswork using solid ink density

Background information - ink film thickness & solid ink density

Offset printing presses are designed to lay down a film of ink, in the presence of water, onto a substrate - usually paper. The ink forms the image while the "water," more accurately fountain solution," prevents the non-image area on the printing plate from accepting ink. For the process to work, there needs to be a critical ink/water balance with the goal of having an ink film thickness between one micron.

If the ink film thickness is too great, the result can be "ink tailing/misting." In addition, the non-image background may take on ink resulting in "catch-up" (sometimes mistaken for "scumming"):
On the other hand, if the ink film thickness is too thin, the result can be a breakdown of the ink on the sheet causing low contrast, loss of sharpness, and mottle:
So, from a color tolerancing point of view, because the function of ink is to filter light and allow us to see color and because its thickness also effects the integrity of the printing process - ink film thickness on the sheet becomes an important metric to measure and tolerance in presswork.Top: CMYK at high ink film thickness/solid ink density.
Bottom: CMYK at low ink film thickness/solid ink density.
There is no practical way to directly measure the ink film thickness on a press sheet. However, there is an indirect way and that is to measure the solid ink density (SID) using an instrument called a densitometer.

Color tolerancing through densitometry

Measuring SIDs in the solid ink patches in the color bar with a densitometer does not actually provide information about the color being printed. However, because it indirectly provides information about ink film thickness (which impacts color and tone reproduction) SID values are valuable for process control and defining variation during a press run where the instrument, ink, and substrate remain the same.North American (Status-T) high-low specifications for acceptable SID variations measured with ink dry.
Top: Commercial sheetfed, Middle: Magazine/heatset web,
Bottom: Newsprint/coldset web.
From a color point of view, the assumption is that all three chromatic colors vary in the same direction and therefore remain in relative balance. When that happens there is a shift in color saturation (higher SIDs = higher saturation) as well as tone reproduction (higher SIDs = higher dot gain/TVI). If one color, e.g. Cyan, is at the maximum low point while another color, e.g. Magenta, is at the maximum high then the result may be a visible color bias in the presswork.
Typical SID variation in presswork graphed by measuring color bar patches every 10 sheets through the press run.
While a densitometer can also be used to monitor variations in non-process, i.e. spot/Pantone colors, usually a printed sample of the target color, including a high/low density tolerance reference, is used instead since this helps both print specifier and supplier visualize the acceptable range of color change as SIDs naturally vary during the press run.Checking for spot color variation
Addendum: Densitometer set up - "Status" condition

Densitometers are set by their manufacturers to an industry defined "Status" which defines the total response of the instrument including light source, optics, filtering, and receptor for given wavelength. The primary responses for the print business are "Status E" and "Status T" (ANSI PH2.18 and DIN 16536). In addition, densitometers are available with or without polarizing filters. Dry ink density readings from polarizing and unpolarized densitometers as well as those set to Status E vs Status T will not agree. Typically European instruments are set to Status E and use polarizing filters while North American instruments are set to Status T and do not use polarizing filters.

The important thing to be aware of is that if SID information is shared outside of the printshop - then the Status of the instruments that were used to determine SID values must be known. In addition, it is critical that all instruments within the printshop are set to the same Status. In North America, where many of the presses and their closed-loop color control systems are from Europe, it is not unusual to find the press set to Status E polarized while the handhelds are set to Status T unpolarized which can easily result in quite a bit of confusion in production.

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

Printing at DMaxx - maximizing the CMYK gamut

The published standard solid ink density (SID) targets and their associated CIEL*a*b* values are designed to be be achievable by the majority of printshops with equipment in reasonable working order. They are great for standardizing presswork across the industry. However, they do not represent the best that can be achieved on press nor do they allow for competitive differentiation. Printing to "DMaxx" - the maximum, stable, SIDs the press is capable of while using standard process inks - provides a quick and relatively easy way to add visual impact to presswork, as well as a competitive edge, when appropriate, for specific print projects.
This process is best suited to sheetfed printers with an inline coater as this helps prevent setoff caused by the heavy ink densities. To prevent sheet distortion, use coated sheets in heavy text or cover basis weights. CMYK separations done with UCR rather than GCR work best with this process.

The basic recipe for DMaxx printing:

1) Benchmark your presswork.

You'll need about 2,000 sheets of paper and two hours press time. Create a target strip consisting of 12 tone patch steps – 1, 2, 5, 10, 25, 50, 75, 90, 95, 98, 99, and 100%, for each process color. Include four gray balance patches – typically, highlight (10C/7M/7Y), quarter-tone (25C/16M/16Y), mid-tone (50C/39M/39Y), and three-quarter-tone (75C/63M/63Y). These tone patches should be located near their nearest black tone equivalents (10K, 25K, 50K, 75K) to allow for visual comparison.
Include a standard CMYK image for visual evaluation.
Arrange the targets "in-line" on your press sheet and run to your standard house SIDs.
Mark the paper delivery stack "house standard" at the point the press is up to color and pull a few sheets for reference. Then gradually increase the ink densities. Try and go in 10 point steps. It is not unusual to be able to increase SIDs by 15-25 points and still maintain press stability. Don't worry about how the images and tone targets look at this point - just maintain gray balance.
Maximum SIDs are arbitrarily reached at the point where the press is still stable, ink/water balance is stable, dots are still sharp (no slinging or tailing), and set-off is not a threat. Mark the paper delivery at that point as "Max Ink" and pull several sheets for reference.

2) Chart a dot gain curve

Measure the target tone areas of your house standard sheet and plot them on a graph comparing requested tones vs actual press sheet values. Draw a "best fit" curve through the data points. This charts the dot gain characteristic curve of your press work at normal SIDs.
Now measure the tone values of the Max Ink sheet and plot them on a similar graph. Draw the best fit curve. This charts the dot gain characteristic curve of your press work at maximum SIDs.
This information will be used to create a dot gain compensation curve that will be applied to plate output to compensate for the increased dot gain caused by the boosted ink density.

3) Create the dot gain compensation curves

A dot gain compensation curve is derived from a plot of requested tone values against the resulting tone values. For example, if the house standard 50% requested tone resulted in a dot area of 67%, but The Max Ink measured 67% tone value corresponded to a 35% requested tone – then one of the points on your compensation curve would be: 50% = 35%. Put another way, we build a transfer curve that maps a requested tone of 50% to 35% because we know that 35% produces a 67% dot area at maximum ink density (which is what we are trying to achieve). This process would be applied to all the specified data points in order to align the tones of the High Ink presswork to the house standard.

4) Update your proofer

If you have a digital proofer you may want to build a profile of your DMaxx press work in order to properly set customer expectations in your proofs for this process. Alternatively you might stay with your current proofing and just tell customers that color will be boosted on press. This gives your press operators more leeway in how high they will run their SIDs.