Color Bars

Color bars are printed in the trim area and are used for quality control purposes by the printer.

Squares of colour are printed on the area of the page to be trimmed off, which the printing press operator uses to check colour density and consistency is maintained. This checking process is automated by some printers, with digital scanners tracking the colour bars to ensure quality and consistency is maintained.

• 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.
• Color bars can be tailored to meet the needs and measurement capabilities of individual print shops.
• 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.
• Color bars can reveal issues with ink hue, blanket condition, impression cylinder pressure, etc.
• They can be used forensically to help understand why a specific job did not meet expectations.
• 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.

An offset printing press is essentially a complex machine for laying down a specific film thickness of a specific color of ink onto a substrate. The digital version uses toner electrically charged then fused onto paper. The ink or toner is metered out in zones across the width of the press sheet according to how much coverage is required for each color in each zone.

The trick is that if you are producing a critical spot color with a build of process inks, those inks and all associated print attributes need to be “spot on.” The best way to control critical press factors like density, dot gain, print contract, etc. is to measure as many color bar patches as possible.

Color bars can serve many purposes. They can be used to determine color accuracy against a given standard or to determine a proof’s accuracy against a final print.

Color bars also can be used to measure consistency for the duration of a long print run, from job to job when a print job is reprinted, or between two similar printers.

So, ranked in order of importance, here are the patches we suggest for a four color press color bar.

1. Unprinted substrate patch (to zero out substrate when necessary)
2. Solid patch for each process (and spot) color (needed for solid ink density (SID))
3. 3/Color Grey patches at multiple tone values
4. Mid-tone patch for each process color (to gather dot gain (TVI) values)
5. Process color over prints (needed to measure trapping efficiency of inks)
6. Additional highlight and shadow tone patches of each process color (for a 75% patch to calculate print contrast)
7. GATF Star targets or microline targets (used to visually evaluate for press slur and doubling issues)

The items listed above are important and truly necessary, but if space is an issue, start with No. 7 and work backwards removing items until the bar fits. Also, items 3 and 4 could be easily reversed, but having one without the other makes diagnosis of grey balance issues difficult at best.

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.

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.

Color bars are available from a variety of sources, including IDEAlliance (at no charge) and iStockPhoto.

Many imposition packages also include color bars in their imposition templates.

Another option supported by industry standards is the Ugra/FOGRA Media Wedge CMYK, which monitors the quality of digital proofs. It also can serve as a digital control aid to monitor the effect of imaging in CMYK mode and other prepress work.

The CMYK tonal values of the Ugra/FOGRA Media Wedge are based on ISO standards. The Ugra/FOGRA Media Wedge is available for purchase from FOGRA.

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Email me at reilley4color@gmail.com