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The match game

Jan 1, 2006 12:00 AM


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Prepress

Many printers have successfully implemented color management programs. They’ve profiled monitors, proofing devices and presses and proven they can get a good color match from one device to another. But despite these programs, some printers still are struggling to maintain consistent color throughout a press run. Have you ever had a customer fan out a job, revealing colors that don’t match from sheet to sheet? It’s a common problem, one that all printers eventually encounter.

Why is maintaining consistent color so difficult? At any given time, the press operator is contending with print variables such as ink density, dot gain, gray balance and print contrast. And that’s just the press; using less than optimum paper stock further compounds the color challenge. This article can be used as the initial steps toward ensuring the first good sheet matches the last good sheet of a press run as closely as possible.

Roots of consistency are planted in prepress There are two basic steps you can take to maintain consistent image color during a press run. First, evaluate your color separation process. Converting images from an RGB or LAB to a CMYK color space involves more than merely changing Photoshop modes. You have to consider the ink and paper as well as the condition of the press. (See “Does painless RGB to CMYK conversion exist?” June 2005.)

Gray component replacement (GCR) and undercolor removal (UCR) determine how much cyan, magenta and yellow are replaced with black when an image is converted to the CMYK color space.

UCR reduces the amount of cyan, magenta, and yellow primarily in the shadow areas of an image and increases the black. UCR will affect only the neutral areas of the image—it has no effect on the color areas of a printed reproduction. It helps alleviate potential printing problems associated with heavy ink coverage such as set off or blocking.

By contrast, GCR is more aggressive—it affects the neutral and color areas throughout the entire image. GCR replaces the gray component of the trichromatic colors with black during color separation. (A trichromatic color is any color that is made up of all three primary printing colors: cyan, magenta and yellow.) The gray component of the trichromatic colors is the level to which all three primaries are equally present. Applying GCR replaces the tertiary color with black (Figure 1).

A GCR separation has all of the advantages of a UCR separation with one important additional benefit: A GCR separation will print more consistently throughout a press run. This is especially useful for high-speed web presses, where a color shift on press could affect many sheets in a short period of time. Long-run sheetfed work also will benefit, yielding consistent, repeatable color during the entire press run. Additionally, there are cost savings associated with GCR because the more expensive cyan, magenta and yellow inks are replaced with the cheaper black.

The ultimate challenge: four-color gray
Four-color gray is one of the most difficult images to keep in color balance throughout a press run. A four-color gray is a black-and-white image reproduced using cyan, magenta, yellow and black. Figure 2 illustrates the color stability benefits of GCR vs. UCR. Figure 2a was separated using UCR; Figure 2b was separated using GCR. The images immediately below have a five percent magenta shift in the midtones, simulating a dramatic color shift on press. Note the shift in color on the UCR separation, while the GCR separation is still neutral.

GCR relies on the black separation (instead of equal amounts of cyan, magenta and yellow) to achieve neutral grays within trichromatic colors. The result is more consistent color throughout the press run. Apply GCR to a “normal” color image, and color shifts on press will have little effect on the images.

The major benefit of GCR is maintaining thecolor balance of images during a press run. Note, however, that GCR reduces or eliminates the operator’s ability to make color adjustments on press. GCR shouldn’t be used if you have customers who expect press operators to “correct” color during press checks.

Following guidelines to control color on press
Conforming to print specifications is the second step toward achieving consistent color. Specifications define target reference values for press variables such as paper type, solid ink density, dot gain and print contrast.

Many printers do not include color bars along with the work on the printed sheet, preferring to rely on the press operator’s assessment of color match and consistency. But measuring the color bar eliminates guesswork, providing measurable numeric values, or aim points, for the press operator.

SNAP, GRACoL, and SWOP
In the late 1960s and early 1970s, web offset printing was growing by leaps and bounds. Nonetheless, printers were struggling with films and proofs that were impossible to match on press. The problem was that the prepress houses providing these materials didn’t know exactly what the printers required—they essentially took their best guess and hoped for the best. Color issues escalated—printers couldn’t color match advertisements supplied from various sources on the same side of the press sheet. Needless to say, the advertisers were unhappy.

In late 1974, various industry experts formed a committee to create specifications for material being supplies to web offset printers. Guidelines known as Specifications for Web Offset Publications (SWOP) were developed.

The SWOP organization’s specification represents an industry-wide agreement and is a result of various organizations and industry committees. Changes are constantly being made to the SWOP specification as the printing industry evolves, and it is updated on a regular basis.

Subsequently, other guidelines have been defined for newspaper printing: Specifications for Newspaper Printing (SNAP); and for commercial offset printing: General Requirements in the Application of Commercial Offset Lithography (GRACoL) (see chart).

Print Characterization Chart
Suggested Input Variables Output Print Characteristics
Paper/
Sub-
strate
LPI=
Line Screen
TAC=
Total Area Cover-
age
SID=
Solid Ink Density
PC=Print Contrast TVI=Total Dot Gain %
K C M Y K C M Y K C M Y
Grades 1 and 2 premium gloss/
dull coated
175 320% 1.70 1.40 1.50 1.05 40-45 35-40 35-40 30-35 22 20 20 18
Grades 1 and 2 premium matte coated 150-175 300-320% 1.60 1.30 1.40 1.00 40-45 35-40 35-40 30-35 24 22 22 20
Premium text and cover (smooth) 150-175 260% 1.30 1.15 1.15 0.90 35-45 30-40 30-40 25-35 26 22 22 20
Grade #3 coated 150 310% 1.65 1.35 1.45 1.02 45 40 40 35 22 21 22 18
Grade #5 coated (SWOP) 133 300% 1.60 1.30 1.40 1.00 35-45 30-40 30-40 25-35 22 20 20 18
Supercal SCA+ 133 280% 1.50 1.25 1.35 1.00 23 21 21 20 28 26 26 24
Supercal SCB 120 240-260% 1.35 1.10 1.15 0.95 20 19 19 18 28 26 26 24
Uncoated offset 110 240-260% 1.25 1.00 1.12 0.95 20 17 16 17 28 26 26 24
News-
print (SNAP)
85 240% 1.05 0.90 0.90 0.85 16 13 12 15 30 30 30 30
News-
print (heatset)
100 240% 1.20 1.08 1.15 0.95 16 13 12 15 32 32 32 32

Conform or else!
Prior to digital proofing, analog color proofs were made from sheets of high-contrast film. The colorants and dot gains were built into the proof and could not be adjusted. The analog proof was given to the press operator with very specific orders: Force the press to match the proof. Generally speaking, the press operator did not have to conform to a specification. It didn’t matter how the press operator got there, as long as the color on the printed sheet matched the proof. In this workflow, the analog proof was the control from which the rest of the process was built.

Today, regardless of the specification used, the most critical component of the process is ensuring that your operators actually conform to that specification. Printing to the numbers will result in more consistent color—but only if you follow the specification from job to job, shift to shift and day to day.

In the digital workflow, all calibration in prepress is defined by how the press prints. The platesetter (by building curves) and the digital proof (via a profile or by building curves) are calibrated based on the press’s print characteristics. In the digital workflow, the press sheet is the control from which the rest of the process is built. Failure to run consistently to a specification in the pressroom will render digital proofs and plates useless.

The specifications’ Print Characterization Chart groups commercial printing into various categories that correlate directly with the grade of paper to be printed. The idea is that the paper types defined in the Print Characterization Chart will cover the vast majority of market needs, improving communication between client and printer.

The print characterization chart can be broken into two major sections: input and output variables. The input variables, or prepress aim points, consist of the line screen and recommended total area coverage in which the job will print:

  • 
  • Line screen per inch (LPI) refers to the number of halftone dots per linear inch in which the job will be printed. On smooth, coated stocks, a higher line screen is recommended (175 lpi). On more porous paper stock, the line screen is much lower. Because fine line screens exhibit more dot gain, trying to print higher line screens on uncoated paper would result in dark printed images with loss of detail.
  • Total area coverage (TAC) refers to the sum of all four process-color printing inks. The value is obtained by adding the dot percentages of cyan, magenta, yellow and black in the heaviest (darkest) printing area in a digital file. TAC is defined during the color separation process (converting images to CMYK) using UCR and GCR.
The output variables, or press aim points, consist of solid ink density, tone value increase and print contrast:
  • Solid ink density (SID) is the numerical measure of how much complementary light is absorbed by a solid patch in a color control bar as measured with a densitometer.
  • Tone value increase (TVI), also known as total dot grain, is the enlargement of halftone dots during the image transfer process. TVI is the result of the combination of absorption of ink by the paper (mechanical dot gain) and the light scatter around and under the printed dots (optical dot gain). The TVI values in the Print Characterization Chart refer to the 50 percent dot for each process color. For example, a tint densitometrically measured on a press sheet with a value of 72 percent, but that had a value of 50 percent in the digital file, exhibits 22 percent TVI.
  • Print contrast is determined using a calculation that compares the density readings of a three-quarter tone area (75 percent) to the density of a solid area (100 percent) of the same process color. Good print contrast indicates a printing system’s ability to hold open the shadow areas while maintaining high ink densities.
Measurement and acceptable variation
Measuring these press aim points to maintain print consistency requires two tools: a color bar and a color measurement device, such a densitometer. The color bar must contain patches that correlate to the press aim points. At a minimum, the color bar should contain individual solid CMYK patches to measure SID, individual 50 percent CMYK patches for measuring TVI, and individual 75 percent CMYK patches for measuring print contrast.

Variation is a natural part of the print reproduction process. The important thing is to measure and control these variables as closely as possible. If these variations can be controlled within an acceptable range, then you will have control of color consistency. For specific information on acceptable variations for these specifications, see www.swop.org and www.gracol.org.

Prepress and press must cooperate
Controlling color variation on press requires prepress and the pressroom to work closely together. If you decide to start separating color using GCR, it’s important to discuss the change with your press operators. Start by running a set of test images on press. Select images that are representative of typical jobs, separate them using various amounts of GCR, and evaluate the results on press.

Additionally, if the decision is made to print to a specification in the pressroom, it is important for management to provide the press operators with the necessary tools. Equip your pressroom with the proper color measurement devices (such as a densitometer) and provide color bars with the printed job. Implementing both of these concepts—separating color using GCR and printing to a specification—will dramatically improve color consistency on press.



GCR at a glance
Gray component replacement (GCR) is total undercolor removal. GCR reduces cyan, magenta, and yellow in the neutral and trichromatic color throughout an image. Note that:

  • GCR improves color consistency on press. GCR separations will produce more consistent, repeatable color throughout a press run. The disadvantage of this, however, is the reduction in the ability to make color changes on press.
  • More GCR results in more black within an image. Since most text on a press sheet is black, densities often are run higher to make the text blacker. Black densities that are too high can adversely affect GCR separations.
  • High amounts of GCR require the use of undercolor addition (UCA). Applying UCA to GCR images will result in more dense blacks in the printed reproduction.


FOR FURTHER READING
SWOP, IDEAlliance join forces

SWOP (Specifications for Web Offset Publications) (Alexandria, VA) has announced a merger with IDEAlliance (Intl. Digital Enterprise Alliance) (Alexandria, VA), an association of advertising agencies, printers, publishers and technology partners. Working within the IDEAlliance infrastructure, SWOP plans to chart a new course through training initiatives, technical specifications and SWOP certification programs.

According to Nubar Nakashian, chairman of SWOP, “Merging with IDEAlliance will provide synergy for both SWOP and IDEAlliance programs. Working together, we can extend the reach of specifications and guidelines across the printing industry.”

SWOP and IDEAlliance began working together in early 2004 to develop common marketing, publishing, ordering and fulfillment mechanisms. The new IDEAlliance SWOP Program is composed of the IDEAlliance SWOP Advisory Board (formerly the SWOP Board of Directors) and the IDEAlliance SWOP Review Network (formerly the SWOP Review Committee).

SWOP: Specification for Web Offset Publications (10th Edition)
The updated booklet, which features a bold new design and colorful graphics, provides the latest information on the use and handling of digital files (including CTP) and the SWOP Proof Certification Program. SWOP 2005 incorporates new digital specs and a complete new Reference Section. See www.printtools.org.

Like a BRIDG over troubled cyan
BRIDG’S “Color Handbook” has been updated with eight pages of new material and a complete redesign. The book is an introduction to color, explaining what it is, and how it is measured and managed. Other BRIDG’s guides cover halftone screening, digital workflow, color management, proofing and computer-to-plate. See www.printtools.org.

Learn the basics
Abhay Sharma’s book, “Understanding Color Management,” explains the basics of color science and reviews a wide range of profiling hardware and software options. The book also details International Color Consortium (ICC) color management—what it is, how it works and how to use it. See www.amazon.com/colormanagement.

In the real world
Peachpit has published the second edition of “Real World Color Management,” by Bruce Fraser, Chris Murphy and Fred Bunting. It covers everything from color theory and color models to understanding how devices interpret and display color. You’ll find expert advice for building and fine-tuning color profiles for input and output devices (digital cameras and scanners, displays, printers, and more), selecting the right color management workflow, and managing color within and across major design applications.

According to one online reviewer: “The book empowers you to make thoughtful choices on every Photoshop setting (black-points, colorspaces, rendering intents, etc). The section on evaluating profiles is excellent.” See www.peachpit.com.


Joseph Marin is a senior prepress technologist and instructor for PIA/GATF. Contact him at jmarin@piagatf.org.