Complete colour management for large-format printing

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After it was introduced in the early 2000s and popularized by Apple’s ColorSync system-level software, ICC colour management, or calibrated workflow, was embraced by various segments of the graphic arts industry—some faster than others. The large-format printing industry was quick to adopt colour management as a way of coming to grips with the wide variety of printers, inks, media, RIPs, and settings. Likewise, photographers, whose darkrooms got replaced with inkjet printers, quickly appreciated the potential for accurate on-screen previews and matching output.

What Is Colour Management?

Colour management uses industry-standard ICC profiles to characterize the colour of different production devices, including digital cameras, monitors, printers, and proofers. Due to the plug-and-play nature of desktop publishing, colour cannot be expected to match from one device to another, especially when they use different colourant systems (e.g., RGB vs. CMYK) and a multitude of colourants. Given a set of ICC profiles and workflow software, such as Photoshop or a RIP, colour-managed workflow attempts to match colour as closely as possible among different devices.

Why Do You Need It?

Commonly cited reasons for interest in colour management include:

1. Consistency. A key word in the graphic arts, consistency means the ability to get colour correct “the first time, every time” (to quote the famous rice commercial) — and also to get colour that matches throughout a print run and from run to run.
2. Soft proofing. Monitor previews that match printed output are referred to as soft proofs. Contrary to popular belief, a calibrated monitor does not necessarily guarantee a match with printed colour. It only shows that “what you see is what’s in the file” (WYSIWIF). For accurate soft proofs, both a printer and monitor profile are required, along with a “proofing” or “simulation” workflow.
3. Hardcopy proofs. Clients want to see proofs that match production prints. Whether you are printing digital, offset, large format, or using another process, proofs refer to preprinted samples that show how production prints will look. They are generally printed on faster, lower cost, and easier-to-operate printers, such as inkjet, which may not inherently match the production process.

Some additional interests in colour management:

4. Calibrated captures. These are digital photos that match the original scene or object. This could be especially important in catalogue and product photography. Also, what if a large job is farmed out to several photographers, how could it be assured that all of their results will match?
5. Standardized files. Digital cameras that capture JPEG format can output to standard profiles, generally sRGB and the larger-gamut Adobe RGB. Having all job files in the same colour profile ensures they will have optimum colour and match closely when output.

Here Come the Geeks!

First born in the ivory tower, colour management has benefited from considerable philosophical thought. However, these principles, while they may seem academic, help guide colour management implementation.
The four “C”s of Colour Management. The procedures necessary to calibrate a device could be thought of as the four “C”s:
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Consistency refers to setting up a device to achieve optimum colour. For an inkjet printer, these steps could include setting ink limits and total ink coverage for optimum ink usage and colour gamut.
Calibration refers to aligning all devices to a known standard or specification. A common standard for inkjet printers is linearization, meaning tones, or halftone dot values, increase in sequence. This ensures consistent performance and images with good contrast and tone separation.
Characterization means profiling a device using a profiling program and a colour target that’s printed, captured, or displayed.
Conversion – for colour to match from one device to another, colours must be converted, or changed in value, to create the match. This conversion is done by the workflow software, whether it’s Photoshop or a RIP.
Source-destination-simulation workflow. A model of workflow for calibrated devices, it means that each image came from somewhere (the source, e.g., digital camera, digital file) and will go somewhere (the destination, e.g., a monitor, printer, proofer, or even a digital file). In addition, the simulation workflow allows the output to be altered to match a different device. This could be true when an inkjet printer (the destination) is used to make hardcopy proofs for a lithographic press (the simulation).
Standard working space profiles. Apple introduced a series of standardized monitor profiles as so-called “standard working space” profiles. These profiles, widely available and distributed with system software and applications, provide a standard of reference for images. The most widely used standard working space profiles are Adobe RGB (larger) and sRGB (slightly smaller). Others include ProPhoto RGB, ColorMatch RGB, and Apple RGB.
Profile embedding. Once applied to images, ICC profiles can be embedded into the image files in JPEG, TIFF, and PDF format. The embedded profiles act like a “nametag” to show where the image came from, such as a digital camera or a standard working space. Embedded profiles can be read by workflow applications including Photoshop, InDesign, and software RIPs.

Tools of the trade

The equipment and software you need depends upon what type of production devices you want to profile.
Emissive colorimeter. This is a low cost instrument necessary for profiling monitors. Emissive means the instrument reads radiant light from monitors, as opposed to reflected light from prints. A colorimeter is a colour measurement instrument that reads red, green, and blue channels and maps colour to a mathematical model of human vision known as CIELAB.
Emissive/reflective spectrophotometer. Unlike a colorimeter, a spectrophotometer is a more sophisticated instrument that reads the full spectrum of colour, from 380–720 nm in wavelength. Readings, generally at 10 nm increments, provide a true “fingerprint” of colour. These can be used for profiling both monitors and printers. Spectrophotometers are available in two types:
x-scanning, or semi-automated, instruments. Manual instruments that read one colour patch at a time are not very productive for reading test charts with hundreds or even thousands of colours.  These instruments are designed to scan rows of colours so that a target can be read in a few minutes. They are useful for profiling up to several printers per day.
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x/y-scanning, or fully-automated, instrument.  These instruments are designed to read an entire test chart unattended. They are useful for profiling several printers per day, such as in a high-volume print shop.
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Colour profiling software. Profiling software is available for monitors, digital cameras, and printers. These are often separate modules that can be unlocked by a key code.

What You Need and How It Works

The following is an outline of the equipment and software you need and the procedures necessary to profile monitors, printers, and digital cameras.

Monitor

What you need:

emissive colorimeter or spectrophotometer for monitors
colour management software with monitor-profiling capability

How it works:

Software displays known colour values that are measured by instrument.
Calibrates monitor to known contrast (gamma) and colour balance (white point); loads calibration curve into video card.
Makes profile used by workflow software to convert displayed colour to match that of file (destination workflow) or printer (simulation workflow)
 

Printer/Proofer

What you need:

reflective spectrophotometer—fully-automated (x/y-scanning) or semi-automated (x-scanning)
colour management software with printer-profiling capability

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How it works:

Print colour target of known values.
Measure target with spectrophotometer.
Software calculates profile that converts file to match on printer.
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Digital Camera

What you need:

target of known values (Macbeth ColorChecker, ColorChecker SG)
colour management software
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How it works:

Set the camera’s exposure and white balance to get a good capture of the colour management target.
Software compares captured colour values with those on target.
Creates profile for camera.
Workflow software converts captured colour to closer match to original subject.
The same profile can be used for different lighting conditions, provided that the camera is white-balanced for each.
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Putting It All In Place

Colour matching really comes into play when an image is output from one process to another, whether from digital camera to standard working space, from file to display, or from file to print or proof. Apps could be divided into two categories:
File Creation apps. Applications for creating or editing images, illustrations, or pages. Photoshop, InDesign, and Illustrator are all ICC compliant and support colour-managed workflow. These programs can read embedded ICC profiles, render colours for accurate display, and soft-proof files to preview final output. Some of the key settings are:
Convert to profile—tells where the image is going, i.e., converts colour (see “The four ‘C’s of colour management”) to the output profile.
Assign profile—tells where the image came from, i.e., digital camera or standard working space.
Read embedded profile—most applications do this by default, however if there is no embedded profile, default profiles can be set using either:
Colour settings—used to specify default profiles if none are present, or,
Monitor profile—applications automatically read the default system profile and use it without operator intervention.
Software RIPs. Raster image processors used to output digital files to printers and image carriers are virtually all ICC-compliant. Look for places to specify source, destination, and (in the case of proofing) simulation profiles.
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Further Reading

Adams, R. M., A. Sharma, and J. Suffoletto. 2008. Color Management Handbook. Printing Industries Press, Sewickley, PA.
Fraser, B., C. Murphy, and F. Bunting. 2004. Real-World Color Management. Peachpit Press.

International Color Consortium, www.color.org
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