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How to Achieve Color-Accurate Proofs
A color-accurate proof allows a job to go to press. Color accuracy is critical because the proof is relied upon to communicate the color expectation between the printer and the print buyer. Inaccurate proofs that can’t be achieved on press can result in costly remakes and dissatisfied customers.
Several important factors affect the color accuracy of a proof, and impact whether the printer will be able to consistently and accurately reproduce the color that is produced on the proof.
Proofing Environment
The first important factor is the environment within which the proof is produced. Temperature and humidity have a large impact on how color is produced. Temperature and humidity affect how the ink is absorbed into the paper and how quickly the ink dries on the paper, both of which can affect how the inks interact on the paper, and ultimately impact the way the color is produced on the proof.
A stable proofing environment is essential in contributing to consistent color. For example, a room that maintains the same temperature and humidity every day provides a good proofing environment. Consider what happens when a proofer sits beside a window in a room without temperature control. Throughout the day, the humidity and temperature changes in the room can affect how the ink and paper interact and color variations in proofs may appear. A temperature-controlled environment removes these fluctuations and provides a stable environment for producing proofs with consistent color.
Using Color Standards
Print to color-accurate proofs that represent an in-house standard or an external standard, such as FOGRA or SWOP. The advantage of using a standard is that you have a repeatable and recognized process for producing proofs.
Process Control
Process control is essential to help ensure that inkjet proofers produce color accurately and consistently. Process control includes the printer calibration, workflow process, color measuring device, and more.
Process control should always ensure that the correct elements were used to output a proof:
• Media and ink are correct
• Profile is signed and correct for media
• Proofer calibration is valid
By ensuring the above, you can avoid incorrect output due to an incorrect setup, which saves you time and money.
Before presenting proofs to print buyers, printers check that the content is correct, verify that the correct media and color management setup were used, and measure the color bar to help ensure that the color is within tolerance. Only then is the printer confident the proof is ready for the print buyer.
Proofing Media Match
For the best visual color simulation of a press target, choose a proofing media that reflects the amount of brighteners used in the press paper. Brighteners are added to media to make it appear whiter and to increase the range of colors the media and ink can produce.
You can identify the level of media brighteners two ways. One way is to view the proofing media and press paper under black light. For example, compare a sample of your inkjet proofing media with your press paper under black light to find which proofing media best resembles the amount of brighteners used in the press paper. A second way is to measure the proofing media and press paper with a spectrophotometer. Measure each proofing media and press paper twice: measure once with an ultraviolet (UV) filter and once with a U (unfiltered) filter (the GretagMacbeth Spectrolino Spectrophotometer lets you change its filter). The difference between the UV-filtered and unfiltered measurements identifies the amount of brightener in the proofing media or paper stock. The closest two measurements of paper white indicate similar levels of brighteners.
Press stock and proofing media with similar amounts of brighteners let the inkjet proofer provide the best visual simulation and numerical match of the press target. A significant mismatch of brighteners requires profile editing to improve the visual color simulation. Consistent media gives reproducible color.
Optimized Media Configurations
Kodak Media configurations characterize proofing media. Kodak tests and evaluates how Kodak Media perform on Kodak Inkjet Proofers, and we use the test results to create media configurations that control ink volume, offer gray balance, help ensure linear output of CMYK inks, and control the gamut of the media. Media configurations also define the absolute baseline target numbers used to calibrate the media on the device. All of this testing allows Kodak to provide media configurations for each combination of proofer, resolution, media, ink, and spectrophotometer that we support, providing the best proofing solution for the media we sell.
Accurate Printer Calibration
Calibration specifies and controls how a proofer prints media and ink. A calibrated proofer consistently prints the same color. It’s important to note the difference between calibration and color management. Whereas calibration is required to ensure that the inkjet proofer consistently prints color on a specific combination of media and ink, color management uses ICC profiles and/or DeviceLink ICC profiles to simulate a target color space. ICC profiles and/or DeviceLink profiles characterize the calibrated color output of a device. When used with workflow software that supports ICC profiles, they translate color output from one device to another. Even though calibration and color management have different purposes, successful color management requires reliable calibration. With unstable calibration, color management may miss its target due to the fluctuating color characteristics of the target or source color condition.
Once calibrated, you should periodically check the calibration to ensure it remains within tolerance. How often you check calibration depends on your desired level of process control. Only when the inkjet proofer fails the calibration check do you need to recalibrate the device.
The ability to check the calibration is particularly powerful when you have an inkjet proofer calibrated for one media and setup to simulate three different color targets. You only need to check the calibration for the proofer, not each setup, to ensure that the device remains calibrated and can continue to consistently simulate all three color targets.
Using Spectrophotometers to Measure Color
Spectrophotometers measure color by taking spectral readings of color wavelengths. Spectrophotometers with UV filters offer the most consistent readings of color by eliminating the effects of media brighteners. Proper viewing conditions are recommended. For more information, contact your local Kodak representative to receive the white papers on Color Measurement with a UV Filter and Color Measurement for Profiling with PressWhite Papers.
Each type of spectrophotometer interprets spectral data slightly differently, even different models of the same spectrophotometer. For this reason, it is important to use a software application that provides media configurations optimized for its supported spectrophotometers. These media configurations allow the best visual color simulation even when different models of the same spectrophotometer are used.
Tip: Use the same spectrophotometer for calibration, and check calibrations when you have more than one proofing device. This helps ensure quality control when load-balancing between devices. Also use the same spectrophotometer to measure proofs generated by different devices (this does not apply to inline spectrophotometers).
Spectrophotometer measurements are also affected by ink drying time. Different ink drying times can produce different spectrophotometer measurements. For the best color measurements, establish an ink drying time standard, and wait the specified amount of time before measuring proofs.
Some inkjet proofers have a spectrophotometer attached to the print head. These are called inline spectrophotometers, and they offer several advantages. Inline spectrophotometers automate the measurement process, removing proof handling errors from the measurement process and keeping your hands free to do other tasks. They also offer consistent color readings because the inline spectrophotometers always wait the same amount of time for the ink to dry before measuring the color bar at exactly the same angle and speed. Even though each inkjet proofer uses a different spectrophotometer, the consistency of the color measurements overrides any measurement differences between the inline spectrophotometers.
Confirm Color Output
Once the proof is produced, it is important to check that the color produced on the proof is within the specified tolerance to the color that was specified for the proof.
This can be done through a process where a standard color bar is printed on the proof (either an international standard color bar from FOGRA or SWOP, for example, or a custom color bar). This color bar can be read by a spectrophotometer, and then the color compared to the known color standard.
The color produced can then be confirmed to be within tolerance of the tolerance specified, providing you with confidence that the correct color was produced on your proof.
Use Proofing Software from the Same Vendor as your Workflow Software
Using proofing software which is from the same vendor as your workflow software will help ensure color accuracy in your proofs.
Using the same software vendor for proofing and workflow helps ensure that color can be accurately communicated from one software application to the other, without the loss of color information.
If the two software applications are based on the same RIP technology, then this has the benefit of using the same RIP for plating and proofing to help ensure files are interpreted consistently.
Color Management
A combination of ICC profiles, DeviceLink ICC profiles, and PANTONE Color Libraries to color-manage proofs and simulate spot colors.
Black Reproduction
Inkjet proofers use combinations of CMYK ink to print black text, and profiles control the color. The type of profile can affect black text reproduction.
For example, when you use a pair of ICC profiles, overprints of CMYK inks generate black text. If the ink nozzles are misaligned, the inks may print slightly beside each other instead of on top of each other, making text look fuzzy because the edges are not crisp.
In contrast, when you use a DeviceLink ICC profile, also known as 4D technology, you can better control the generation of black text. DeviceLink ICC profiles can be configured to use more black ink and less of the CMY inks to print black text, producing text with a rich black color and crisp edges. The text is less likely to look fuzzy because fewer inks overprint to create the text and the opportunity for misaligned nozzles lessen.
Define and Achieve Color Targets
The first step towards establishing a color target that your proofs can simulate is to choose a good target. The target can be printed using an inkjet proofer, analog halftone proofer, digital halftone proofer, or press. Regardless of the device used to generate the target, ensure that the color is consistent across the target and that it represents how the device prints on a regular basis.
For example, if your target is a press sheet, ensure that that it represents the center of the press printing range. Avoid using a press target that the press struggles to reproduce on a regular basis. Contact your local Kodak sales representative for the whitepaper: Targeting and Aligning Presswork and Proof White Paper.
Proofers generally generate good targets. Many shops use a halftone proofer to generate a target. When properly calibrated halftone proofers generate good targets. Also, manufacturers of halftone media have put much effort into ensuring that the media simulates press inks. It is much less costly to characterize a halftone proofer compared to a press.
With the use of ICC profiles and/or DeviceLink ICC profiles, a properly calibrated inkjet proofer can also simulate a color target generated by halftone proofers or another device.
With a target identified, the next step is to select a proofing media that can reproduce the gamut of the target media as well as possible. Consider the tint and brighteners used in the proofing and target media.
With a proofing media and target selected, you now need to select profiling software. Profiling software generates the ICC profiles and DeviceLink ICC profiles that your inkjet proofer uses to simulate color. Select profiling software that supports the open International Color Consortium (ICC) standard and that generates ICC profiles and DeviceLink ICC profiles. Kodak recommends DeviceLink ICC profiles for inkjet proofing because of how they reproduce black text. Kodak Profile Wizard Mio Color Management Software meets all of these requirements.
Professional Color Services
For additional support with profiling, it may be worthwhile to have professional color services assist with profiling your color devices and your inkjet proofers:
• On-Site CMYK Profiling
• On-Site Profile Wizard Software Training With On-Site CMYK Profiling
• Remote CMYK Profiling
The color services provide you with a customized profile for your inkjet proofer and Kodak Media to help you simulate your proofing target.