G7 and Digital Print

By Ron Ellis

 
In the post-recession print market, digital print has taken a large share of printing formerly handled of offset presses. Even though digital print systems are typically closed loop systems, they typically vary more than offset presses and have a reputation for low quality. This is sometimes puzzling to the observer because it has been demonstrated time and again that these machines are normally capable or targeting almost all offset characterization data sets with ease.

It is often said that the difference between a copier and a digital printer is that the digital printer is calibrated. Most of the machines doing digital print today do not fully calibrate themselves and require operator intervention to achieve the best results. These devices can do some limited machine calibration but require and operator to complete the calibration.

Calibration is not always a good thing. For one thing it takes time, and in the digital market time is valuable. Another downside is that calibration can limit the gamut of the device, making colors less vivid and pleasing. Many digital devices have a larger gamut than offset and can print a prettier print than the standard four color offset press.

There is a critical reason for calibration though – with calibration we can achieve repeatability with the digital press. This means we can print predictably, and that we can do reprints without manual edits on the press. Many digital press operators say they are faster at doing manual edits than they are at performing tedious calibration procedures. This is seldom the case, but even if it were it would make exact reprints nearly impossible.

There tend to be 3 types of devices:

       The first are devices with poor CMS systems. These rely on manual LUT corrections. For these devices we use the G7 method to create 2-d curves to move the device to a G7 print condition based on gray. In this case we do not achieve overprints or a match to the colorspace. It makes the color predictable, and neutral but the results are not as accurate as they could be. Often this result is a function of the RIP.

       The second device is a device that can be manually calibrated and has good CMS. These devices contain both a LUT capability as well as an adequate CMS engine. This means that with the combination of the LUT and color profiles we can create a good color match. These devices do not automatically calibrate themselves and require manual intervention to achieve a good color match. Devices in this category include the Indigo, iGen 3, and many color copier engines with a RIP such as Fiery, Creo and others on the front.

       The third type of device is a device that can automatically calibrate itself and has good CMS. This means that the device has manual tools such as LUTs but in general can auto calibrate itself sufficiently so that no manual operator intervention is required other than initiating the calibration. The iGen 4 is an example of this type of device.

Calibrating a digital device typically involves three steps:

       Machine Calibration, during which you initiate a calibration of the machine on the control panel. During this step the machine attempts to pull itself back to a repeatable condition.

       LUT Correction, during which 2 d LUT curves are optimized to move the machine as close as possible to the reference print condition. Typically during this the overprints, solids and other metrics may not match the desired characterization data set.

       CMS, during which two profiles are used to adjust the device to match the desired characterization. One profile represents the device and the second profile represents the target characterization data set. In addition to helping achieve overprints, solids and other parts of the data set the profiles bring additional benefits. By increasing black generation in the profiles the printy results can be improved. When this occurs the images are reseparated and color is taken out and black increased. This helps stabilize the prints makes calibration even more effective, reducing the effect or color change on the machine. The profiles can also help compensate for coating or other print elements that change the color of the page.

One of the biggest problems of digital recalibration is the time it takes. Printing a large test chart, reading it in, and creating a profile can easily take 20 minutes, which is way to long to take a machine off-line. For this reason if possible the use of nested profiles are advised. With this scenario a LUT is adjusted using the G7 method to calibrate the machine back to color. The profiles then sit up this LUT to achieve the optimum color accuracy. When the machine drifts the LUT is simply adjusted back to the same position and the profiles then become valid again. The difference in this is important because a LUT adjustment may take as little as 5 minutes compared to the 20 or more minutes required to reprofile. Some RIPs have an automated LUT adjust in them that helps you return the machine to the same position. Other do not work as well and in the case the G7 Method is a great way to adjust the LUT to pull the machine back to color.

Almost all digital presses can be calibrated with accuracy but it is seldom done in many plants. Calibration only needs to be done when the device drifts. A key question for the digital print is does these prints need to be accurate? If not then calibration is a waste of time. If they do need to be accurate then a combination of quality control and calibration is important.

Like calibration, QC for the digital press needs to be fast and not hinder the press operator. There are several ways to do this but one successful way is to locate HR patches on the sheet outside of the work and have the operator periodically check these patches, typically be reading CMY density. These readings are very fast and intuitive, and give the operator a quick check on how his machine is doing. When the tolerance is exceeded the operator stops and recalibrates his device. Another method is to have a small control strip on the jobs. While the operator uses the HR patch the calibrate a customer or QC manager may use the control strip to trend and track color, typical by looking at random jobs, or perhaps by pulling a top sheet off each job. This will give a good indication of the variation and accuracy of print jobs as well as detect problems.

Digital print holds great promise for increased accuracy and efficiency. A great deal of my work involves training digital operators on basic process control and qc procedures. As equipment improves the calibration will become automatic, but the need for qc and process control will still remain.



 Ron Ellis is a Boston-based consultant specializing in color management, worflow integration, and press calibration. He has provided installation and training services to dealers, manufacturers, and content creators since 1986. An IDEAlliance G7 Expert and chair of the GRACoL Committee, Ron has performed over 100 G7 calibrations. In addition to calibrating pressrooms for customers such as Pantone, Ron also specializes in creating internal working spaces for brands and agencies that allow them to work more efficiently with vendors, saving both time and money. Ron is published frequently in industry magazines, and has produced training materials for numerous printing industry vendors and publishers. He can be contacted at 603-498-4553 or through his web site at www.ronellisconsulting.com.



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