Calibrating the Presstek DI to G7 Specifications:
Methods of Calibration, Results, and Process Controls
Ron Ellis Consulting LLC
The Presstek Di is a unique combination of offset and digital technology. This combination allows rapid calibration like a digital press, as well as the stability and speed of traditional offset printing. In this whitepaper we will discuss methods of calibrating the Presstek DI press to G7 specifications such as GRACoL and SWOP, as well as the results and methods needed to maintain the calibration.
This document will examine using three methods to calibrate the DI:
Calibration using G7 Method
The G7 method is a calibration procedure based on gray aim points. The calibration method involves targeting ISO solid LAB values, an NPDC Curve, and gray balance. Typical G7 calibration requires 2 press runs and is often performed with software such as the IDEALink Curve software. (Other manufacturers also make software to perform this calibration). Results of this calibration typically result in a deltaE of 3.0 average, with a peak of 8-10 deltaE.
Calibration using ICC Devicelink
Calibration using ICC Devicelink involves using 2 icc profiles to calibrate from one to another. The procedure involves using an input profile, which characterizes the desired color condition, as well as an output profile which characterizes the current output condition. ICC Devicelink calibration can be performed using software such as Alwan Color Optimizer, GMG ColorServer, and ORIS Pressmatcher. Results of this calibration typically result in a 2.0 average deltaE with a peak of 6 deltaE.
Calibration using Iterative Devicelink technology
Calibration using iterative Devicelink technology involves using software that links the input and destination color spaces, and then calculates and improves the color result. Typically this type of technology will require two or three press runs to refine the output result. For example the first may have an 3 deltaE average from the data set, the second may have a 2.0 average deltaE, and the third may have a 1.5 deltaE average and so on until the desired deltaE has been achieved. Iterative calibration can be performed using software such as CGS ORIS Pressmatcher or GMG ColorServer. CGS ORIS Pressmatcher can iterate and create an icc devicelink profile that can be used by CGS ORIS Pressmatcher as well as other applications. GMG ColorServer creates a proprietary devicelink file (MX4) that can be used only by GMG ColorServer. Results of this calibration typically result in a 1.1 deltaE average with a peak of 5 deltaE.
Results of the calibration methods are as follows:
- All results are using deltaE76.
- Measurements are based on using an It8/7.4 and comparing the the GRACoL 7 2006 data set.
Method Avg. Peak Worst 10%
Calibration using G7 Method 3.0 9.5 6.04
Calibration using Devicelink 2.0 5.9 4.0
Calibration using Iterative Devicelink 1.1 6.0 2.6
Calibrating to G7 involves achieving all the conditions required for G7 Qualification – during a fixed moment in time. The G7 calibration is a snapshot in time showing you can print to G7 conditions. But what about a few days later? Are you still able to match the same proof or was it just a passing moment when the press conditions were just right? An even more important question is what happens a few months later. Is the press still able to print to G7 conditions, or was it just a useless exercise? The Presstek DI52 was able to maintain stable print conditions during the testing, not moving more than .5 deltaE during the course of each testing day.
Print conditions are highly subject to environmental conditions such as temperature, humidity and other factors. In addition press conditions are subject to maintenance and normal wear and tear. It recommended that users monitor a color bar and control strip to detect when press conditions require recalibration. Once press conditions have changed to the point where recalibration is required, one of the above calibration methods can be used to pull the press quickly back into calibration.
How Close is Close Enough? Implications of Calibration Methods
Accuracy when printing to print standards and specifications is important, however for different print users have differing tolerances for the accuracy required. In addition to the tolerance required, the time required to calibrate is also important. For example one print buyer may demand the printer print within a certain deltaE for their work, or a printer may be splitting quality work across multiple presses. In this instance a low tolerance would be appropriate to make sure the jobs match and the print buyer is happy. Another print buyer may be less critical, and paying less and in this instance the printer can relax the tolerances and concentrate on the quality of work that particular customer is willing to receive and pay for. The point is that printing is a custom business, and many printers and buyers work with different pricing models and expectations. If you spend all your time calibrating then you may not have enough time to spend printing and making money!
- For many printers a deltaE of 3 when targeting a G7 specification such as GRACoL is considered very good. This can be achieved using the G7 method of calibration.
- A deltaE of 2 is considerably by many to be extremely accurate in offset printing, and beyond what can be done in a normal press calibration. While curves and ink density can provide a good calibration there are certain things that curves and ink density alone cannot handle. A Devicelink can perform transforms to the data that cannot be achieved otherwise. For example overprints and other areas in ¾ and above areas on press can be difficult and impossible to control conventionally. In addition non-standard ink rotation and other techniques can make trap and overprint color unobtainable. By using a Devicelink these colors can be artificially compensated for. Using a Devicelink is a good way to quickly calibrate the DI and move it to an accurate calibration state.
- A deltaE of 1 is considered to be only achievable by proofers, and is extremely accurate. This accuracy comes with a cost however, because the multiple pressruns required to do this when using Iterative Devicelinks usually are too time consuming and take the press away from production for to long. The Interative Devicelink approach when coupled with the stability of the DI is extremely accurate. In this scenario the DI is calibrated to the same accuracy as a typical Epson or other inkjet proofer.
The calibration method needs to be determined by the individual printing plant. As shown above there are several simple calibration methods that will help the user maintain an accurate calibration and quickly pull the press back to target conditions when needed. The DI can also be calibrated to even tighter ÔproofingÕ conditions when needed, although that would not normally be cost effective nor desired in a production environment.
Time Required for Calibration on DI52:
G7 Method: 2 press runs, approximately 35 minutes
Devicelink Method: 2 press runs, approximately 35 minutes
Iterative Devicelink Method: 4 press runs, approximately 2 hours.
Note: In daily production with a good control strip users may be able to skip the second confirmation press run and confirm calibration from the control strip.
Software and Tools Used for This Testing
Press: Presstek DI52
Inks: Toyo Aqualess Ultra Series
Spectrophotometer: eye-one iSis 19Ó
RIP: Presstek Momentum RIP (Harlequin-based)
Paper: RIS Value Gloss
Auditor: Ron Ellis. Ron Ellis Consulting LLC
Calibration Using G7 Method
IDEALink Curve Software Version 1.1
Calibration Using Devicelinks
Alwan Color Optimizer v3.5.2
Calibration Using Iterative Devicelinks
CGS ORIS Pressmatcher v5.