Calibrating the Presstek DI to G7 Specifications:
Methods of Calibration, Results, and Process Controls
Ron Ellis Consulting LLC
Introduction
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.
Methods
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
Results of the calibration methods
are as follows:
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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
Process
Control
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!
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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
Press Scanning:
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.