IMAGE 2: Adams/Archer ZONE SYSTEM gray card
How does IJC/OPM help the QC process?
If you were to print a gradient starting with white and ending with black, using only one black ink, you would most likely get an image that resembles IMAGE 3A. This image shows an uneven distribution of tonal range. Not only that, but if you were using only one ink, the dot distribution on the paper would be very visible.
Now, if you divide this test strip into 26 segments from light to dark, you would notice that only the first 10 steps show variation and the rest of the strip is one continuous patch of black. The INK LIMIT of this ink is equal to 10. Printing a photograph on this printer using this one ink "as is" will have very little detail in highlights, midtones and shadows. ICJ/OPM can help you print an evenly distributed 26-Step Grayscale using multiple inks to achieve detail with a MIDPOINT of approximately 18% gray .
Each combination of printer, inks and paper corresponds to a unique 26-Step Grayscale strip that can be saved as a custom profile in the software. The software helps convert an uneven gradient [IMAGE 3A] into an evenly distributed gradient of 26-steps [IMAGE 3B] by tweaking inks and performing a linearization. Although the software provides several profiles, you have to know how to create a new one in order to evaluate the quality of any test strip.
IMAGE 3A: an uneven 26-Step Grayscale
IMAGE 3B: an evenly distributed 26-Step Grayscale
WHITE POINT= the shade of paper with no ink applied to it
BLACK POINT= the combination of paper with full ink coverage
a. How does Ink Tweaking work?
This method allows you to visually determine how to mix different inksets. The goal is to try to determine how the inks should be mixed together so that you can achieve an even distribution of dots. The process is time-consuming and it involves printing a sample after each tweak and examining it carefully. Examples of things to look for include noticeable dots on highlights and detail in shadows.
You can start by printing out a Test Patches Target using your desired combination of inks. IMAGE 4, left is a set of test strips from a 6-ink printer. The combination of 1 Black, 2 Dark Gray, 2 Middle Gray and 1 Light Gray has the potential of becoming a perfect 26-Step Grayscale. For this to occur, you have to first determine the INK LIMIT (C) for each ink, (e.g. Step 18 for Middle Gray) and then enter these values in the Ink Tweaks Window [IMAGE 5].
---- Test Patches Target ------------ Tweaked Inks Target ----
IMAGE 4: 26-Step Grayscale strips using 6 inks before
Now you can tweak the Ink Curves (D) for each ink separately so that the 26-Step Grayscale looks more even. You can print a Tweaked Inks Target [see IMAGE 4] to inspect how the inks blend together or pring a photograph and inspect it.
& after they have been tweaked
If you are happy with the results, you can save this ink tweak as a new profile. You have now established a measurable standard. In reality, there is no right or wrong way to blend the curves. Everyone does it a little different. The important thing is to try to achieve an evenly distributed tone.
IMAGE 5: The Ink Tweaks Window where the IC software determines how the inks should blend together after you select an INK LIMIT for each ink (left)
& the LINEARIZATION Window (right)
b. How can I perform a Linearization?
This is a power feature of the IJC/OPM software than can help (a) normalize the intervals of a 26-Step Grayscale you created with the ink tweaking method, (b) setup multiple printers to produce the same image without drift, (c) update a profile when a variable has changed, e.g. paper or printer.
Since human vision is limited, eyeballing the gradient while tweaking inks results in an imperfect gradient. To perfect a tweaked ink profile, you have to first take a reading of each of the 26 steps using a DENSITOMETER, and then enter each value in the Linearization Window. In the latest version of IJC/OPM we have added features that allow you to perform a LINEARIZATION without the use of a DENSITOMETER.
The generate button automates the listing of the aims of each step, which are the the ideal values for your paper and inket combination. A manual way of finding out what the aims should be is by looking up the value of each step in printed tables of data that are organized by PAPER RANGE. The AIMS curve is a depiction of ideal values and by linearizing you attach to the profile an offset curve of what needs to be adjusted for the values to become ideal. This curve becomes attached to your profile behind the scenes: you will not actually see any differences if you go back into the Ink Tweaks Wwindow and you shouldn’t tweak inks further after this point. Re-tweaking destroys the validity of the linearization process.
In the case of trying to create a profile for a photograph that didn’t have one associated with it, you can do this only if you have a 26-Step Grayscale card that has been linearized. By entering the BLACK POINT and WHITE POINT in the Linearization Window the software will generate the appropriate values for each step. Also, if you need to update an existing profile because something has changed in your setup, you have to re-measure every step.
IMAGE 4: a 26-step grayscale before (top) & after (bottom) LINEARIZATION
GLOSSARY OF TERMS
CALIBRATION is the act of ensuring the accuracy of the instrument (e.g. a printer or monitor), by comparison with an established standard
The ZONE SYSTEM is a method of understanding and controlling the exposure and development of the negative, and a guideline to vary that exposure to get the results you want
an INK LIMIT is the usable range of an ink, defined by noting the point at which an ink fails to achieve more density on a particular paper, and may begin to bleed
AIMS data points are the ideal values for each of the 26 steps for a particular inkset and paper combination
LINEARIZATION the process of normalizing the range of values for each of the 26-steps
PAPER RANGE can be determined by subtracting the value of the WHITE POINT from the the value of the BLACK POINT (measured by a densitometer)
Marientina Gotsis has been involved with information technology, art and
design for both industry and academia since 1995. Her interests and
expertise include consulting for creative companies, higher education
teaching, technical writing and academic research. She has received an MFA
in Electronic Visualization from the University of Illinois at Chicago. Marientina is currently employed by the University of Southern California, School of Cinema-Television, Interactive Media Division and is a member of ACM, IEEE, STC and ECAI.