By Marientina Gotsis

Let us consider the following scenario:

You have received a final print that is darker (or lighter) than what you think it should be based on a proof that you have in hand from the same image file. This proof was previously printed in-house by the same computer and monitor combination, yet it seems different than what was produced the second time around. How can you tell which print is the good print?

Well...There is actually no such thing as a good or bad print. Judging whether a print is good or bad is a subjective task, which means that a bad print for one person may seem perfect to someone else.

The term quality is often used to describe a subjective judgement such as a good or bad overall appearance. In the printing industry, quality refers to a tangible and measurable property. A good quality print is one that is identical to one produced when a printer was properly CALIBRATED.

So what is Quality Control, or QC? Very simply, it is the ability to produce consistent and repeatable results. This is the most critical step to ensure that we can produce a good, print after print, after print...

How do you ensure QC?

The truth is that we can't tell you if a print is good unless we check the CALIBRATION status of our devices.

Sure we may know if the second print was printed from the same computer and monitor combination as the first one. However, a good quality print is not one that looks most identical to the original digital file as viewed on the monitor of the computer it was printed from.

In fact, a file can appear different on the same monitor at different times during the day, depending on how long the monitor has been powered on. Therefore, it is pointless to check monitor calibration as a starting point for quality control. This situation calls for some tedious work in trying to measure the differences in the two prints to guess where the problem lies.

In order to avoid print mismatches from ever happening, one has to have a point of comparison. The solution is the a 26-Step Grayscale card which should be included with every print job so that there exists a measurable point of comparison. This eliminates the guesswork in trying to produce identical prints and it provides a standard that is independent of any monitor or printer.

What is a 26-Step Grayscale and how do you make one?

InkJet/OpenPrintMaker Control (ICJ/OPM) can output a 26-step Grayscale to establish the tonal scale of a paper and inkset combination. This method is loosely based on the 10-step grayscale used in the ZONE SYSTEM (developed by Ansel Adams and Fred Archer). The 13th step in IJC/OPM’s grayscale corresponds roughly to Zone V in the ZONE SYSTEM [see IMAGE 2].

With this method in mind, the average tonal range of a properly exposed scene should result in an gray that reflects 18% light (Zone V on IMAGE 2) when read on a densitometer [see IMAGE 1]. A properly exposed scene will reveal a greater range of detail in the highlights, midtones and shadows of a photograph.

Creating a 26-Step Grayscale card is a process necessary (a) for perfecting the printing of an image, (b) for reproduction of an image at different times with the same results and (c) to have as a reference point when there is a drift in quality.

IMAGE 1

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
& 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)