Improved Profiles for Printing Digital Negatives with Quadtone RIP

Improved Profiles for Printing Digital Negatives with Quadtone RIP
Improved Profiles for Printing Digital Negatives with QuadTone RIP:
Using all seven inks to make negatives results in smoother print tones.
Ron Reeder
QuadTone RIP (QTR) is a printer driver that was originally written to improve the
ability of Epson printers to print quality black and white images. As it turns out, QTR is
also the best available driver for making digital negatives with Epson printers. The
reason is that with QTR it is relatively easy to make custom profiles which control the
printer’s ink settings. With these controls one can adjust the maximum amount of ink
laid down (thus setting the contrast range of the negative) as well as adjusting the
linearity of tonal distribution between maximum black and pure white. Making these
adjustments in the ink settings avoids having to apply distorting correction curves to the
image file itself.
We introduced the use of QTR for making digital negatives in Chapter 10 of our
recent book, “Digital Negatives: Using Photoshop to Create Digital Negatives for Silver
and Alternative Process Printing”. Nearly a year has now passed since writing that
chapter and during that time I have made hundreds of prints from QTR negatives. I
remain convinced that QTR is the best current technology for making digital negatives.
But, I also have become aware of a serious problem with QTR negatives made as
described in the book. Fortunately, I also have learned the solution to the problem. The
purpose of this article to to acquaint you with both the problem and how to fix it.
The Problem. Prints made from QTR negatives as described in the book often
show unsightly banding in areas of smooth light tones. It took a while to become aware
of the problem because many of my images are of a darker key and the banding does not
appear in tones darker than about 50% gray. But occasionally I would try to print an
image with a large area of clear sky, and then unsightly dark and light bands would
appear in the sky. The bands were spaced about half an inch apart and the periodicity
changed as I changed the resolution at which the printer operated. The banding was
clearly visible in the negative itself (and not in the computer screen image) and appeared
to be due to the printer having difficulty in laying down an even coat of ink as the
darkness (and amount) of ink increased. Out of despair I tried printing a negative using
the Epson driver and --- the banding disappeared! After all the opprobrium I had heaped
upon the Epson driver, was I going to be forced to use it after all to make negatives for
high key images?
The Solution. Once again, Roy Harrington to the rescue. Roy wrote the original
QTR and tutored me in writing the QTR profiles that we provide elsewhere on this
website. Recently I met Roy on a photo trip to Ireland and spent half a day telling him
my banding woes. After some thought he opined that the problem might stem from the
fact that all the QTR profiles I had written so far used only two inks, K and LK, to print
negatives. Perhaps, if I used all seven ink colors (K, LK, M,C,Y,LC and LM) the result
would be smoother tones and little or no banding. So, with some further help from Roy, I
learned how to write QTR profiles to print negatives using all seven inks. And I am
delighted to report that doing so essentially eliminates the banding problem. In
retrospect, this makes total sense because the Epson driver uses all seven colors to print
black and white (that is the source of its difficulty in printing good black and white) and
it also made unbanded negatives.
Figure 1 shows how the K and LK inks are used in a QTR profile I wrote for the
Epson 4000 over a year ago. This profile was designed to make negatives for printing on
a palladium emulsion. The light tones (in the negative) are printed solely by the LK ink
up to about 38.5% gray. Then the darker matteK ink starts making a contribution. In the
darkest negative tones (which correspond to light tones in the print) the majority of the
contribution was also from a single ink, the K ink. Apparently the printer is unable to lay
down an even coat of a lot of K ink and thus negatives printed with this ink profile show
banding in the high print tones.
Figure 1. Ink distributions for the QTR profile RR4000-UCmk-OHP-Pd.
Figure 2 shows ink distributions for a profile I recently wrote that uses all seven
of the printer’s inks (matteK, LK, C, M, Y, LC, and LM). In this profile the curve which
defines how the LK ink is used is copied over and also used to control the use of both the
LC and LM inks. Since the ink limits are set slightly different for each of these inks the
curves have the same general shape but do not superimpose (the ink limits were chosen
rather arbitrarily and other limits would probably work just as well). Likewise, the curve
that controls the distribution of the matteK ink was copied over and used to also control
the distribution of the C, M, and Y inks (again, the ink limits for the C, M, and Y inks
were chosen rather arbitrarily).
Figure 2. Ink distributions for the QTR profile RR4000-UCmk7-OHP-Pd which
uses all seven inks.
Once I had all inks contributing, I printed out a test step tablet with the default ink
limit set rather high (75%) and got a surprise. The darkest step of the tablet had an
optical density in the UV of 4.6. This is the highest UV density I have ever seen on a
digital negative. Apparently combining all the inks can lead to extremely high UV
densities. The UV density that is needed to print pure white on a palladium emulsion is
only about 3.2. So I kept dialling the default ink limit down until I reached a setting of
44% -- which gave me a UV density in the darkest step of just over 3.2. This QTR
profile prints negatives that yield the smoothest tones on a palladium emulsion that I have
ever been able to achieve. I conclude that an additional reason why this profile prints
such smooth tones is that no one ink is used to a very high amount. But together they
easily achieve the needed UV density.
I will now go through the textfile for a QTR profile named RR4000-UCmk7OHP-Pd, designed to use all seven inks on the Epson 4000 to print negatives on Pictorico
OHP destined for making palladium prints. I will try to explain the function of each line
of instructions. Lines of textfile are in regular type. My comments on the textfile
instructions are in italics.
QuadToneRIP curve descriptor file
for Epson4000 using matte black and all seven inks
Placing a # sign at the beginning of a line turns that line off. Thus, these lines are
just for your information and convey no instructions to the printer.
Specifies the printer, says that this profile is not for calibration purposes, and
arranges that an ink distribution graph (like those shown in Figures 2 and 3, above) will
be drawn each time the profile is installed.
#number of inks must be 4, 6, or 7
#the ink limits are percentages
#usually they are all the same but they can be individually set
N_OF_INKS=7 Specifies the total number of inks being used.
This is a very important number. It sets the maximum
amount of any one ink that can be deposited and thus controls the contrast range of the
negative. This number is altered empirically until the darkest patch on a negative will
just print as pure white on the emulsion being used.
On this and succeeding lines an ink limit can be individually specified for
each ink. If the number specified is 0, then the ink is effectively turned off. If the
specification is left blank, then the limit for that ink is controlled by the DEFAULT INK
BOOST_K= The BOOST function is useful in making black and white digital prints
but is not useful for making digital negatives. Leave it blank which effectively turns it off.
LIMIT_LM=32 The limits on the C, M, Y, LC, and LM inks are fairly arbitrary. I just
wanted them high enough they would all make a significant contribution to the overall
negative density.
Again, leaving this blank means that the limit of the LK ink is specified
Gray Partitioning Information
N_OF_GRAY_PARTS=2 Two gray inks, K and LK, will be used.
The above four lines specify that the LK ink will be used exclusively to print tones
from 0 to 38.5% black. Beyond 38.5% black the K ink will make an increasing
contribution until only the K ink is used at 100% black. This is the ink usage that is
graphed in Figure 1, above, when only the two gray inks are used. The number 38.5
comes from the fact that the LK ink, printed at the maximum the printer can print, has an
optical density in the UV equivalent to the K ink printed at 38.5%. Instructions for
determining this number for other printers and inksets are given in the tutorials that
download with QTR.
For reasons not clear to me, overlap of the K and LK inks changes dramatically
when the profile is written to use all seven inks (see the graph in Figure 2, above) even
though the above four lines of instructions did not change.
The next 10 lines, GRAY_INK_3= through to GRAY_VAL_7= may be
dispensable but I have not yet had the courage to delete them. Leave ‘em alone.
GRAY HIGHLIGHT controls tones in the highlights of the negative (or the
shadows of the final print). Increasing the number lightens the negative and darkens the
print. GRAY SHADOW controls tone in the shadows of the negative (or highlights of the
print). Higher numbers lighten negative density.
GRAY GAMMA controls midtones of the negative, higher numbers making them
lighter, lower numbers making them darker. When writing a new QTR profile, the usual
proceedure is to play with GRAY HIGHLIGHT, SHADOW, and GAMMA, largely by trial
and error, to make a negative that yields a print with a reasonably linear tonal
distribution. Then you turn on GRAY CURVE (below) and use that function to complete
the linearization process. Instructions for how to do this are given in Chapter 10 of our
GRAY_CURVE=”0;0 2;10 6;30 15;50 47;70 72;80 83;85 95;90 98;95 100;100”
The paired values in GRAY CURVE essentially describe a PhotoShop curve that
completes the linearization process.
These three lines of instruction tell the printer to use the distribution curve for the
K ink to determine the distribution of the C, M, and Y inks as well.
These two lines tell the printer to use the distribution curve for the LK ink to
determine the distribution of the LC and LM inks as well.
I believe that using QTR to print negatives with all seven inks represents the
current pinnacle of technology for making negatives with a desktop inkjet printer. When
such negatives are used to make platinum/palladim prints on a matte surfaced fine art
paper, and are viewed at normal viewing distances, I think the prints rival anything made
from an in camera negative. In short, I think we have arrived at the point where we can
stop worrying about the quality of ink jet negatives and get on with the primary business
of making beautiful, expressive prints. And, even though I am pleased with the current
quality of ink jet negatives, it is certainly possible that they will get even better in the
near future. Printer manufacturers may continue to improve the quality of their print
heads, someone may find an even better substrate than Pictorico OHP, who knows where
the next advance will come from? But for now I intend to concentrate on trying to make
some good images.
What about other printers and inksets? I expect there are a number of printerinkset combinations out there that will make excellent negatives. I am only biased
toward the Epson printers because I find QTR to be so useful and at present QTR is only
written for Epson printers. As to inksets, most of my experience is with the Ultrachrome
inkset used by older printers such as the 2200 and the 4000. I am certain, however that
equally good negatives can be made with the K3 ink set used by the newer 2400, 4800,
and 7800 printers. The K3 inset has a third gray ink, LLK, which is said to produce
smoother light tones in digital black and white prints. Unfortunately, the LLK ink would
be contributing to smoother tones in the shadow areas of a digital negative, and I kind of
doubt that any improvement it might make would be visible on the final print. But it
certainly would not hurt. QTR profiles have also been written for the K7 Piezography
inkset that uses seven different gray inks to print black and white. I have not used the K7
inkset but I expect it could also be programmed to make excellent digital negatives.
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