Editor’s
Note: Chromaticity experts will address questions from the pressroom, plateroom,
prepress, workflow, proofing, digital photography and color management. Submit a
question by going to www.chromatech.com and choosing the trial question option.
Please make sure to note that you are submitting a question for Newspapers &
Technology.
Q:
What is the YuleNielsen factor for Epson’s semimatte professional media
proofing paper?
A: There really is no “correct” YuleNielsen factor
(Editor’s note: the factor accounts for how light is scattered on paper) for
any media, paper, plate or process.
To
qualify that statement, we need to better understand what the YN equation
actually does to the MurrayDavies dot area equation and how the Nfactor is
calculated for any given process. (Editor’s note: The MurrayDavies equation
computes a dot from density measurements.)
The
YN equation simply adds a “fudgefactor,” correctly called the Nfactor, to
the MD equation. This exponent adjustment to the MD equation simply adjusts
the result of the dot area equation  much like a plate compensation curve
changes the dot gain result on press.
If
you want your instrument (densitometer or spectrophotometer) to read a
particular dot pattern differently than it does now, simply change the Nfactor
and the numbers reported by the instrument will change.
The
“zero” state of the Nfactor is 1.0 because it is an exponent in the
equation. (Raising a number to the power of 1 in effect does nothing.) If you
increase the Nfactor, the densitometer’s dot area values will go down. If you
decrease the Nfactor, the densitometer’s dot area values will go up.
The
Nfactor is primarily used to make a densitometer react like a planimeter. It
will make the instrument measure a visual dot area more like a mechanical dot
area. Again, it does this based
upon a user adjustment.
The
way you determine the Nfactor is to visually determine what you think is a
mechanical 50 percent dot area on your plates or paper. You then adjust the
Nfactor in the instrument until it reads this area as 50 percent.
(Editor’s
note: You can use a CCDbased plate reader or planimeter.)
Determining
what the Nfactor should be for the Epson semimatte paper really begs the
question: “What are you trying to do?”
If you are attempting to read dot gain on an inkjet proof as compared to
a press sheet, do not use an Nfactor.
In
other words, use the same Nfactor for both the proof and the press sheet.
If
you are really attempting to read a mechanical dot on the inkjet media, follow
the instructions above. You must determine what you think a 50 percent dot is on
the proof, then adjust the Nfactor until the instrument reads it as 50 percent.

Mike DiCosola
mike@chromaticity.com
Q:
We recently switched to inkjet technology for generating proofs. A coworker
told me he understands that we need to make sure our clients compare the inkjet
proof to the printed piece under 5000 K lighting conditions. He said if the two
are compared under office or some other lighting that the proofs may not match.
Is this true and, if so, why?
A:
Yes, it is very important that inkjet proofs are viewed under standard 5000K
lighting conditions, especially when compared to a proof generated by something
other than inkjet technology. The reason for this is that certain printing
technologies, inkjet in particular, suffer from a phenomenon known as metamerism.
Metamerism
causes certain colored pigments to shift in color disproportionately with each
other under lighting generated by different color temperatures.
For
example, suppose that you have two grayscale images printed on the same page.
One is printed with black ink only while the other is a fourcolor gray image
printed with cyan, magenta, yellow and black ink.
Let’s
say that when viewed under 5000K lighting the images match.
Now, if we move the proof to warmer or cooler lighting conditions, the
two images may no longer match each other; the fourcolor gray image may shift
from warmer to cooler when compared to the black inkonly image. Further, even
within the same image the shadows and highlights and various colors may shift
when compared to each other.
This
is because the inks suffer from metamerism with different pigments shifting in
different directions.
The
same thing can happen if an inkjet proof is compared to a proof made using
another technology or with the press sheet. Your client might approve the proof
under the proper lighting conditions while rejecting the proof if viewed under
improper lighting conditions.
To
help identify this problem, the Graphic Arts Technical Foundation (www.gatf.org)
sells a product called a rhem indicator. The rhem indicator is a small sticker
that can be placed on proofs or in a light booth. When the color temperature of
the viewing light is 5000K the indicator appears to be an even solid color.
If
the lighting conditions are anything other than 5000K, stripes will appear in
the indicator. This is a very inexpensive way to ensure proper viewing
conditions.