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Dec.
2003








 

 

 

 













 

 

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Color Source Help Desk
Understanding Yule-Nielsen factors and when to use them  


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.chroma-tech.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 Yule-Nielsen factor for Epson’s semi-matte professional media proofing paper?

A:  There really is no “correct” Yule-Nielsen 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 Murray-Davies dot area equation and how the N-factor is calculated for any given process. (Editor’s note: The Murray-Davies equation computes a dot from density measurements.)

The Y-N equation simply adds a “fudge-factor,” correctly called the N-factor, to the M-D equation. This exponent adjustment to the M-D 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 N-factor and the numbers reported by the instrument will change.

The “zero” state of the N-factor 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 N-factor, the densitometer’s dot area values will go down. If you decrease the N-factor, the densitometer’s dot area values will go up.

The N-factor 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 N-factor is to visually determine what you think is a mechanical 50 percent dot area on your plates or paper. You then adjust the N-factor in the instrument until it reads this area as 50 percent.

(Editor’s note: You can use a CCD-based plate reader or planimeter.)

Determining what the N-factor should be for the Epson semi-matte 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 N-factor.

In other words, use the same N-factor 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 N-factor until the instrument reads it as 50 percent.

 

- Mike DiCosola
mike@chromaticity.com

 

Q: We recently switched to inkjet technology for generating proofs. A co-worker 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 four-color 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 four-color gray image may shift from warmer to cooler when compared to the black ink-only 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.

 

- John Nate
jnate@chromaticity.com