Types of Color Variation

[SteveSuffRIT]

I'm developing content for a lecture for my Graphic Arts/Printing college students at Erie Community College near Buffalo, NY.
Your feedback is welcome.
Steve Suffoletto

 

[Deleted member 16349]

Why are you asking these questions? You have a lot of experience and I would think you have a good idea about how to present this material. Just curious.

 

[SteveSuffRIT]

Gordo recently posted a note suggesting industry groups assist in peer reviews.
Thought this would be a good opportunity to have practical industry patriciate in theoretic academia.

 

[Deleted member 16349]

Theoretic academia? Sounds like it is understood well. I am guessing that this means you have theories and that they can be shown to be valid.

 

[gordo]

Here is the reviewed document edited primarily for clarity using sticky notes - expand to read. I hope it makes sense. Questions? You can email me at pritchardgordon (@) gmail (dot) com

Thanks for giving me the opportunity!

At Erik. It never hurts to have a fresh pair of eyes to review tech documents.

 

[Deleted member 16349]

Here is the reviewed document edited primarily for clarity using sticky notes - expand to read. I hope it makes sense. Questions? You can email me at pritchardgordon (@) gmail (dot) com

Thanks for giving me the opportunity!

At Erik. It never hurts to have a fresh pair of eyes to review tech documents.

Gordon, I could be wrong but I think what Steve was wanting was added content, from a practical industry perspective, to the topics he outlined and not an edit of his list of topics.

IMO the use of the term "stability" is not used properly. In control theory, stability or instability are very specific issues. The offset process is a stable process. But it is not a consistent process. A stable process, if not disturbed will settle to a stable steady state condition. Then when a disturbance is applied, it will drift to another stable steady state condition. Instability is when the values being measured oscillate with increasing amplitudes or just go off the chart.

I think Steve should stick with "consistent" for a term and avoid the use of stable. Just my personal picky view.

 

[gordo]

Gordon, I could be wrong but I think what Steve was wanting was added content, from a practical industry perspective, to the topics he outlined and not an edit of his list of topics.

I didn't understand that from what he wrote. I'd need more info - e.g. context - before I could add content. Maybe Steve can elaborate if that was what he was looking for.

IMO the use of the term "stability" is not used properly. In control theory, stability or instability are very specific issues. The offset process is a stable process. But it is not a consistent process. A stable process, if not disturbed will settle to a stable steady state condition. Then when a disturbance is applied, it will drift to another stable steady state condition. Instability is when the values being measured oscillate with increasing amplitudes or just go off the chart.

I think Steve should stick with "consistent" for a term and avoid the use of stable. Just my personal picky view.

You are quite correct about "stability" which is why I changed:

"It can be thought of as Stability." (which it is not)

to

"It may also be referred to as "stability"." (which is how it is often referred (incorrectly) to.)

Being picky is appropriate when it comes to these kinds of documents.

I also had a problem with the term "match" - a term I hate - and never use - when it's applied to presswork and proof alignment. But I let it go in this instance.

 

[SteveSuffRIT]

Hi Gordo & Erik,

Thank you for your feedback. It is welcome and respected.
My first objective is to simply list the different kinds or types of color variation. Classify & categorize at a macro level. Did I miss or forget any thing major?

Next, define the terms, maybe with some context examples. How is consistent same/different than stable? What is a steady upward or downward tend? Do I use the term "precise" or "precision"?

Lastly, add detailed content, perhaps about causes. For example, litho inking system uses a reciprocal doctor and limited roller train (number, size), causing potential color variation from starvation, ghosting, rerolling, not seen in gravure and flexo because of unlimited inking supply versus demand of image carrier.

Remember, my audience and scope is basic and introductory. Perhaps these conversations are too advanced already?

 

[Deleted member 16349]

Lastly, add detailed content, perhaps about causes. For example, litho inking system uses a reciprocal doctor and limited roller train (number, size), causing potential color variation from starvation, ghosting, rerolling, not seen in gravure and flexo because of unlimited inking supply versus demand of image carrier.

This is a big subject but it is an important one. It is also too big to fully discuss here. One would like to make it simple to suit your students abilities but often simplification also results in misinformation.

The problem with offset lithography is that there are several faults in the design of the press technology and this causes a lot of confusion which makes it hard for people to pin down the causes and to design out the fundamental sources of variation in the process.

I am glad you commented on the ductor roller. It is a huge problem. This 170+ year old technology, which transfers ink inconsistently, IMO is the fundamental cause of ink/water balance density variation seen on press. It has been tested to confirm this view.

Roller trains are also very important but we see little change in how they are designed over many years.

I view two modes of variation. Variation during transients and variation during steady state conditions. Yes, one could redesign the press so that both of these modes are improved so that transients are very short and the different kinds of variation about the average density, during steady state conditions are small.

Have a look at my 1997 TAGA paper on the subject. Most of it is still valid but it is not comprehensive since it was limited to just discussing some of the issues. It tried to outline what steps were needed to solve these kinds of variation problems. It was only a starting point. Much can be done.

 

[gordo]

Next, define the terms, maybe with some context examples. How is consistent same/different than stable? What is a steady upward or downward tend? Do I use the term "precise" or "precision"?

An analogy (which I've found helpful when speaking with press operators and students):

This is a graph of a beating heart:

It shows stability - but not consistency.

If you chart natural solid ink density variation over time for an offset press, you will see a similar result. When the press is operating normally, each press manufacturer will have a unique stability/consistency profile for their presses. It is sometimes referred to as the "heartbeat" of the press. And just like a human heart's cardiogram it can tell a technician a great deal about the mechanical condition of the press.

More on that can be learned here: On-press stability and consistency

Lastly, add detailed content, perhaps about causes. For example, litho inking system uses a reciprocal doctor and limited roller train (number, size), causing potential color variation from starvation, ghosting, rerolling, not seen in gravure and flexo because of unlimited inking supply versus demand of image carrier.

Remember, my audience and scope is basic and introductory. Perhaps these conversations are too advanced already?

If your audience is basic and introductory then IMHO they need a basic understanding of the offset process to give context to your definitions.
And please, make sure they understand that offset presses do not print color. They are designed to lay down a film of ink and a film of water. The major cause of perceived/measured color variation in presswork results from variations in the thickness (measured indirectly by density) of the film of ink and water. You can drill down the rabbit hole as to the causes of the variation of ink film thickness (as Erik does) if you like - but that might not be appropriate for your audience.