Dot gain question?

[tmiller_iluvprinting]

Knowing that the Murray-Davies equation uses the density of the substrate, the density of the solid, and the density of the sample dot %, and assuming that your plates are imaging correctly, is it safe to assume that the diameter of the dot on the plate really doesn't effect dot gain?
Regards,
Todd

 

[gordo]

Knowing that the Murray-Davies equation uses the density of the substrate, the density of the solid, and the density of the sample dot %, and assuming that your plates are imaging correctly, is it safe to assume that the diameter of the dot on the plate really doesn't effect dot gain?
Regards,
Todd

Dot gain happens at the perimeter of the dot. Smaller dots (i.e. smaller diameters) have larger perimeter to area ratios and hence more dot gain.
Does that answer your question?

Gordo

 

[tmiller_iluvprinting]

Dot gain happens at the perimeter of the dot. Smaller dots (i.e. smaller diameters) have larger perimeter to area ratios and hence more dot gain.
Does that answer your question?

Let me re-phrase, since the size of the the sample dot %(plate dot) isn't used in the Murray-Davies equation, does it have any relevance when dot gain is calculated?
Regards,
Todd

 

[Deleted member 16349]

Let me re-phrase, since the size of the the sample dot %(plate dot) isn't used in the Murray-Davies equation, does it have any relevance when dot gain is calculated?
Regards,
Todd

The M-D equation is meant to determine the dot size. It does not calculate the dot gain.

When using the densitometer and the M-D equation and reading the printed screen, the result is the Apparent Dot size. This is then compared basically to the plate dot size. The difference is the dot gain.

Apparent Dot % minus the plate dot % = Dot Gain % eg. 65% - 50% = 15%

So for this measurement it would be important that if you are measuring a 50% printed screen, that the screen was not modified with tone correction curves. This could be getting into that old argument about whether one should linearize or not. I will stay out of that one.

As I understand it, in the past when using film, the Apparent Dot % was compared to the file dot %. There was not only dot gain as a result of ink spreading and optical effects but also the dot gain or loss due to the exposure of the dot onto a positive on negative plate.

I might have some of the details wrong and I am sure Gordon will correct any errors in my description.

 

[Alith7]

...snip
This could be getting into that old argument about whether one should linearize or not. I will stay out of that one.

As I understand it, in the past when using film, the Apparent Dot % was compared to the file dot %. There was not only dot gain as a result of ink spreading and optical effects but also the dot gain or loss due to the exposure of the dot onto a positive on negative plate.

I might have some of the details wrong and I am sure Gordon will correct any errors in my description.

I was thinking the same thing... don't want to start that argument, but this a more scientific approach to it...

My understanding...and like you said, I could be wrong...was that the dot gain from film to plate was assumed. the negs were essentially "linear". Aside from that, what I learned about that process was when I knew nothing. so, I'm just going to lurk on this one.

 

[rich apollo]

I think what Todd is alluding to is the fact that Murray-Davies is a comparison of density, or reflectance, not a geometric measure of the dots.

Dot gain going from film to plate was/is not assumed. It was/is real IF you were using negative film. Think of how a flashlight beam spreads as it goes forward. Light coming through the clear part of the film, the dot, spreads out until it strikes the plate surface - more so for e-up film than for e-down. And the thicker the film, the greater the gain. Positive film has the opposite characteristic - the plates will tend to sharpen.

 

[Deleted member 16349]

I think what Todd is alluding to is the fact that Murray-Davies is a comparison of density, or reflectance, not a geometric measure of the dots.

Dot area "A" is the unknown in the M-D equation. It is determined by measuring the reflectance of the paper, the reflectance of the solid and the reflectance of the screen. It is a geometric relationship.

The problem with the M-D equation is that with actual printing of the dots, the reflectance of the dot is assumed incorrectly to be the same reflectance as the solid. This is not necessarily the true. Also there is the optical effects around the dots. Dots also don't tend to have a uniform ink film but can have peaks of ink in the center of the dots.

M-D equation

R halftone = (1-A)Rpaper + A * Rsolid

This says that the reflectance of the halftone is the sum of the reflectance of the paper from the non print area plus the reflectance from the dot from the printed dot area. A is the dot area fraction and Rsolid is the assumption that the dot has the same reflectance as the solid, which one can measure.

Solving for A, the M-D equation can be rewritten as

A = (Rhalftone - Rpaper)/(Rsolid - Rpaper)

This is the area as a fraction. To get the percentage multiply by 100%.

 

[meddington]

I think what Todd is alluding to is the fact that Murray-Davies is a comparison of density, or reflectance, not a geometric measure of the dots.

What Rich said.
Murray-Davies doesn't care about dot size, shape, lpi and cannot know what you've done with your plates curves. Its a strength and a weakness. Measure responsibly.

 

[tmiller_iluvprinting]

What Rich said.
Murray-Davies doesn't care about dot size, shape, lpi and cannot know what you've done with your plates curves. Its a strength and a weakness. Measure responsibly.

This answers my question, thank you all!
Best,
Todd

 

[Alith7]

I think what Todd is alluding to is the fact that Murray-Davies is a comparison of density, or reflectance, not a geometric measure of the dots.

Dot gain going from film to plate was/is not assumed. It was/is real IF you were using negative film. Think of how a flashlight beam spreads as it goes forward. Light coming through the clear part of the film, the dot, spreads out until it strikes the plate surface - more so for e-up film than for e-down. And the thicker the film, the greater the gain. Positive film has the opposite characteristic - the plates will tend to sharpen.

well...yes. But a given shop, for the most part, used the same process: film thickness, emulsion, mask, plate, burner, burn length, etc. from job to job, so the dot gained was "assumed" to stay stable and a given shop printed the color they printed. Yes, I know it was more complex than that, just didn't want to go too far into something that was off topic.

 

[meddington]

Hi Eric,

Dot area "A" is the unknown in the M-D equation. It is determined by measuring the reflectance of the paper, the reflectance of the solid and the reflectance of the screen. It is a geometric relationship.

Not sure I understand how this is "geometric"? Shape isn't really a consideration, or are you referring to a relationship?

The problem with the M-D equation is that with actual printing of the dots, the reflectance of the dot is assumed incorrectly to be the same reflectance as the solid. This is not necessarily the true. Also there is the optical effects around the dots. Dots also don't tend to have a uniform ink film but can have peaks of ink in the center of the dots.

I don't doubt the attributes you describe, but how is the M-D equation making an assumption that the reflectance is the same between the solids and screens. If the ink isn't uniform in the screens, would this not be expressed by a difference in the reflectance measurement, and thus a different "dot gain" result than if it were uniform in the screens?

 

[meddington]

The problem with the M-D equation is that with actual printing of the dots, the reflectance of the dot is assumed incorrectly to be the same reflectance as the solid. This is not necessarily the true. Also there is the optical effects around the dots. Dots also don't tend to have a uniform ink film but can have peaks of ink in the center of the dots.

Upon reflection (pun intended) I do see your point on this. The equation does assume uniform ink, just as it would assume that the tint you measure hasn't been tweaked by plate curves. I'll retract my question here.

 

[gordo]

A few things:

Let me re-phrase, since the size of the the sample dot %(plate dot) isn't used in the Murray-Davies equation, does it have any relevance when dot gain is calculated?
Regards,
Todd

No, the size of the plate dot does not have relevance. Note also that the densitometer that is used to measure dot gain does not "see" the dots. It can only measure reflectance values so it cannot be a geometric measure of the dots. You tell it that you are measuring a given % dot (tone area) and the formula then derives a dot area tone value based on the relationship between the measured paper reflectance, the measured solid relfectance, and the measured reflectance of the patch that you've told it is, for example, a 50% patch. The dot area can then be compared with the original tone request (i.e. measured 50% on the film or requested 50% in the authoring application in the case of CtP) and a simple subtraction provides the dot gain value. I.e. I requested 50% in the file but in the presswork the dot area tone value measures 64%. 64-50=14% dot gain.

For the purposes of dot gain measurement it does not matter what the actual dot % value on the plate is, nor the size, shape or frequency of the halftone dots are. It also does not matter whether the plates/film are linear or not.

The Murray-Davis equation inherently assumes (like its predecessor the Demichael and Neugebauer equations) that the amount of reflection is linearly dependent on the relative amounts of ink and paper over the surface and that the halftones dots are well defined and that only direct reflection in the surface occurs, which in reality is far from true.

The issue of dot gain measurement is very complex, with a long history of attempts (did I mention the two-flux approximation and Kubelka-Munk theory?).
The best resource on the topic - if you want to hurt your brain is "Dot Gain in Colour Halftones" by Stefan Gustavson (ISBN 91-7871-981-X) - it's hard to find but for dot heads worth the search.

best, gordo

 

[Alith7]

A few things:



No, the size of the plate dot does not have relevance. Note also that the densitometer that is used to measure dot gain does not "see" the dots. It can only measure reflectance values so it cannot be a geometric measure of the dots. You tell it that you are measuring a given % dot (tone area) and the formula then derives a dot area tone value based on the relationship between the measured paper reflectance, the measured solid relfectance, and the measured reflectance of the patch that you've told it is, for example, a 50% patch. The dot area can then be compared with the original tone request (i.e. measured 50% on the film or requested 50% in the authoring application in the case of CtP) and a simple subtraction provides the dot gain value. I.e. I requested 50% in the file but in the presswork the dot area tone value measures 64%. 64-50=14% dot gain.

For the purposes of dot gain measurement it does not matter what the actual dot % value on the plate is, nor the size, shape or frequency of the halftone dots are. It also does not matter whether the plates/film are linear or not.

The Murray-Davis equation inherently assumes (like its predecessor the Demichael and Neugebauer equations) that the amount of reflection is linearly dependent on the relative amounts of ink and paper over the surface and that the halftones dots are well defined and that only direct reflection in the surface occurs, which in reality is far from true.

The issue of dot gain measurement is very complex, with a long history of attempts (did I mention the two-flux approximation and Kubelka-Munk theory?).
The best resource on the topic - if you want to hurt your brain is "Dot Gain in Colour Halftones" by Stefan Gustavson (ISBN 91-7871-981-X) - it's hard to find but for dot heads worth the search.

best, gordo

All I can picture is the old school superman standing with hands on hips and red cape blowing just slightly to the left. "Dun-dah-dah-DUN! Gordo to the rescue!"

That is one of the clearest explanations I've seen yet on dot gain and color scanning.
Am I understanding this right? to calculate dot gain...
1. The scanner reads the paper and gets a reflective value it pegs as "Point A" on the color gamut and calls it "0%".
2. The scanner reads the 100% patch you give it and gets a reflective value it pegs as "Point C" on the color gamut and calls it 100%
3. The scanner reads say the 50% patch you give it and gets a reflective value it pegs as "Point B" on the gamut.
4. Then, it compares the "distance" from "Point A" to "Point B" and "Point C" to "Point B", and on a scale returns the value of "Point B" based on that comparison.

Now, to get that reflective value, it essentially "blurs" it's vision so it sees a "solid" color based off the paper/ink combination that is in front of it? Kind of like stepping back so that you can no longer see the dot, but only the illusion of solid color?
If that is right, then really, it doesn't even "see" the dot or care on the dot shape/size? Or have I completely jumped the train off the track and off the bridge on this one? If I did, just say so, and I'll edit this out so I don't confuse anyone else.

 

[Deleted member 16349]

For the purposes of dot gain measurement it does not matter what the actual dot % value on the plate is, nor the size, shape or frequency of the halftone dots are. It also does not matter whether the plates/film are linear or not.

I would disagree with this. The M-D equation is quite clear on what it is trying to do. Shapes and frequency of dots are not important but area of the dots are.

If one measures the screen and it is not a result of a 50% screen on the plate, then when comparing the measured/calculated apparent dot area with the expected 50%, one will get different values of dot gain depending on what the actual dot percent was on the plate. If one does not use a 50% screen on the plate for this measurement, one does not have a datum.

Maybe for practical reasons, this is not such an issue these days but it seems to me that if printers generate dot gain values that are not measured in a similar way, how can they adjust their plates to some standard that requests a specific dot gain.

I seems clear to me but I won't argue the point if there are methods now used that don't respect the intention of the calculation. I remember the time over twenty years ago, we paid a lot of money for Brunner bar film strips that had certified patch screen percentages. The point was that one could not get accurate readings if one did not have accurate screen patches.

 

[gordo]

Am I understanding this right? to calculate dot gain...
1. The scanner reads the paper and gets a reflective value it pegs as "Point A" on the color gamut and calls it "0%".
2. The scanner reads the 100% patch you give it and gets a reflective value it pegs as "Point C" on the color gamut and calls it 100%
3. The scanner reads say the 50% patch you give it and gets a reflective value it pegs as "Point B" on the gamut.
4. Then, it compares the "distance" from "Point A" to "Point B" and "Point C" to "Point B", and on a scale returns the value of "Point B" based on that comparison.

Now, to get that reflective value, it essentially "blurs" it's vision so it sees a "solid" color based off the paper/ink combination that is in front of it? Kind of like stepping back so that you can no longer see the dot, but only the illusion of solid color?
If that is right, then really, it doesn't even "see" the dot or care on the dot shape/size? Or have I completely jumped the train off the track and off the bridge on this one? If I did, just say so, and I'll edit this out so I don't confuse anyone else.

Close, but not quite. There is no "color gamut". This is tone reproduction. It also doesn't blur it's vision. It just measures reflected light, like a light meter used in photography. It does not have an ability to "see" dots or lines or anything else.

So, it goes like this:

1. The scanner reads the paper and gets a reflective value it pegs as "Point A" and calls it "0%".
2. The scanner reads the 100% patch you tell it it is a solid 100% patch and gets a reflective value it pegs as "Point C" and calls it 100%
3. The scanner reads say the 50% patch you tell it is a requested 50% patch and gets a reflective value it pegs as "Point B".
4. Then, it compares "Point A" to "Point B" and "Point C" to "Point B" using the Murray Davis formula (there are other formulas it could use), and returns the value of "Point B" as a dot area tone value based on that comparison. It can also subtract the requested tone value (e.g. 50%) from the calculated dot area tone value (e.g. 64%) and display that difference value as "dot gain" (e.g. 64%-50% = 14% dot gain).

best, gordo

 

[nikunj]

Is the dot gain is an advantage or an issue for a printer..?????

 

[Alois Senefelder]

Dot Gain

Dot Gain

Hello Pre - Press and fellow Lithographers,


"The effects of dot gain on the print results" - PDF, I hope you will find of interest and value.



Regards, Alois

 

[gordo]

Is the dot gain is an advantage or an issue for a printer..?????

Dot gain is simply a natural part of the printing process. It is actually more productive to think of it as representing your tone reproduction from 1% to 99%. You can use tone reproduction curves during film or plate imaging to achieve whatever tone reproduction you want in your presswork. You can also build it into an ICC profile.
Dot gain (or TVI (Tone Value Increase)) values have historically been stated in published print standards - but this is a failing of the industry standards people and it was based on a film workflow. Instead they should have been publishing target tone reproduction curves - the printer's task would be to achieve those target tone reproduction curves using whatever dot gains were needed to do so.
Dot gain is only an issue if it is variable and/or out of control in which case it is looked at as a process control metric/indicator.

best, gordo

 

[Alois Senefelder]

Dot Gain # 2

Dot Gain # 2

Hello fellow Lithographers and members of the Pre-press Brigade,


PDF's - "Dot Area, Dot Density and Tone Value" hopefully of interest and value.



Regards, Alois