Calcutate dot gain based upon readings with stochastic screen

[screenpixie]

Hello everybody,

I've been told that it's impossible to calculate dot gain, based upon readings from printed stochastic screens. Only AM-screens (round, elliptical, ...) should do the trick.

I find that very hard to believe since tint values (and thus dot gain) are calculated using both a blank paper measurement and a 100% measurement. If these two readings are known to the measuremet device, it should be quiet simple to calculate the relative positive of the measured tint since the measured spot (diameter about 4mm) is averaged.

Am I mistaking? And what can be the cause of inaccurate readings for stochastic screens?


Thanks in advance for your feedback.

screenpixie

 

[gordo]

Hello everybody,

I've been told that it's impossible to calculate dot gain, based upon readings from printed stochastic screens. Only AM-screens (round, elliptical, ...) should do the trick.

Whoever told you that is ignorant. The device typically used to measure dot gain is a densitometer. Densitometers don't know anything about dots. They measure and compare density. So, they can measure dot gain irrespective of what halftone screen dot is being used.

Best, gordo

 

[kewlbigdan]

Dot Gain is a math formula and is based on Density. Since AM and FM are both based on a dot shape and ink denisity there should be no difference.
This website has the Murray-Davis formula. Welcome to Bruce Lindbloom's Web Site
BUT
In agreement to Gordo, who I don't know but serious respect whenever I read his posts, these 'people' that said there will be a difference between AM and FM dot gain could very well be talking about Mechanical (Murray-Davis) and Optical (Yule-Nielsen) dot gain. Both are present but low end densitometers only measure Mechanical. It is widely believed by printers that you cannot measure Optical dot gain, which is false, but tell my boss that.
I do know that the size of the dot at the same lpi will change the dot gain in FM screening. The smaller the dot the larger the sum of all circumference of all the dots in a given area, the more dot gain. There is a math thing for this too but I can't find it.

 

[gordo]

these 'people' that said there will be a difference between AM and FM dot gain could very well be talking about Mechanical (Murray-Davis) and Optical (Yule-Nielsen) dot gain. Both are present but low end densitometers only measure Mechanical. It is widely believed by printers that you cannot measure Optical dot gain, which is false, but tell my boss that.

Jut to clarify - Densitometers don't know anything about dots, they compare tones: paper, solid ink area, tone patch and use the Murray-Davis formula to determine the dot area or dot gain. So, both mechanical and optical dot gain are included. This is the typical way that dot gain is measured and reported since we see the combined effect of mechanical and optical dot gain when we look at presswork.

The Yule-Nielsen formula is a modification to the Murray-Davis formula. It introduces an "N" factor to attempt to factor out the optical part of dot gain in order to provide an approximate correlation between the measured dot and the physical dot size. This is not how we see tones in presswork. The Yule-Nielsen formula is typically used when densitometers are used to measure printing plates. Modifying the N factor helps to zero the device by defining the threshold between the printing dot and the non-image area of the plate.

If you are using a plate reader or dot reader that takes a picture of the plate or printed dot rather than a densitometer then there is an internal tone thresholding to separate the plate's printing dot from the non image area of the plate. It's akin to the N factor in the Yule-Nielsen formula. It does the same thing when measuring the dot on the press sheet. Because, unlike a densitometer, a photo imaging device it does have resolution limitations. Not all plate readers or dot readers have the resolution to capture an image of an FM screen. Or if they do, the thresholding may affect the integrity of the reported values. Perhaps this is what the OP was referring to?

I do know that the size of the dot at the same lpi will change the dot gain in FM screening. The smaller the dot the larger the sum of all circumference of all the dots in a given area, the more dot gain. There is a math thing for this too but I can't find it.

Basically right, but not just for FM screens. The smaller the dot diameter is - the greater the perimeter to area ratio is. Since dot gain occurs at the perimeter of the dot, then the smaller the dot the greater the dot gain. So a high lpi AM or XM screen will have basically the same dot gain profile as an FM screen. But that can be factored out with a dot gain compensation curve applied to the plate.

To find the "math thing" for this you have to see the movie Life of Pi :-D

best, gordo

 

[screenpixie]

Thanks for your feedback.

The reason for asking was that we will be using an off-line measuring device on our new press. Since we are using different dotshapes throughout our jobs, I was wondering if I should allways use the same dotshape in the control strip. Or should I use the same dotshape for the control strip as I use in that job. It seems obvious to me that the control strip should use the dot as the job. But, if the technician of the new press is right, I would allways have to use the same control strip, no mather what the dot in the job is.
The reason for using an offline device is that the same device can be used for multiple presses. This way we can rule out the differences between results measured by inline devices on different presses. The offline device will be used for setting up a color management. Once this has been done, we will try to 'translate' these results to the inline measurement systems.

I'm aware of the disability of some plate readers to measure dots. That's what set me in doubt. But now I know why this is. It will have something to do with the formula used on the plate reader for calculating the dot.

My God, what a great forum this is! Thanks to all the valued feedback of fine people througout the world.

screenpixie

 

[gordo]

The reason for asking was that we will be using an off-line measuring device on our new press. Since we are using different dotshapes throughout our jobs, I was wondering if I should allways use the same dotshape in the control strip. Or should I use the same dotshape for the control strip as I use in that job.

You should use the same dot shape in the control strip as is used in the live image area. This is because things like slur and doubling also affect dot gain and dot shape affects how slur and doubling affect dot gain.

Why are you using different dot shapes in your jobs?

gordo

 

[kewlbigdan]

See this is why I like to read Gordo's posts!
I know just enough to be extremely dangerous in a print shop!

 

[screenpixie]

As a packaging printer, we typically print lots of repeat jobs. Jobs we have been printing and reprinting for a long time still have their original jobshape, which was elliptical. New jobs are typically printed with stochastic screen. Once the color management has been set up properly, we should be able to convert jobs from AM to FM without to much color difference.

screenpixie

 

[gordo]

I know just enough to be extremely dangerous in a print shop!

The only extremely dangerous person in a print shop is a customer.

Best, gordo

 

[rich apollo]

<I've been told that it's impossible to calculate dot gain, based upon readings from printed stochastic screens.>

That is completely untrue. If you're using a video device, then you need something with adequate resolution.