Stochasdtic Screens

[gtmo]

Need some help, trying to generate film with Harlequin rip Stochastic screens. Running on Avantra 44.

In out putting Super Fine, Fine, Medium and Coarse they appear to be all the same.

Any Help is much appreciated.


Thanks in advance.

 

[Ulrich]

hi

i don´t know what "Super Fine, Fine, Medium and Coarse" means, but i know that stochastic screens and film is not a fitting combination (computer to plate yes, but not computer to film, because via film you will get lost superfine dots in 20my size by exposering a plate with that).

For what business you want to exposure the film?

If there will be no more response here, post some pictures of the dialog to get an idea of the possible opportunities...


Ulrich

 

[gtmo]

Hello Ulrich,

This would be for silk screening.

Thanks

 

[gordo]

Hello Ulrich,

This would be for silk screening.

Thanks

You can certainly use FM screening in a film to plate situation (before CtP Canada's postage stamps were printed 10 micron FM in a film workflow). What you have to be careful with is the draw down in the vacuum frame when you expose the film to the plate or other media to make sure that the film is in perfect contact for exposure.

With HDS, the terms "Super Fine, Fine, Medium and Coarse" just means the size of the minimum dot.

These are the minimum dot sizes, in microns, for HDS screening on a 2400 dpi device. If you lower the device dpi then the minimum dot size changes even though the same screen has been specified. For example, if you specify "HDS Fine" then on a 1200 dpi device the minimum dot size is 30 microns, at 1800 dpi it's 20 microns, at 2400 dpi it's 15 microns. Same screen but different results.

If memory serves, the minimum dot size happens at the 20% tone. Lighter tones are achieved by removing halftone dots rather than making them smaller.

At 2400 dpi:
HDS Super Fine 1x1 pixel - 11
HDS Fine 2x1 pixels - 15

HDS Medium 2x2 pixels - 21
HDS Coarse 2x3 pixels - 26

HDS Super Coarse 4x4 pixels - 42

For silk screen printing you will probably use, at 2400 dpi, HDS Coarse or HDS Super Coarse. At 1200 dpi you'll probably use HDS Fine or HDS Medium.

I know that doesn't answer your specific question, however, I hope it helps you diagnose what might be happening.

 

[gtmo]

Hello Gordo,

Thank you very much for the information, it helps us.

 

[Ulrich]

That´s what i call GREAT(!) teaching:

...What you have to be careful with is the draw down in the vacuum frame when you expose the film to the plate or other media to make sure that the film is in perfect contact for exposure.

With HDS, the terms "Super Fine, Fine, Medium and Coarse" just means the size of the minimum dot.

These are the minimum dot sizes, in microns, for HDS screening on a 2400 dpi device. If you lower the device dpi then the minimum dot size changes even though the same screen has been specified. For example, if you specify "HDS Fine" then on a 1200 dpi device the minimum dot size is 30 microns, at 1800 dpi it's 20 microns, at 2400 dpi it's 15 microns. Same screen but different results.

If memory serves, the minimum dot size happens at the 20% tone. Lighter tones are achieved by removing halftone dots rather than making them smaller.

At 2400 dpi:
HDS Super Fine 1x1 pixel - 11
HDS Fine 2x1 pixels - 15

HDS Medium 2x2 pixels - 21
HDS Coarse 2x3 pixels - 26

HDS Super Coarse 4x4 pixels - 42

For silk screen printing you will probably use, at 2400 dpi, HDS Coarse or HDS Super Coarse. At 1200 dpi you'll probably use HDS Fine or HDS Medium.

I know that doesn't answer your specific question, however, I hope it helps you diagnose what might be happening.

...and that´s what i call producing with laboratory conditions:

You can certainly use FM screening in a film to plate situation (before CtP Canada's postage stamps were printed 10 micron FM in a film workflow).

Ulrich

 

[Slammer]

Works yeees-ish, but as Ulrich says, lab conditions (übrigens Servus Uli) depending on what mesh you are using the best results will be with coarse and even then you will get pixels "falling" through the web, if I remember correctly there was a addition to the FM screenings that you added to the code and you could get such things as super coarse. It worked but in the end it was too much trouble to get decent results.

 

[gordo]

Not lab conditions - but the 10 micron that Canada Post uses was extreme for a film workflow (they now use CtP to do it). In the late 1980s I worked in a prepress service bureau and supplied film halftoned with 20 micron FM screening to many print shops in the city where I worked. They certainly did not have lab conditions when burning plates. BTW, having CtP is not a guarantee that you can use FM screening. There are some CtP devices that are not as good as film to plate systems.

 

[Ulrich]

Not lab conditions...

bitmapping an 1%-value screen-dot of a 175lpi AM screen exposured with 2540 dpi is a square in a size 0,00057 inch (=1,4 microns), right?

1 line is 0,0057 inch, (the reciprocal of 175, so 1 divided through 175)

to write a 1%-value screen-dot there is needed only the tenth part of that 0,0057 inch width, that means at least 0,00057 inch (1,4 microns rounded)

In the early and mid 90ies i worked in the prepress still glueing films on a mounting foil.

The second vacuumized UV-exposuring for each plate with a diffusion foil had to brake out the cutting edges resulting from the assembly and dust, eye lashes etc, so that not only the 1% dots got lost, even many of the 2% dots got broken, no chance for a 10 micron dot FM and a very hard fight for a 20 micron dot, too.

And working without the second exposure with a diffusion foil is meant with Lab-condition.

Of course, if you have an imposed film in a larger size there is no need for braking the cutting edges via a second exposuring process, but still facing dustproblems, i wouldn´t call it a daily temple of joy to work in that way ;-)


best, thankfully and with all my respect!

Ulrich

 

[gordo]

bitmapping an 1%-value screen-dot of a 175lpi AM screen exposured with 2540 dpi is a square in a size 0,00057 inch (=1,4 microns), right?

1 line is 0,0057 inch, (the reciprocal of 175, so 1 divided through 175)

to write a 1%-value screen-dot there is needed only the tenth part of that 0,0057 inch width, that means at least 0,00057 inch (1,4 microns rounded)

1,4 microns (or 1.4 microns outside of continental Europe) is not correct. The formula is more complicated than that (it's on my blog).

2540 dpi means the addressability of the CtP device - not its resolution. Theoretically it is using a 10 micron laser spot. (In practice it is usually a larger spot of energy).

In any case, at 2540 dpi and 175 lpi a 1% dot would be 15 microns.

If you have a 10 micron laser spot to work with then you have a problem. How do you expose a 15 micron pixel? That would mean you have to expose 1 1/2 laser spots. But how do you expose a 1/2 laser spot?

There are two possible solutions to this problem that are commonly used.

The solutions are based on the fact that you are creating a 1% tone that is made up of many halftone dots. It does not matter what size the individual dots are as long as you end up with a 1% tone.

So...

1. You dither. This means that you alternate between 10 micron (single pixel) dots and 20 micron (4 pixel dots). The tone effect of the average of 10 and 20 microns is the 15 micron dot that you needed to create the 1% tone.

or

2. You remove a percent of 20 micron (4 pixel dots). So you alternate between an imaged dot and a non-imaged dot in order to create the 1% tone (this is how hybrid AM screens like Sublima work)

A properly maintained film to plate system should be able to hold a 20 micron halftone dot (which is a 2% sized dot at 175 lpi on a 2540 dpi device.) At least my clients back then had no trouble or they wouldn't have bought film from me LOL!

 

[Ulrich]

In any case, at 2540 dpi and 175 lpi a 1% dot would be 15 microns.

harrgh!!! i forgot to move the comma from cm to mm in my account...

getting an PANTONE Rubin red face and walking away moving in the rear rows

;-)

Ulrich

A properly maintained film to plate system should be able to hold a 20 micron halftone dot (which is a 2% sized dot at 175 lpi on a 2540 dpi device.) At least my clients back then had no trouble or they wouldn't have bought film from me LOL!

You sold them larger, already imposed Films?

Before CTP-times nobody expected 1% and 2% tone-values on the paper, now sometimes they are badly surprised to find them... ;-)

 

[gordo]

harrgh!!! i forgot to move the comma from cm to mm in my account...

getting an PANTONE Rubin red face and walking away moving in the rear rows

;-)

Ulrich

You sold them larger, already imposed Films?

Before CTP-times nobody expected 1% and 2% tone-values on the paper, now sometimes they are badly surprised to find them... ;-)

LOL - very funny! I sold 4 up film imaged on an Agfa Selectset 7000. I was one of the beta sites for Agfa's CrystalRaster FM screening and I also was developing my own FM screening system (a plug-in to PhotoShop) at the time.

 

[aqazi81]

Just made a video for fun.
It's a 2400 dpi, 150 lpi square dot screen.
​​​​​​
​​​​​

 

[Ulrich]

hi,

nice to see in a video that the growing happens in a spiral movement!

Ulrich

 

[Repro_Pro]

Generally, in silkscreen printing the smallest dots are lost (while at the darker end of the scale the smaller "white" dots get plugged).
Compared to Offset printing, these are limitations inherent to the process.
If Direct Emulsion is used, the smallest dot must be supported by at least 4 threads of the mesh, so mesh count as well as thread type and thickness are the basic limiting factors (thread color too, BTW).
Some other factors are mesh tension, squeegee type, ink consistency (liquidity), as well as many others.
So, using Stochastic screens or any higher LPI screens for that matter, will require a long learning curve...

 

[gordo]

Generally, in silkscreen printing the smallest dots are lost (while at the darker end of the scale the smaller "white" dots get plugged).
Compared to Offset printing, these are limitations inherent to the process.
If Direct Emulsion is used, the smallest dot must be supported by at least 4 threads of the mesh, so mesh count as well as thread type and thickness are the basic limiting factors (thread color too, BTW).
Some other factors are mesh tension, squeegee type, ink consistency (liquidity), as well as many others.
So, using Stochastic screens or any higher LPI screens for that matter, will require a long learning curve...

It can be pretty straightforward.
1 - do a test to determine your minimum printable dot size for highlight and shadow.
2 - set that dot size using a hybrid AM of FM screen at the lpi you wish to run.

 

[Repro_Pro]

Basically it certainly is straightforward.
Surely if one settles for printing comparatively large minimum size dots.
It gets more complicated when one needs to get smaller dots to print consistently.
Identifying and controlling the variables once you need to "raise the bar" is where the learning curve getting steeper...