HDS Screening question

[Jbarrie5]

I'll try to be brief with this question. We are currently using Harlequin's HDS FM screening. Additionally, when we chose the FM package (many years ago), we went with 2400 dpi Coarse screening, but... even though we thought it would equate to 25 micron... it seems to be much smaller.
Now... I want to choose the correct FM screening that will be a true 20-25 micron dot(or around a 200 line screen equivalent), do I need to adjust my dpi in order to get a 25 micron? I made a test sheet and printed several dot patterns, one of which was what I thought would be the equivalent to 200 line screen... 1524 dpi using HDS medium dot pattern, is this correct? Should I be concerned about using such a low dpi in order to achieve the 20-25 micron dot (or 200 line screen equivalent)?
The Global Graphics white pages on the subject is a bit confusing and I would appreciate some assistance in choosing the right dpi/to FM dot pattern to hit my target.

Thanks,
Jeff

 

[Ferran]

Hi Jeff, I don’t have any experience with HDS (Harlequin Dispersed Screening), but I do with other FM screening solutions from other vendors. I think you are wrong when you say 20-25 microns is 200lpi equivalent. Having read the whitepaper from Global Graphics®, I would say that HDS Coarse at 2400 dpi produces a 26 microns spot size, and if you low your resolution you will get a bigger dot size. I am not sure if you can increase the platesetter resolution to 3600 dpi in order to get for instance 17 microns, but I guess you won't be able switch to 3600 dpi. So, if you want to get a smaller dot size like f.i. 21 microns, I think you will have to go for the HDS Medium option. Please don't take my word for granted, if I were you I will contact directly with Harlequin support and ask for help. Thankyou, regards.

 

[Jbarrie5]

Only thing about the Global Graphics instructions, is that it talks about medium at 2X2 pixels. I have tried 2400 dpi using medium HDS and the result was a very small dot (probably about the size of 300-350 line screen AM dot)... the meaning of 2X2 is puzzling to me, but I think it means 2 dots.

 

[Ferran]

I have said before you were wrong thinking 20-25 microns is 200lpi AM equivalent. Imaging at 2400dpi, you get 21 microns spot size with HDS Medium and 26 microns spot size with HDS Coarse. The table at the end of the whitepaper that shows resolution and spot size (microns) is very useful. But now, I am not sure what you want to accomplish…

You say now 21 microns is very small, okay then, Which dot size (microns) do you want to get?

 

[Jbarrie5]

Sorry if I'm not very clear... I have put in a call to Global Graphics for assistance as well. Before using this screening we used Fuji's Taffeta at 25 microns. HDS medium at 2400 dpi is half the size of the 25m Taffeta.. I believe the chart you have posted is misleading. No way is HDS medium at 2400 dpi rendering a 21 micron dot. I think the 2X2 pixels has something to do with it.

 

[gordo]

Ferran omitted the qualifier in his excerpt from the white paper. The micron sizes quoted are for a 20% HDS screen. Because HDS is a second order FM screen the dot size varies through the tone scale. When one says "I want a 25 micron screen" the 25 micron is typically the average of the dot sizes rather than the specific dot size throughout the tone scale. There are several ways of calculating the lpi equivalent so different sources will give different values.
It will be interesting to hear what Global's response is.
Here are the figures that I have:
HDS Medium @ 2400 dpi - 550 lpi equivalent, minimum highlight dot 10.6 microns, midtone dot 15 microns
HDS Medium @ 1200 dpi - 325 lpi equivalent, minimum highlight dot 42.3 microns, midtone dot 48 microns
Don't adjust your dpi to change dot sizes as it could cause artifacts or compromise with image elements. Go with 2400 or 1200 dpi according to your (unmentioned) application.
You should tell Global what your application is - heatset, coldset, commercial? And they should be able to advise which screen at which dpi is supported and recommended for your application.

 

[SteveSuffRIT]

Maybe some basic math will provide a reference.
At 1200 spi (addressability), a single spot is 0.00083" or 21.1 microns (um).
Obviously, at 2400 spi, a single spot is half the size, 0.00042" or 10.6 microns.
At 200 lpi (AM), a single row of dots is 0.005" or 127 um.
A 10% highlight dot at 200 lpi @ 2400 spi is a 4x4 spot, which is 0.0017" or 43 um.
Steve Suffoletto

 

[gordo]

Maybe some basic math will provide a reference.
At 1200 spi (addressability), a single spot is 0.00083" or 21.1 microns (um).
Obviously, at 2400 spi, a single spot is half the size, 0.00042" or 10.6 microns.
At 200 lpi (AM), a single row of dots is 0.005" or 127 um.
A 10% highlight dot at 200 lpi @ 2400 spi is a 4x4 spot, which is 0.0017" or 43 um.
Steve Suffoletto

But that alone does not determine AM (LPI) to FM equivalency if that is your intention.

 

[Ferran]

Ferran omitted the qualifier in his excerpt from the white paper. The micron sizes quoted are for a 20% HDS screen. Because HDS is a second order FM screen the dot size varies through the tone scale.

Yep, you are right Gordon, thank you for pointing that out.
I thought this screening uses dots of the same size all over the density range. But now, when I look at your figures I have to admit I am a bit lost

It would be nice if Jeff can share with us his findings after having contacted Global Graphics.

 

[gordo]

when I look at your figures I have to admit I am a bit lost

How so?