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Fixed dot size in halftone?

4537 Views 7 Replies 2 Participants Last post by  spreadingink
Hey everyone, firstly this is a great forum.

The thread on using ghostscript for halftones was fantastic, I can now do halftones. My question now is, I had a chat to a screen printer and he suggested that it would be ideal if I can fix the dot size in my halftones and instead increase white space inbetween the dots to get a better gradient textile print. After printing my first halftone film I can see where he is coming from, as the gradient gets to white the dots become really small and may not expose correctly or at all.

Has anyone come across this practice and how does one go about achieving fixed dot size?

Cheers in advance.
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This is more of the idea used in index printing, which used 'square' dots of a non-varying size. Halftones use a variable dot size to give you a gradient.

Index can give a really good print, but will usually require more colors to print the same design compared to printing it with halftones and sim-process seps.

Hope this helps.

Thanks Dave,

Im not having any luck with googling index printing. Is there another term that it falls under? And when you say it requires more colour, do you mean more ink or more screens?

Do you have any problems printing the finer end of a gradient, ie losing some of the smaller dots on the artwork? So far my halftones are set to 55lpi 22.5deg ellipse.

Thanks again
We usually print @ 55 lpi or 65 lpi depending on the artwork. As for losing the dots at the low end of the spectrum that is only one of the problems you can actually 'fill in' the dots at the higher end as well say from 90 percent to 100 percent where 90 percent fills in and become 100 percent even though it shouldn't be. A lot of how good you can hold the dots at both ends of the spectrums depends on many factors among them are: the accuracy of your output device and rip, the screen mesh that you are printing through, the type of light source you are using to burn your screens. Another factor that should be considered is how you have photoshop set up to deal with dot gain. If you have a densitometer you can do an pretty good test by doing the following: make up a set of squares that goes from a 1% dot all the way to 100% print this out on your rip (at any lpi that you want to) and then burn a screen and expose it. You will see where you are not holding the dots correctly at both ends of the spectrum just by doing this (ie 5% becomes totally washed out). You can then create a set of custom curves to apply to your channels seps in photoshop that will effectively adjust your 5% dot to 0% and say your 92% to 100%. Once this is done you can take this same file and re-output it with this custom curve. It should now allow you to have a percentage difference across the spectrum from 1 - 100%. To take it all a step further you will need a densitometer. Take the adjusted screen and put it on the press and print it. You can now take the densitometer and measure your print across the spectrum to further refine your custom curves. For instance if you read the 50% box and find that with dot gain from priniting it is now showing a 55% fill patern, you can pull back the curve 5% at the 50% part of the box (so that your rip is giving you a 45% dot at 50% percent and test again to remeasure. It can be a lot of work, but you really can refine this if you want to. Reality is that most sim process prints will look good if you are holding in the range from 5% up to 93% or so which you can figure pretty well by doing the above tests without a densitometer, if you happen to be lucky enough to have a meter though you can really tweak it all in. Once you have your curves set you can then use those curves to adjust your final seps prior to output or some rips will allow you to have them use a custom curve during the output (which will allow you to view the sep on screen the way you want to without having the custom curve affect the way it looks on screen, but yet have it applied when you do the actual file output).

Why do all of this? Why not use someone else custom curves? You will find that while using another's set of curves may work for you it may not too - the variables are too high - screen tension, mesh counts, point exposure light source, and even your press and off contact can all effect this so having your own set is the best route as it will be set to your conditions. Understand though that if you don't use newman's and keep your screen tensions the same and the variables under control then it's probably not worth the time to do all of the above testing - it only takes some slight variation and it could throw your curves off. Most printers find that they get acceptable results for most clients without ever doing all of the steps I mention here, but I thought you might be interested in what is attainable and how to get there though, so I explained it. :)

Hope this helps, let me know if you have any more questions or I can help further.

For more information on index printing - try google for index separations - you will get a bunch of stuff - it's really the separation method rather than printing it that makes it an index print. Index usually requires more colors (and screens) to get the same look you will with less in the channel separation using halftones.


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Thats a brilliant explaination of how to customise halftones. Strangely something similar popped into mind after my last post but no where near as intricate as what you had explained just now. Nothing beats good advice from someone in the know. I'll have a crack at setting up my own custom range and see how I go with it.

Dropped by the screen printers today to show him my test film. He was quite happy with the result but introduced me to the world of "moire". With the untrained eye I had a hard time seeing what he was talking about untill he put it on 77mesh over a light box and started rotating it. Bizzare concept to get your head around but once you see it in person you understand. Thankfully my 22.5deg setting for the one colour halftone worked well with it. Will be very interesting when more colours are involved.

Stumbled across some index sep info, it was something I played with a while back for a client on Pshop but really never knew what I was doing. Was never really happy with the final artwork, though I'm sure it has its purposes.

Thanks again Dave, I'm sure this will enlighten other forum members on the topic.
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Moire - yep - there are actually two kinds. You can get moire from color to color (sometimes noticeable sometimes not), but the most common kind is simply an interference between the frequency of the openings of the mesh and the threads of a particular mesh and the frequency of the halftone dots. Usually the actual angle of the dots has little to nothing to actually do with it unless you are talking about color to color moire which is usually most notably seen in 4CP work.

One thing interesting to note is that an actual index sep never reallys looks quite like what it will printed when viewed on screen in pshop. The actual print will look much 'smoother' than it will in pshop, but it will almost always take more colors to do the same design with this method.

Glad you found my post interesting - it's really not rocket science, but it can take some time to get it right and controlling variables is where you will see the biggest gains in overall print quality whether you actually create custom curves or not.


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I should correct myself here though - color to color moire usually is evident in 4CP process because you use differrent angles from one color to another and it this moire that you don't want. Understand that when you use different angles from one color another there is always a color to color moire, it's just that you choose angles to minimize the eye's ability to discern this moire and to allow the 4CP colors to overlay each other by different percentages. It is precisely this moire and overlapping that allows the gamut range of 4CP to be as high as it is.

Good catch Fred - the fabric weave can have an effect as well!


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