Halftone Dots and Screenprinting
A user guide to picking the best halftones
by David Foster. Positivity Ltd.
Screenprinters have a bewildering array of halftone dots to choose from when producing their imagesetter films, we look at why you might like to use particular dots and why some should be avoided.
Halftones come in two basic varieties FM (frequency modulated) and AM (amplitude modulated) screens, the same as radio signals.
Frequency modulated have the same basic dot size and vary the ‘blackness’ by printing more dots (i.e. frequency), as used by inkjet printers.
Amplitude modulated have the same number of dots but vary their size to increase the tone. This is what traditional imagesetters produce.
The simplest halftone is a round dot which just gets bigger.
SIMPLE ROUND DOT
This does the job, but as the dots join they create ‘stars’ with sharp corners, these are ideal places for ink to dry in, closing up the dots and blocking the screens.
The dots join at around 71% to four neighbours. This gives rise to an optical illusion, exacerbated by the behaviour of ink, where a gradient shows a sudden jump in density, know as a tone jump.
The next stage of evolution was Euclidean dots.
EUCLIDEAN ROUND
The dots become a chequerboard at 50% and then become negative dots into the shadows – gets over the drying in problem but still suffers from a tone jump albeit at 50%.
If you use elliptical dots rather than circular dots, the long axis meets before the short axis, this gives two smaller tone jumps rather than one big one and looks a lot better.
SIMPLE ELLIPSE
Euclidean elliptical dots can be combined to address both printing problems.
Geometric or Line Screens have been fashionable in recent years, they give very tight rosettes and are very easy to find moiré-free screens.
Their great failings are that the highlights can become detached from the screen and can peel off the rest of the line.
The prints will also have a colour cast if used with standard settings, it is necessary to produce an ICC profile for proper colour.
SIMPLE LINE
A long axis ellipse gives the same effect as a line but can avoid the problem of highlights leading to the line detaching.
SCREEN LINE
FM screens are often used in t-shirt printing. They avoid all moiré, but can appear rough in the highlights.
FM/Stochastic Screen
Traditional Postscript screens are set with the Cyan, Magenta and Black screens 30° apart i.e. 30/60/90° with the yellow plate 15° away from the others.
This leads to unavoidable moiré between the yellow and cyan/magenta screens.
A hybrid of traditional Postscript screens for the Cyan, Magenta and Black plates plus an FM yellow plate avoids such problems.
A sophisticated screenprint orientated RIP such as Wasatch SoftRIP has the capabilities to produce a variety of screening options including the hybrid screening.
It has never been easier to produce professional looking screen positives from low resolution devices such as inkjet printers.
Edliveshere.com has cool information about stochastic vs conventional printing. They also have good info about other printing and paper techniques. Super useful!
http://edliveshere.com/issue/stochastic_conventional/
So it seems there is more to halftones than the simple Photoshop filter? [Does shocked face]. Interesting read, it is really good to look at a technique in a bit more depth and in a practical situation.
Ed really knows his stuff, thanks for the Edliveshere.com link Lizy, this is very interesting, I really like examples and the magnifying tool. I was not aware of such an interest in Stochastic outside of screen printing, it will be very interesting to see how things develop. Although I am pretty sure David Foster of positivity who wrote this article for us is well aware of its advantages for other mediums.
I can’t believe I just read that whole article and was interested in how many types of halftones there are! I must be such a geek to know that I’m going to be able to use a phrase like Euclidean elliptical dots can be combined and there are FM and AM screens.
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Thanks for taking a look Andy, this is just skimming the surface regards dots, Dave has already submitted an article in response to Lizys comment which I shall post soon if you are really interested in knowing more, but I suspect your real interest is in the commentluv, I am interested in seeing how commentluv works out for us. First signs are pretty good and I am really pleased you absorbed some info before contributing.
the latest post about tshirt printing for fundraising is a great idea, I once used tshirts to promote a website I was making at the time. It worked out great, maybe it’s time I did an article about it and linked to here!
I am quite impressed by your site and it’s niche/focus, your quality articles make this place an authority, I’ve only ever seen 1 other cloth printing type site and it was pants compared to this!
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[...] Gradients require a little thinking about when screen printing. Moire patterns are tricky to avoid, many printers have their own rule of thumb calculations to decide on meshes, the most common are 3.5 or 4 X your halftone LPI = the mesh count you should use (imperial). The angle of the dot on the screen is also a big factor in avoiding moire patterns. [...]
I didn’t realise how much thought had to go into screen printing T shirts. I use DTG for my prints in our store which is so simple; but we get someone to screenprint our large orders. Good to know what you have to do to prepare a design for printing. Will keep it in mind when we are designing our graphics for custom printed Tee’s.
[...] used 61T and 77T meshes for this print, 77T is not an ideal mesh for waterbased ink it held the half tone with no problem at all, but they required flood bar and squeegee tweaks on the machine to print [...]
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