Screenprinting Halftone Dots - A User Guide

Tone-jumps, moire and loss of highlights on your screen are all symptoms of using sub-optimal screening.  Most printers take the software default but there are choices to be made.

Dot shapes have evolved over time from the simplest, circles that expand until they touch their neighbours and continue until filling the cell.  That's great and an improvement on what came before but suffers from a couple of failings - firstly, as circles touch they create star shaped holes that allow ink to dry-in and are prone to peeling off.  Secondly, the point where they meet shows a tone-jump, an increase in ink density, part optical illusion, part physical effect.

IMAGE SETTER

SIMPLE ROUND DOT

The dots can be made more robust by using euclidean dots, these start as round dots, become a chequerboard at 50% and take the form of circular holes towards the shadows, (effectively a negative version of the 0-50% dot).

EUCLIDEAN ROUND

That still leaves a tone-jump at 50%.

So - rather than use round dots, we can use elliptical, as long axis meets before the short axis, it gives two minor tone jumps that are less evident than a single jump, this helps to smooth out the effect.

SIMPLE ELLIPSE

Euclidean & elliptical dots can be combined  to combat both problems simultaneously.

It has been fashionable to try geometric screens in recent years, the halftone dots form a line that becomes wider as the tone increases.  There are two major failings of this screen, firstly the thinnest end of the screen can easily become detached from the mesh and because it is connected to the rest of the line the failure can progressively degrade the whole line, and secondly, the prints do tend to have a colour cast.  The second of these is cured by using a custom ICC profile, the first is more difficult and requires a bit of compromise.

SIMPLE LINE

Using a long ratio c.10:1 elliptical dot gives this compromise - the early dots are elongated ellipses and they quickly assume the form af a line.  This dissociates the line from it's highlights and gives a more robust screen.

SCREEN LINE

There has been a  recent fashion for FM (or Stochastic/Random) screening using a dot with a fundamental dot size which increases density by increasing the number of dots rather than the size of those dots.  This contrasts with traditional screens that have the same number of dots per unit area and change their size to increase density.

FM screening is sometimes used for t-shirt screenprinting but has a coarse appearance in the highlights.

FM/STOCHASTIC

Traditionally, screens set the Cyan, Magenta and Black plates 30° apart i.e. 30°/60°/90° and the yellow screen  15° away from others.  This leads to a clash between the yellow and  the cyan & magenta screens.  A 'hybrid' of Postscript screens for the C, M and K plates with an FM screen for the yellow plate avoids such problems.

Wasatch SoftRIP has the ability to produce these hybrid screens automatically.

It is possible to produce professional looking screen positives from low resolution devices such as inkjet printers.

Screenprinting Halftone Dots - A User Guide

IMAGE SETTER