Printers can use a variety of halftones when printing their films, here we look at some dots are better than others.

Screens come in two separate forms - FM stochastic or random) and AM (postscript or traditional). FM have the same size and vary in frequency, i.e. they get more frequent as the print gets darker. AM screens have the same frequency but vary in size, i.e. they get bigger as the print gets darker.

The basic halftone is a round dot, a series of circles that expand until merging together to cover the paper.

SIMPLE ROUND DOT At around a 71% tone the dots join with their neighbours, creating undesirable 'star shapes' with sharp points allowing ink to dry-in, blocking screens. In addition, as they join, prints show a sudden jump in density, know as a tone jump.

Addressing these problems 'Euclidean' dots were developed.

EUCLIDEAN ROUND

'Euclidean Round' screens have highlights that start with round dots become squarer as they increase in size - through a chequerboard at 50% and become rounded holes in the shadows - reducing the problems of drying-in but continuing to suffer from the tone-jump around the mid-point.

Elliptical dots have one axis longer than the other - as opposed to round, longer axes touch before shorter axes. Tone jumps still occur but there are two smaller, less obvious steps.

SIMPLE ELLIPSE

Printing elliptical dots - dots with one axis longer than another - rather than round, the long axis join at a different tone to the short axis. Thus the tone jump occurs but as two less obvious steps.

SIMPLE ELLIPSE

Using a combination of the previous dots can address these problems. Highlights start with elliptical dots, through the chequerboard at 50% and evolving into negative elliptical dots in the shadows - reducing the problem of ink drying-in and smoothing the tone jumps.

Line or Geometric Screens give good 'rosettes' that can appear to have been printed at a higher lpi but their disadvantage is that once the highlights become separated from the mesh threads, the rest of the line can follow. Prints also show a 'colour wash', an ICC profile is necessary to compensate for this.

SIMPLE LINE

Ellipses with an extremely long to short axis ratio of 10:1 plus are similar to lines but because they are non-continuous in the highlights, failure here does not affect the rest of the print.

SCREEN LINE

FM/stochastic/random dots are 'randomly' printed - avoiding moiré, but appear coarse in highlights.

FM/Stochastic/Random Screen

AM screens set the CMK screens at 30° apart i.e. 7.5°/37.5°/67.5°, the yellow plate 15° between the C&M, ~22.5°. This generally results in moiré in greens and oranges. Mixing sets with AM screens for Black, Cyan & Magenta and stochastic screening for the yellow avoids such problems.

RIPs such as Wasatch SoftRIP SP can produce this hybrid screening.

It's easy to print professional-looking screen positives from devices such as inkjet printers