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Posted on October 23, 2008 by Tom Paine | Posted under Advertising
Postscript Halftone Shapes for Screenprinting
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Dot shapes have evolved from the simplest, a circle that expands until it merges with it's neighbours and continues filling the cell. That's OK and a great improvement on its antecedents but it does suffer from a couple of faults - firstly as dots touch they create "star-shapes" that encourage ink to dry-in and are easily peeled off. The point at which they join also shows a tone-jump, a sudden increase in density, partly optical illusion, partly physical effect. SIMPLE ROUND DOT One can make the dots more robust by using euclidean halftones, these start as round dots, become a chequerboard at the midpoint, then resolve into the form of circular holes, effectively a negative of the 0-50% dot. EUCLIDEAN ROUND That still leaves a tone-jump at 50%. Rather than using round dots, one can use elliptical, since the long axis meets before the short axis this gives two minor tone jumps rather than one large one - this helps to smooth out the effect. SIMPLE ELLIPSE Euclidean & elliptical dots combine to give a solution to both these problems. It has recently been fashionable to try "geometric" or line screens, the "dots" form a line that becomes wider as the tone increases. There are two major failings, firstly the thinnest end of the screen can easily become detached from the mesh and because it connects to the rest of the line the failure can spread up the line, and secondly, the final prints tend to show a colour cast the colour of which depends upon which order the screens are arranged. SIMPLE LINE Using an elongated elliptical dot gives a reasonable compromise - the highlight dots are ellipses which quickly assume the form of a line. This isolates the line from it's highlights which helps control progressive failure but it still has a colour cast. This needs to be controlled with a custom ICC colour profile. SCREEN LINE There has been a recent fashion for FM or Stochastic screens using a dot with a fixed dot size and controlling density by increasing the number of dots. This compares with our traditional screens that have the same number of dots but change the dot size to increase density. FM screening is sometimes used for fabric screenprints but tends to have a coarse appearance in the highlights. FM/STOCHASTIC Traditionally, Postscript screens have settings with the C, M and K plates 30° apart i.e. 30°/60°/90° and the yellow screen set at 15° away from two of the others. This leads to a clash between the yellow and (generally) the cyan & magenta screens. A 'hybrid' of Postscript screens for the C, M and K plates with an FM yellow screen avoids such problems. A screenprint-orientated RIP includes sophistications such as hybrid screens, Wasatch SoftRIP has the ability for instance. It is possible to produce professional looking screen positives from low resolution devices such as inkjet printers About The Author: David Foster has worked in the prepress area for 22 years and currently is involved in the sales and support of imagesetters for screenprinters in the UK. He is Technical Manager for Positivity Ltd. There are some pictures that are relevant to the article here. |
Tags: IMAGESETTER, SCREENPRINT, PRE-PRESS, ACETATE FILM, POSITIVES, MASKS, HALFTONE, POSTSCRIPT
