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Changing Screen Angles in Duotone Images, Part 1

Sometimes, when printing Duotone images, the printed result isn’t what you expected. Your images seem to have a strange dotted pattern which wasn’t in your digital file. This is caused by wrongly set screen angles. To understand this, it is necessary to understand what a screen is in printing.

Ink is laid on paper in form of tiny little dots that, combining each other, give you the illusion of continuos colour. You have a number of black dots, cyan dots, magenta dots and yellow dots that are printed on paper and combine to produce your orange, your brown, your red and all the other colours that can be obtained by mixing CMYK.

Creating Printing Plates

To create plates, some printers will use films, others will simply make them straight from a computer (also known as Computer To Plate process or CTP). For ease of understanding, I am going to take up the process which uses films.

Prior to separating your job into cyan, magenta, yellow and black, your printer’s service bureau will plan, or impose, your job so that it can be printed on the press. For example, if you are printing a 20 page magazine, the printer will place the pages so that they fit into the bigger sheet of the press and so that once your pages are trimmed and folded, they all fall back into sequence to give you the final product of your 20 page magazine.

Once your design is planned, the printer will send it to their imagesetter, i.e., a special kind of “printer” which creates the films, as a separated job. The printer’s imagesetter will produce a film for cyan, one for magenta, one for yellow and one for black. With positive films, there will be more dots where the darker areas are and less dots to none in the lighter areas or areas that have no colour. With negative films it’s the other way around.

Films are then placed on the plates that will be used to print your job. The plates are hit with UV light which will go through the areas of the films with less dots in more quantity. The films are effectively screening the plates, allowing the UV light to only hit certain areas of the plate. The resultant pattern of dots is therefore called screen, or halftone screen, as with this system you are able to create tints and shades of colours, instead of just creating solid colours with no shades in between—the bigger and the closer the dots on your sheet of paper, the darker your colours will be.

Screen Angles and Screen Frequency

The dots within a screen also form lines. The number of lines in one inch is called line frequency, or screen frequency, also known as LPI (Lines Per Inch). Generally the higher the LPI the finer your halftone will be, i.e., your colours will blend better with each other. However you need to keep in mind the resolution of the printer that will output the job when setting LPI.

Each screen must have different angles and line frequency to prevent dots from overlapping. If dots overlap too much, colours don’t mix well and you get the so called moire effect.

0420050919screen.gif

When printing CMYK images, you shouldn’t have a problem with the moire effect. Usually the defaults in your layout application will work fine. It’s when you start using spot colours, which it is usually the case in duotone images, that you might get trouble. Look at the screenshot on the left. I took InDesign’s Print window as an example in this case. The “New Swatch Colour” ink has the same angle and frequency of the black plate. That will very likely create the unwanted moire effect. Unless you printer’s service bureau handles this for you (and most of them do), you will have to change the screen screen angle and frequency manually.

In the next article I will explain how to change screen angle and freequency in InDesign, QuarkXPress, Photoshop and Illustrator.

Desktop Publishing & Prepress

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