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In offset printing, CMYK inks can not be vary in density across an image unless halftoning is used to break up the 100% coverage of each colour. In halftoning each colour is printed as dots of various sizes in overlapping grids at different angles. As the dots get smaller the shade of color lightens. Grids are placed at different angles for each of the ink colors. Smaller halftone dots cover less of the white paper therefore density is decreased and the object appears lighter since more of the white is visible.
Screen angles of Cyan, Magenta, Yellow, Black and a combination of all four as used in traditional printing (images enlarged).
The resolution of any image in digital format is measured in dpi (dots per inch). For example an optimised image to be displayed on a computer monitor needs a resolution of 72 dpi whereas an image to be printed on a commercial press needs a higher resolution and the easiest way to work out the optimum resolution for printing is to double the lpi (lines per inch) used by the printers' imaging device. If your local printers' in-house lpi is 150 then the best resolution for files to be printed is 300dpi. Some printers' imaging devices can run at higher lpi so make sure you ask before you submit your files. Anything above the double lpi formula is pointless and will only result in a clumsy, large file with no effect whatsoever. Newspapers typically use 85 LPI because of the absorbency of the paper used. This can be seen with the naked eye. Commercial printers however use 150 lpi but can go upto 300 lpi. You would probaby need a magnifying glass to see the dots at these resolutions.
Written by A. S. Bergant, © Samberg PTY LTD, 2007
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