Commercial printers use their own lexicon of words, but what do they all mean?
An occasional series attempting to throw some light on the question of just what it is that those printers are actually going on about.
What does resolution mean in commercial printing?
Resolution is a term that is often mentioned by printers but, confusingly, it can actually mean several different things. And it doesn’t help that most people (not even printers) consistently use the correct terms to discuss it.
Image resolution in litho and digital print
Image Resolution refers to the number of pixels per inch contained in an image file. Properly speaking, this should be described in ppi (pixels per inch), but is often described as either dpi (dots per inch) or even lpi (lines per inch) which actually mean something different. Most decent quality print requires colour images to be at least 300ppi at the size the pictures are to be reproduced, and black and white images should often be higher. Enlarging or reducing an image file in a page layout programme has the effect of altering the pixel resolution of the picture – making the picture bigger decreases the effective resolution, and vice versa. Black and white images actually divide into two types – greyscale and bitmap. Greyscale files are usually photographs, so should be at the same resolution as colour images (300ppi or above), but if they include text or line work they should be higher than this to allow the type or line work to render as sharply as possible. Bitmap images contain no grey – they are just black and white, and should be saved at quite a high resolution – usually 1200ppi or more.
Of course, all this varies depending on whether you’re printing using litho or digital printing presses. You can learn more about the differences in Litho vs. Digital Printing
LPI, DPI and Dots can confuse a non-printer
Screen Ruling refers to the number of halftone dots applied to an image when producing colour separations. This is usually a lower number than the ppi of file resolution, and can be described in either dpi (dots per inch) or lpi (lines per inch). (This is because it is possible to create halftone screens using both dots and lines, although it is rare that lines are used in modern printing.) Typically most printing would be done with a screen ruling of between 150 and 240dpi, and in general, the higher the screen ruling, the finer the results. It is also true, however, that good modern computer-to-plate (CTP) systems can produce very good quality results without the need to run high screen rulings, since the quality of the dots is better (before CTP, dots had to be exposed onto plates via films, which caused degradation of the image since they were “second generation”. CTP dots are created directly on the plate by an output device, so are “first generation”.)
Device Resolution refers to the number of imaging marks an output device (laser printer, film or platesetter) can make using its smallest imaging dots. This would normally range from 300 dpi (fairly low-quality laser printer) to 2400dpi or higher on modern computer-to-plate systems.
Ask the printing experts
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