momusradar

Scum dots are small points of ink (typically cyan) that appear in areas of a print that should be completely white. These imps can manifest themselves during image compression within the creation of a PDF file.

In Adobe Creative Suite (CS through CS3—I can’t speak for CS4 as I have yet to use it) the default image compression, for both color & grayscale images to be included in a PDF, are set to JPEG. This seems to be where the problem lies.

JPEG is a lossy compression scheme. If you compress a raster graphic that contains a large area of white using JPEG, it can introduce small artifacts into those areas that are not visible to the eye. When a PDF containing JPEG images is sent to a printer, the file is translated by a RIP into data that the printing device can output. The humanly imperceptible artifacts are quite obvious to the RIP and, depending on how it is configured, can be sent to ink resulting in scum dots. To be fair, many printers set their RIPs to hold back such artifacts to prevent scum dots, but there may be one day that you have to work with one that hasn’t taken such precaution.

Fortunately, changing the image compression for both the color & grayscale images to be included in the PDF to None will help you from falling prey to this problem. Simply put, None will not compress the images at all; at least not any more than they have been already (You are using uncompressed images to make your print-ready PDFs, right?).

As for compressing monochrome images, the CCITT, Run Length & ZIP options are all safe since monochrome by definition is only one color; although I also set it to None for consistency’s sake.

addendum July 10, 2009

I should emphasize that the suggestions noted above will only minimize scum dots; not eliminate them completely. In order to do that you need to disable dithering in the conversion options of your application’s color settings. The caveat with leaving this option turned off is that images with gradients could then suffer from banding.