PostScript or PDF Color Separations
Two Degrees of Separation: Composite vs. Pre-Separated
On a printing press each color of ink used in a document is printed one at a time. Each printing plate used on the press is made up only of components of the page in that one color.
Separations are artwork split into component plates of cyan, magenta, yellow, and black in preparation for process printing (CMYK) or into the required number of plates for spot color printing – a plate for each color of ink.
Each separation prints a single process or spot color. Digital PostScript or PDF files take two primary forms: composite or pre-separated. Which you use depends on your software, colors, and the software or RIP your printer or service bureau uses.
A pre-separated PDF or PostScript file contains a separate plate for each color in the document.
A standard process color job would have four plates (pages) containing just the color information for each color of CMYK. For spot colors, you’ll have a page for each spot color.
You’ll have separations for each page in your document so a 4 page CMYK preseparated file would have 16 pages. If you have a preseparated file you can only print the file as separations and on-screen viewing is limited to viewing each individual color plate.
A composite file contains all the color information in one file. It can be viewed or printed as a composite (everything on one page). Your service bureau separates the file into its individual color separations at the RIP.
Which type of file — composite or pre-separated — does your printer need from you? Ask.That’s the best way to know for sure. Each has its benefits. It’s probably obvious that a composite file will be more compact than a preseparated file — fewer pages. With composite files both you and your service bureau can view the document on-screen and check things like graphics, text flow, etc. more easily.
However, there are some types of documents that require pre-separated files for proper handling. At present, composite files don’t contain the necessary information to properly generate separations for EPS duotones, DCS images, colorized TIFFs, and images with spot-color-to-spot-color gradients. These will require pre-separated PostScript or PDF files. Check with your printer if you are unsure.
Even when supplying a composite PostScript or PDF to your printer, you’ll want to know how to create and print color separations to your own printer for proofing purposes. This will help you see if your composite file will separate correctly (all colors on the correct plates, no extra colors) and help you catch other mistakes that might not be apparent looking at the application file or the composite file.
This is a brief overview of composite and pre-separated files — just one aspect of creating color separations. For more in-depth technical specifications and tutorials as well as advice on proper preparation of separations in a variety of programs, see the sidebar resources.