Prepress: preflight, halftone and color management

Long before any printing machine runs, it is decided whether a print run will be a success or an expensive failure. It happens in prepress — the phase where the digital file is prepared for production. As a graphic technician, this is where you catch errors while they are still cheap to fix.

Preflight is the systematic control of a PDF file before it is forwarded in the workflow. The goal is to detect problems that would otherwise only show up on the printing press or in the finished product.

• 01Resolution of images — too low resolution gives a blurred or pixelated print.
• 02Color space — is the file set up in CMYK or special colors, and not just RGB for screen?
• 03Font types — are they embedded in the file so the text doesn't change when the file is opened on a different system?
• 04Overprint and falling colours — is black text set to overprint so it does not become white if mis-registration occurs?
• 05Trim marks and format match the ordered product.

A printed image is actually composed of thousands of small raster dots of varying sizes that fool the eye into seeing a continuous gray tone or color. How closely the dots are spaced is called raster resolution and is typically measured in lpi (lines per inch). The finer the raster, the more detail the print can reproduce—but the higher the demands it places on paper and printing method.

When the file has been checked, it's sent to a RIP (Raster Image Processor)—the software that translates the digital objects into the raster dots the machine actually prints. At the same time, imposition happens: pages are arranged and rotated so they land correctly on the large printing sheet and end in the right order when the sheet is later cut and folded.

Screens, printers and printing machines reproduce colours differently. To get a predictable result, ICC profiles are used that describe how a given device renders colour in the device-independent Lab colour space. In this way an image can be correctly converted from the screen's RGB to the print shop's CMYK without the colours changing character along the way.

Prepress requires patience and a sharp eye for detail. But the time you spend here is the best investment in a print run — because the better the file is prepared, the less you need to fix once the machine starts running.