Registration: How Four Colours Land in the Same Place

A sheet on an offset press passes through one unit for each colour. Cyan goes on first, then magenta, then yellow and black. Between units the sheet travels a short distance, and each plate has to put its colour exactly where the previous one did. That alignment is registration.

What goes wrong without it

Set four impressions slightly apart and the edge of every letter picks up a halo of the colours that missed. Fine type closes up. A solid headline takes on a coloured fringe. On a four colour image a shift of a tenth of a millimetre is enough to soften the whole picture.

The number behind the check

ISO 12647-2, the process control standard for offset, puts the distance between the image register centres of any two process colours at no more than 0.10 mm. That is about the thickness of a sheet of paper. Good sheetfed work normally holds tighter, closer to 0.05 mm.

How it is measured

Small crosses sit in the margins of the sheet, one set for each colour. An operator puts a loupe over them, or an inline camera reads them automatically, and compares the centres. When the crosses sit on top of each other the sheet is in register. The marks are trimmed off with the rest of the waste.

Trapping covers the small shifts

Paper is not rigid. It takes up dampening water in the early units and grows a little by the time it reaches the later ones, so the black printed last can land a fraction wider than the cyan printed first. Prepress works around this with trapping, a small overlap where two inks meet. A trap of 0.05 to 0.10 mm means a light shift leaves no white sliver along the edge.

What causes a shift

Worn cylinder bearings, a loose front or side lay, uneven blanket pressure and plate tension all move the image. Sheet size matters too. A large, light sheet drifts more between units than a small, heavy one, which is one reason very fine register work is easier on a smaller format.

Why the buyer sees it

Nothing on the finished sheet points to registration. It simply looks sharp or soft. Those crosses in the margin were the check that decided which one it would be.

When a reprint comes back looking softer than the original, is registration the first thing you would look at? From commercial projects to specialized publications, book printing plays a vital role in modern print production.