Proofs
Before the job is printed, the designer receives a sample of the printed piece, called a proof. From this, he checks the color, the size of the reproduction, and whether or not the overall quality is acceptable. Proofs can be made from the screened separations (pre-press proofs), or from the printing plates (press proofs).
Pre-press Proofs. Pre-press proofs are made directly from the film separations before the printing plates are made. These proofs can be made from either acetate sheets or photopolymer sheets. Acetate sheets, also called colorkeys, consist of four sheets, one for each color, which when overlayed create the final proof. The color created by color keys is not accurate, but it does give the designer a good idea of how the final printed piece will look.
Photopolymer sheets represent an improvement over acetate sheets in that they are able to reproduce the four-color proof on a single sheet of coated paper. Furthermore, the color is accurate and can be used for color correcting. (In most cases the color is so good, because of the sharpness of the dots, that the printer has a hard time matching it.)
Press Proofs. These are made directly from the printing plates, using the same ink and paper that will be used for the final printing. Although press proofs can be pulled on the production press, they are more often pulled on a small four-color proofing press.
Along with press proofs, the designer will often receive progressive proofs, more commonly called progs. Progs consist of a number of sheets, each showing a process color by itself and in combination with the others. The sequence follows the proper printing rotation: yellow, magenta, cyan, black. The first sheet shows the yellow plate, the second shows the magenta plate, and third sheet shows the two combined; this is followed by the cyan plate, then a combination of the cyan, yellow, and magenta; and finally the black plate, followed by all four combined. Although the designer will have very little use for progs, they are of value to the printer as a guide. Due to the high cost of press proofs, pre-press proofing with photopolymer sheets may well become the standard proofing method. The quality is high, and as the printing plates have not been made corrections are relatively inexpensive. (Once the plates have been made, corrections are not only more expensive, but there is much less the printer can do to adjust the color.)
Color Bar. Most proofs carry a color bar, made up of the four process colors and usually including tints, overprints, slur bars, star targets, etc. Although not of much concern to the designer, to the printer the color bar represents a simple and accurate means of controlling the quality of a job. At a glance he can tell if the colors are accurate and if the proper amount of ink is being carried on each plate. If necessary, he can measure the color strength with a densitometer. Any variation in the color strength will affect the printed piece. The color bar also shows such printing problems as poor trapping and dot gain. Perhaps most important for the printer is the fact that the color bar is independent of the copy being reproduced and therefore represents a consistent guide from job to job.