Color bars are also referred to as color control bars, color control strips, or proofing bars. Color bars are rows of different colored patches printed in the trim area of the press sheet. They are used by proofers and press operators to control the trapping, ink density, dot gain, and print contrast of the proof or the printed sheet. They usually consist of solid and tint blocks of cyan, magenta, yellow, and black; two and three color solids and tints; and additional elements and patterns such as resolution targets and dot gain scales.
Magenta. A color bar are used to measure ink densities.
Solid overprint colors include blue (C&M), green (C&Y), and red (M&Y) Screen tint patches usually 25%, 50%, and 75% of the process colors. A color bar are used to check for dot gain. A color patch made up of 50C/30M/30Y. A color cast in the patches will show an imbalance in one of the colors, either from incorrect ink density or from dot gain.
There are several different types of color bars and which one to use depends on the type of printing being used. Prepress and print suppliers purchase original film or the rights to digital files of color bars and other test images. The film and files can then serve as constant reference points throughout the proofing, platemaking, and print processes. They are available from GATF (Graphic Arts Technical Foundation), DuPont, RIT (Rochester Institute of Technology), and others.
Color bars are an essential troubleshooting tool. They are easy to analyze, both visually and with a densitometer, which is an instrument used for measuring the relative density of any part of an image.
Color bars created for computer-to-plate (CTP) systems may require additional features than color bars generated for film.