colour bar
A pattern of varying tonal density that enables visual and numeric comparisons to be made across multiple printed sheets or pages.
colour bar: density reference strip for print consistency
A colour bar is a strip of printed patches, usually running along the edge of a printed sheet, that shows a range of tonal densities from light to dark. On offset presses and digital printers, these bars serve as a visual and measurable control: press operators and quality inspectors compare the printed bars against a reference standard to verify that ink density, registration, and colour consistency remain within tolerance across the run.
The most common format uses four to seven square patches in a single row, typically cyan, magenta, yellow, and black (CMYK) in solid form, plus stepped patches showing 25, 50, and 75 percent tints of each colour. Some bars include additional patches for spot colours or extended gamut inks. The bars are positioned in the crop margin, outside the final trim area, so they do not appear on the finished product.
Measurement and Control
Press operators use a densitometer to read the optical density (OD) of each patch and confirm it matches the target value set at job setup. A typical target might specify that the solid black patch reads 1.60 OD; if the reading drifts to 1.45 or 1.75, the operator adjusts ink feed or pull to bring it back into the acceptable range, often set at plus-or-minus 0.10 OD. On multi-colour presses, separate colour bars for each printing unit allow detection of individual unit drift.
Colour bars also reveal registration errors: if the cyan, magenta, and yellow patches are not perfectly aligned, the bar will show visible fringing or colour shift. This alerts the operator to mechanical problems such as blanket slippage or impression cylinder wear before the fault spreads across the entire sheet.
In digital printing, colour bars are often embedded in the RIP (raster image processor) data automatically, appearing at a fixed position on every page or every Nth page. In offset lithography, the bars are imposed into the film or directly into the plate during exposure. The bars remain a non-negotiable part of professional print control because visual inspection alone cannot detect the small density shifts that compound across large print runs and deliver visible colour variation to the end customer.