Colour validation tests provide you an objective method to validate whether a printer can simulate the colors of a reference printing condition on a specific media.
A printing condition are all those properties of a print system, including the media (print substrate), that influence the colors of a print. An example of an often used printing condition is FOGRA39: offset commercial and specialty printing according to ISO 12647-2, paper type 1 or 2 (gloss or matte coated media, 115 g/m2).
You perform a color validation test for several reasons:
You want to know if the printer is able to print the colors on the selected media according to a reference printing condition.
You want to know if the printer is able to print the same colors on the selected media over time.
You want to check the color reproduction against digital print certifications, for example, FOGRA Print Standard Digital Certification, FOGRA Validation Printing System Certification or Idealliance® Digital Press Certification Program.
You want to check the color reproduction on the selected media against your own reference printing condition. Therefore, you create your own test and define your own printing condition, quality levels, tolerance levels and metrics.
A typical color validation test consists of a few steps:
Before starting the color validation, you calibrate the printer.
A test chart with a color control strip is printed.
The used color management settings of the print job are such that the reference printing condition is simulated as accurately as possible.
It is possible to initiate printing of the test chart both in PRISMAcolor Manager (download and print workflow) as well as outside of PRISMAcolor Manager (external color control strip workflow). The external color control strip workflow can be used if one of the supported color control strips is already available on the printer. For instance, a color control strip can be part of a standard test job or be printed on the edge of a sheet of an ordinary print job.
The colors of the printed color control strip are measured with a spectrophotometer.
The color difference between the measured and the reference colors is determined.
The reference colors are the colors that would be obtained when printing using the reference printing condition, for instance, the colors on gloss coated media as printed on a standard offset press in case the reference printing condition is FOGRA39. In PRISMAcolor Manager, the reference colors (CMYK values) and their corresponding Lab values are represented in a reference value set.
The traditional side-by-side evaluation method checks if printed colors exactly match the colors of the reference printing condition. The media relative evaluation takes into account that the paper white values of the printed media and the reference media can be different. Therefore, the media relative method uses an algorithm to compensate for the different white points. The SCCA (Substrate-Corrected Colorimetric Aims) evaluation method, which is used by Idealliance® tests, is also based on the media relative evaluation.
There are two measurement modes that are used in color validation: M0 and M1. The M0 measurement means that the UV part of the illumination is not taken into account in the measurement of the color sample. The M1 measurement includes the UV part of the illumination.
If a media contains optical brightener agents to make the substrate look whiter, the M0 measurement significantly differs from the M1 measurement. If a media does not contain optical brightener agents, the M0 and M1 measurement are similar.
Newer reference printing conditions (for example, GRACoL 2013 coated) use M1 to determine reference values while the older once (for example, GRACoL 2006 Coated) use M0. The measurement mode specified in the reference printing condition is used in the spectrophotometer when measuring the printed color control strip.
Colour evaluation methods use color metrics to decide if printed colors are close enough to their reference colors or not. A tolerance level or threshold level is set before the evaluation takes place. The level specifies where a calculated difference is still acceptable and where not. A tolerance level is also expressed by a △(Delta) value.
In case the measured values of the color difference metrics are smaller than tolerance levels specified in the reference printing condition, the test passes. Otherwise, the test fails.
Sometimes multiple tolerance values are defined per metric. These multiple tolerance levels are also knows as quality levels. If the measured metric value is lower or equal to the tolerance level, the metric passes with quality level A. If the measured metric value is higher than all defined tolerance levels, the metric fails. A color validation test only passes when all metric values are lower or equal to the tolerance levels. The quality level of the test result equals the lowest quality level of the individual metrics.
In PRISMAcolor Manager, color metrics and their corresponding tolerance levels are defined in a metric tolerance set.