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ISO 18621-11 implementation

ISO 18621-11 Graphic Technology - Image quality evaluation methods for printed matter - Part 11: Colour gamut analysis describes methods of analysing and comparing the gamuts of colour prints. In Annex A it describes example test charts in RGB and CMYK which can be used in the analysis and volume calculation procedure described in the body of the standard.

Alternate test charts

An alternate patch set based on CMYK gamut boundary colours has been derived. The chart has 382 faces on the gamut surface. This patch set has been shown to give a gamut volume that is a close match to the reference values in Annex B when computed according to the method in ISO 18621-11.

The CMYK coordinates of the faces are given in this spreadsheet. This set of CMYK coordinates is present in a number of standard test charts used for printer characterizations:

  • ISO 28178:2009 Graphic technology - Exchange format for colour and process control data using XML or ASCII text
  • ECI2002 - see ANSI/CGATS.17:2009 (R2015) Graphic Technology - Exchange Format For Colour And Process Control Data Using XML Or ASCII Text
  • ISO 12642-2:2006 Graphic technology - Input data for characterization of 4-colour process printing - Part 2: Expanded data set
  • ISO 12642-2:2006 and IT8.7/4.
Where the CMYK values are not present in the test chart used, colour values corresponding to these CMYK may be calculated by interpolation. Where this is done, details of the interpolation method should be provided.

Errata in ISO 18621-11:2020

Users of ISO 18621-11:2020 should be aware of two issues in the published standard.

  • Equation 1 should read V=-(a·(b⨯c))/6 instead of V=(a·(b⨯c))/6
  • The RGB image in Annex A has 36 uniform steps in colorant combination ratios and 21 uniform steps in colorant amounts, while the CMYK image has 36 uniform steps in colorant combination ratios and 22 uniform steps in colorant amounts.