Overview
ISO/TS 18621-11:2022 - Graphic technology - Image quality evaluation methods for printed matter - Part 11: Colour gamut analysis - defines standard procedures to measure, describe and compare the colour gamuts of RGB and CMYK printing processes. It specifies how to represent a gamut as a closed triangulated surface in a CIE-based colour space (normally CIELAB), how to compute gamut volumes and intersections, and which metrics to use for comparing reproduction systems. This Technical Specification is expressly applicable only to RGB and CMYK printing processes.
Key topics and technical requirements
- Gamut representation: The gamut surface is encoded as an nx3 vertex array and an mx3 face array (triangular faces). Each vertex is given as CIELAB L*, a*, b* values computed from spectral reflectance or tristimulus data per ISO 13655 (or with metadata if another CIE-based space is used).
- Topology rules: Faces must enclose the volume without gaps or overlaps; faces are defined in clockwise order when viewed from outside and each edge is shared by two faces.
- Procedures to derive gamuts:
- From an ICC profile (ISO 15076-1) - for usable gamuts in workflows.
- From a characterization model or measured printed gamut targets.
- From characterization data or direct measurement (Annex A provides well‑spaced device coordinates and triangulation guidance).
- Volume computation: Gamut volume is calculated by segmenting the surface into tetrahedra and summing tetrahedron volumes; verification steps are described.
- Intersection methods: Two approaches for computing intersecting gamut volumes are given - triangulated intersection and voxel-based intersection.
- Comparison metrics: Includes GCI (Gamut Chromaticity Index), gamut coverage, and out-of-gamut volume proportion, enabling quantitative comparisons between processes.
- Encoding & metadata: Requirements for reporting colour space, observer/illuminant, measurement mode, precision (recommendation: 16-bit or greater), and whether the description is device or media‑relative.
Applications and users
ISO/TS 18621-11:2022 is intended for practical use in:
- Print technologists and prepress engineers comparing print systems
- Colour management specialists and ICC profile developers
- Quality assurance teams performing gamut verification and proofing
- Manufacturers of printers, RIPs and colour management software
- Research labs assessing RGB vs CMYK reproduction capabilities
Typical applications include assessing whether one printing process can reproduce the colours of another, optimizing workflows for colour fidelity, checking profile accuracy, and standardizing gamut reporting for procurement or certification.
Related standards
- ISO 13655 - Spectral measurement and colorimetric computation for graphic arts
- ISO 15076-1 - ICC profile format and architecture
- ISO 12642-1 - Input data for characterization of four‑colour process printing
- Other parts of the ISO 18621 series (image quality evaluation methods)
Keywords: ISO/TS 18621-11:2022, colour gamut analysis, CIELAB, ICC profile, gamut volume, RGB printing, CMYK printing, graphic technology.
