ISO 18314-4:2024

Overview

ISO 18314-4:2024 - Analytical colorimetry - Part 4 specifies a formalism to quantify illuminant metamerism for pairs of solid surface colours. The standard defines a metamerism index (M_t) calculated as a colour difference (ΔE in CIELAB) between two samples under a test illuminant when they match (or have been corrected to match) under a reference illuminant. The document focuses exclusively on metamerism caused by a change of illuminant (not geometric, observer or field‑size metamerism) and limits direct application to non‑effect (non‑pearlescent) coatings unless metrical adaptations are applied.

Key topics and technical requirements

• Reference and test illuminants
  • Reference illuminant: D65 (default). Other reference illuminants may be specified for special cases.
  • Typical test illuminants: Illuminant A, fluorescent types such as FL11; the chosen test illuminant must be reported as an index to M (e.g., M_A, M_FL11).
• Colour space and observers
  • Metamerism index is based on CIELAB L*, a*, b* coordinates derived from standard tristimulus values (X, Y, Z).
  • Standard tristimulus values use the CIE 1964 10° observer or the appropriate CIE observer per ISO/CIE 11664 parts.
• Calculation and corrections
  • The metamerism index M_t is the ΔE (CIELAB) under the test illuminant when ΔE = 0 under the reference illuminant.
  • When small residual differences exist under the reference illuminant (paramerism), ISO 18314-4 provides three correction methods: additive correction, multiplicative correction, and spectral correction. Annex A supplies calculation examples.
• Measurement guidance
  • Spectral sampling must comply with wavelength rasters in ISO/CIE 11664-2 and CIE 015; missing spectral values should be interpolated/extrapolated.
  • Fluorescent samples must be measured under appropriate illuminants (measuring under only one illuminant can produce erroneous indices).

Applications and users

ISO 18314-4:2024 is practical for:

• Quality control and tolerance setting in paint, coatings, pigments, printing, textiles and plastics where colour matching under multiple illuminants is critical.
• Color matching and reproduction workflows, where M_t is used as a metric to specify allowable metameric shifts between a sample and a match.
• Color scientists, QC engineers, formulation chemists, instrument manufacturers, and color management specialists who require a standardized method to quantify illuminant‑induced metamerism.

Benefits include objective specification of metamerism tolerances, reproducible testing across labs, and clearer communication between suppliers and customers about colour performance under different lighting.

Related standards

• ISO/CIE 11664-1 (CIE standard colorimetric observers)
• ISO/CIE 11664-2 (CIE standard illuminants)
• ISO/CIE 11664-4 (CIELAB)
• CIE 015 (Colorimetry)