Overview
ISO 20654:2017 specifies a metric and measurement method for assessing intermediate tones of a spot colour ink. The standard defines the Spot Colour Tone Value (SCTV), a substrate‑relative metric that maps tones from the unprinted substrate (0 %) to the spot ink solid (100 %) and produces approximately uniform visual spacing of halftones. SCTV can be calculated from either spectral reflectance (or transmittance for back-lit media) or from CIELAB colorimetric measurements.
Key topics and requirements
- Definition of SCTV: A normalized metric (0–100 %) based on three value components derived from tristimulus data (X, Y, Z) using a compression function analogous to CIE L*.
- Measurement inputs: Spectral reflectance (or transmittance) of the substrate, the spot ink solid, and one or more intermediate tone patches; selected illuminant (normally D50 for graphic arts).
- Calculation routes:
- From spectral reflectance: compute X, Y, Z following ISO 13655, convert to the Vx, Vy, Vz components, then apply the SCTV formula.
- From CIELAB: measure L*, a*, b* (D50/2° recommended) and convert to the V components using the specified algebraic formulae; these are equivalent to the spectral route when CIEXYZ and CIELAB are consistent with ISO 13655.
- Communication: The standard recommends using CxF/X-4 (ISO 17972-4) to exchange spot ink characterisation data where possible.
- Scope notes: Applicable to non‑process (spot) inks; note on use for back-lit materials (use transmittance).
Applications and who uses it
- Ensures consistent halftone appearance for spot colours across prepress, printing suppliers and substrates.
- Practical uses:
- Brand and packaging colour control where spot inks (brand colours) are critical.
- Quality control and production acceptance criteria for halftone ramps and gradations.
- Color management, prepress proofing, and specification of spot ink tone values in tender/spec documents.
- Typical users:
- Print production managers, packaging converters and brand owners.
- Color technologists, spectrophotometer manufacturers, and color labs.
- Prepress and ICC/profile specialists who manage spot colour workflows.
Related standards
- ISO 13655 - Spectral measurement and colorimetric computation for graphic arts (used for converting reflectance to CIEXYZ).
- ISO 17972-4 (CxF/X-4) - Spot colour characterisation data format (recommended for communicating spot ink data).
- ISO 12647 (context) - general graphic technology/process colour definitions.
Using ISO 20654 improves reproducibility and perceptual uniformity of spot colour tones, helping suppliers and brand owners achieve consistent printed appearance across workflows and substrates. Keywords: ISO 20654, SCTV, spot colour tone value, spot color, spectral reflectance, CIELAB, CxF/X-4, graphic technology.
