Overview
ISO 5631-1:2022 specifies a standard method for measuring the colour of paper and board by the diffuse reflectance method under indoor daylight conditions (C/2°). The procedure eliminates specular gloss and defines how to obtain CIE tristimulus values and CIELAB coordinates using the CIE illuminant C with the CIE 1931 (2°) standard observer. The standard is intended for non‑fluorescent materials and for papers containing fluorescent whitening agents when the UV content of the illuminant is adjusted to match illuminant C.
Key topics and requirements
- Measurement principle: Colour is measured by analysing light diffusely reflected from a sample with either a tristimulus‑filter colorimeter or an abridged spectrophotometer and calculating results for C/2° conditions.
- Scope limits: Not applicable to coloured papers/boards that contain fluorescent dyes or pigments. Use with fluorescent whitening agents requires UV adjustment.
- Instrument and geometry: Reflectometers must meet the geometric, spectral and photometric requirements of ISO 2469; filter sets or spectrophotometer weighting functions must give responses equivalent to CIE X, Y, Z for illuminant C.
- UV control: For fluorescent whitening agents, the instrument’s UV content must be adjusted using an IR3 fluorescent reference standard as prescribed in ISO 2470‑1.
- Reference and working standards: Includes non‑fluorescent reference standards for calibration, fluorescent reference standards for UV adjustment, working opal glass plates and a fluorescent tablet, plus a black cavity for zero/black reference.
- Sampling and conditioning: Sampling for lot evaluation per ISO 186; conditioning recommended per ISO 187 (avoid elevated preconditioning that could alter optics).
- Calculations and reporting: Defines calculation of CIE tristimulus values, CIELAB L, a, b*** coordinates, dispersion/precision, and required elements of the test report. Annex A supplies spectral characteristics for reflectometers.
Applications and users
ISO 5631-1:2022 is used by:
- Paper and board manufacturers for quality control of whiteness and shade.
- Independent testing and certification laboratories measuring colour properties.
- Instrument manufacturers and calibration labs specifying reflectometer/spectrophotometer performance and UV adjustment procedures.
- Converters, printers and procurement professionals who require standardized colour data for material selection and acceptance.
Practical benefits include repeatable, comparable colour data across suppliers and labs, controlled treatment of fluorescent whiteners, and clear guidance for instrument calibration and result reporting.
Related standards
- ISO 2469 - Measurement of diffuse radiance (diffuse reflectance factor)
- ISO 2470‑1 - Measurement of diffuse blue reflectance factor (ISO brightness)
- ISO 186 - Sampling to determine average quality
- ISO 187 - Conditioning atmosphere for paper and board
- CIE Publication 015:2018 - Colorimetry
Keywords: ISO 5631-1:2022, paper colour measurement, diffuse reflectance, C/2°, CIE illuminant C, CIELAB, reflectometer calibration, fluorescent whitening agents.
