ISO 5631-3:2022

Overview

ISO 5631-3:2022 - "Paper and board - Determination of colour by diffuse reflectance - Part 3: Indoor illumination conditions (D50/2°)" specifies a standardized method to measure the colour of paper and board using diffuse reflectance with elimination of specular gloss. The method defines measurement under the CIE D50 illuminant with the CIE 1931 2° standard observer (D50/2°), intended primarily for the graphic arts industry. It includes provisions for handling papers that contain fluorescent whitening agents (FWAs) by adjusting the UV content of the illumination to CIE illuminant C levels using an IR3 fluorescent reference standard.

Key Topics and Requirements

• Measurement principle: Colour is determined by analysing light reflected from a sample (tristimulus-filter colorimeter or abridged spectrophotometer) and calculating CIE X, Y, Z tristimulus values and CIELAB L, a, b*** coordinates for D50/2° conditions.
• Specular gloss elimination: Measurements are made with specular component excluded to ensure consistency for matte and coated papers.
• UV control for FWAs: When measuring papers with fluorescent whitening agents, the instrument illumination must be adjusted so UV content corresponds to CIE illuminant C using a fluorescent reference standard meeting ISO 2469/ISO 2470‑1 requirements (IR3 level).
• Apparatus and standards: Requires calibrated reflectometers (filter-based or abridged spectrophotometers) with specified spectral/geometric responses, working standards, black cavity, and UV-adjustment capabilities.
• Sampling & conditioning: Sample selection per ISO 186; conditioning recommended per ISO 187 (avoid preconditioning that could alter optical properties).
• Calculations and reporting: Includes procedures for computing tristimulus values, converting to CIELAB coordinates, assessing result dispersion, and preparing a test report. Precision and instrument calibration requirements are specified.

Applications and Users

• Who uses it: Paper and board manufacturers, print buyers, graphic arts and prepress professionals, color/quality control laboratories, and instrument manufacturers.
• Practical uses:
  • Quality control of white papers and boards used in printing and packaging.
  • Contractual colour specifications between paper mills and print customers.
  • Comparative evaluations of paper brightness, tint, and colour consistency.
  • Calibration and verification of color measurement instruments for indoor/graphic-arts conditions.

Related Standards

• ISO 5631 series (other parts for C and D65 illuminants)
• ISO 13655 - Graphic technology - Measurement and specification of colour
• ISO 2469 - Measurement of diffuse radiance factor (diffuse reflectance)
• ISO 2470‑1 - Measurement of diffuse blue reflectance factor (ISO brightness)
• ISO 186 / ISO 187 - Sampling and conditioning of paper and board
• CIE Publication 015 (Colorimetry) and ASTM E308 for calculation practices

ISO 5631-3:2022 is essential when consistent, industry-recognized indoor colour measurement (D50/2°) is required, especially for papers influenced by fluorescent whitening agents in graphic arts workflows.