ISO 5631-2:2022

Overview

ISO 5631-2:2022 specifies a standardized procedure for determining the colour of paper and board using the diffuse reflectance method under outdoor daylight conditions, specifically CIE standard illuminant D65 and CIE 1964 (10°) observer parameters. This method eliminates specular gloss, enabling consistent and accurate measurement of colour, especially for uncoated or mat-surfaced materials. It is particularly useful for evaluating the colour of white papers and boards, including those utilizing fluorescent whitening agents, provided that the UV content of illumination is properly calibrated as described in ISO 11475. The standard does not apply to coloured papers or boards that incorporate fluorescent dyes or pigments.

Key Topics

ISO 5631-2:2022 covers the following essential aspects:

• Measurement Method: Utilizes diffuse reflectance with specular gloss elimination to ensure true colour characterization.
• Instrument Calibration: Emphasizes the importance of calibrating reflectometers and spectrophotometers to match CIE standard illuminant D65 UV conditions using authorized reference standards.
• Applicable Materials: Applies to paper and board, especially white grades with or without fluorescent whitening agents. Excludes samples with fluorescent dyes or pigments.
• Colour Metrics: Uses CIE X, Y, Z tristimulus values and CIELAB (L*, a*, b*) coordinates for precise colour representation.
• Testing Procedures: Detailed protocols for sampling, conditioning, preparation of test pieces, measurement, calculation of colour values, and reporting of results.
• Result Expression: Colour is to be reported in terms of L*, a*, b* averages and the mean colour difference from the mean, ensuring repeatable and meaningful data.

Applications

ISO 5631-2:2022 is highly relevant for industries and laboratories that:

• Manufacture or Process Paper and Board: Ensures that product colour meets quality and consistency standards.
• Perform Quality Control and Assurance: Enables standardized, reproducible colour assessment, vital for product acceptance and compliance.
• Compare Materials with Fluorescent Whitening Agents: Provides a framework to measure paper whiteness accurately under realistic, daylight-like conditions.
• Support Research and Development: Useful for developing and testing new paper types or coatings requiring precise chromatic evaluation.
• Comply with International Requirements: Facilitates trade and certification by aligning colour measurement practices internationally.

The standard is especially critical when visual matching under outdoor or natural daylight is required, and for materials intended for high-visibility or esthetically sensitive applications.

Related Standards

ISO 5631-2:2022 references and aligns with several related international standards:

• ISO 11475 – Paper and board - Determination of CIE whiteness, D65/10° (outdoor daylight)
• ISO 2469 – Measurement of diffuse radiance/reflectance factor for paper, board, and pulps
• ISO 5631-1 – Colour measurement under indoor daylight (C/2°)
• ISO 5631-3 – Colour measurement under D50/2° indoor illumination, essential for graphic arts
• ASTM E308 – Practice for computing the colours of objects using the CIE system
• CIE Publication 015:2018 – Colorimetry foundational document
• ISO/CIE 11664 Series – Various documents further defining colorimetric observers, illuminants, and colour spaces

By adhering to ISO 5631-2:2022, organizations ensure their paper and board colour measurement practices are globally recognized, consistent, and reliable for outdoor daylight conditions. This ultimately enhances product quality, customer satisfaction, and international market access.