Overview
ISO 2471:2008 specifies a standardized laboratory method for determining the opacity (paper backing) of paper and board by the diffuse reflectance technique. The method quantifies how well one sheet visually obscures printing on underlying sheets by measuring luminance factors under controlled illumination. It is applicable to papers and boards that contain fluorescent whitening agents provided the instrument UV output is adjusted to match the CIE illuminant C using an ISO/TC 6 fluorescent reference. It is not applicable to coloured papers or boards containing fluorescent dyes or pigments.
Key topics and requirements
- Measurement principle: Opacity (%) = 100 × (single-sheet luminance factor R0) / (intrinsic luminance factor R∞).
- Instruments: Use a calibrated reflectometer with geometric, spectral and photometric characteristics per ISO 2469 and capable of reporting luminance factor (C). Instruments must allow UV adjustment to UV(C) conditions for materials with fluorescent whitening agents.
- Reference standards: Non‑fluorescent photometric standards and a fluorescent reference standard (for UV adjustment) supplied by an ISO/TC 6 authorized laboratory (per ISO 2470-1).
- Test pieces and sampling:
- Cut rectangular test pieces ≈ 75 mm × 150 mm.
- Assemble a pad of at least 10 sheets (thick enough that doubling does not change R∞).
- Mark orientation (top side) and measure both sides if necessary.
- Procedure highlights:
- Adjust UV content with fluorescent reference.
- Measure intrinsic luminance factor of the pad (R∞) and single‑sheet luminance factor over a black cavity (R0).
- Perform five paired measurements (alternate or grouped) and repeat for the opposite side.
- Results & reporting:
- Report mean opacity and standard deviation (or separate side values if difference > 0.2%).
- Include instrument type, conditioning details, and any deviations from the standard.
Applications and users
- Paper and board manufacturers: Quality control of opacity for newsprint, writing papers, coated board.
- Printing and converting industries: Assess paper suitability to prevent show‑through and ensure print legibility.
- Testing and certification laboratories: Routine conformance testing and interlaboratory comparison.
- Instrument manufacturers: Design and calibrate reflectometers to meet ISO 2469/2471 spectral and UV requirements.
- Buyers and specification writers: Define acceptance criteria for opacity in procurement contracts.
Related standards
- ISO 2469 - Measurement of diffuse radiance (luminance) factor
- ISO 2470-1 - Measurement of diffuse blue reflectance factor (ISO brightness) and UV adjustment procedures
- ISO 186 - Sampling of paper and board
- ISO 187 - Conditioning atmospheres for testing
Keywords: ISO 2471:2008, opacity determination, paper and board, diffuse reflectance, reflectometer, luminance factor, fluorescent whitening agents, CIE illuminant C, ISO 2469, ISO 2470-1.
