Overview
ISO 22891:2013 defines a standardized method to determine the transmittance of white and near‑white translucent papers indirectly by diffuse reflectance measurement. Rather than measuring light transmitted directly, the standard uses luminance (reflectance) data and a Kubelka‑Munk–based calculation to produce a single, total transmittance value compatible with paper opacity testing practices.
Key topics and technical requirements
- Scope: Applicable only to white and near‑white translucent papers; fluorescent whitening agents require elimination of fluorescence via a 420 nm UV cut‑off filter.
- Principle: Measure luminance factors (C) of a sheet over a white backing and over a black cavity, then calculate transmittance from those readings using the method described (Kubelka‑Munk based).
- Instruments: A calibrated reflectometer (filter colourimeter or abridged spectrophotometer) meeting the geometric, spectral and photometric characteristics of ISO 2469. Spectrophotometers should be fitted with the 420 nm UV cut‑off where needed.
- Standards & references: Uses ISO 2469 measurement procedures and cross‑references opacity measurement (ISO 2471). Sampling guidance follows ISO 186; colour computations may use ASTM E308 tables.
- Sampling & preparation: At least five rectangular test pieces (approx. 75 × 150 mm), avoiding defects and stored between protection sheets; orientation of sheet (top/wire side) must be consistent.
- Calibration & traceability: Use ISO/TC 6 authorized reference standards and working standards; maintain black cavity and white backing conditions per the standard.
- Reporting: Test report must state mean transmittance, standard deviation, instrument type, whether a 420 nm filter was used, and any deviations from the method.
Applications and users
- Paper and board manufacturers – quality control of translucency and light transmission.
- Independent testing laboratories – certified reporting of transmittance for specifications and compliance.
- Converters, printers and packaging specifiers – assessing show‑through, printability, and end‑use visual performance.
- Instrument manufacturers and calibration labs – validation and calibration of reflectometers and spectrophotometers.
Practical benefits include a reproducible transmittance metric that complements opacity data and supports product specification, acceptance testing, and process control.
Related standards
- ISO 2469 - Measurement of diffuse radiance factor (reflectance)
- ISO 2471 - Opacity of paper and board
- ISO 186 - Sampling to determine average quality
- ASTM E308 - Colour calculation practice
Keywords: ISO 22891:2013, transmittance, diffuse reflectance, paper transmittance, luminance factor, Kubelka‑Munk, reflectometer, UV cut‑off filter, fluorescent whitening agents.