Opacity describes how effectively paper or packaging prevents a contrasting background from influencing appearance. It matters for show-through, duplex printing, labels over colored containers, white films and reverse-printed packs. Because it is derived from measurements over different backings, sample presentation is fundamental.
What opacity measures
A common opacity result is a ratio of reflectance over a dark or single-sheet backing to reflectance over an opaque white or paper stack, expressed as a percentage. Higher values indicate less show-through. The exact spectral weighting, backing and geometry depend on the named standard, so a result must never be reported as just “opacity” without the method.
Choose the method for the material
ISO 2471 specifies paper-backing opacity by diffuse reflectance for paper and board. Packaging specifications may instead use contrast ratio over defined white and black standards, or a spectral opacity calculation suited to film and coating systems. Follow the contract method and do not combine results from dissimilar procedures. ISO 2471 also places specific constraints on fluorescent and colored papers.
Prepare and orient the sample
- Condition paper or board as required and identify the measured side.
- Cut flat, undamaged specimens larger than the aperture.
- Keep machine direction and instrument orientation controlled.
- Use clean, stable backing standards with known reflectance.
- For printed or laminated structures, state whether opacity applies to the substrate, white ink layer or finished construction.
For nonuniform films or corrugated materials, measure multiple representative areas. Creases, trapped air and poor contact can alter results.
Measure the two backing states consistently
01
Verify
Standardise the instrument and confirm geometry, aperture, UV condition and calculation.
02
Read
Measure the same specimen area over each backing without changing orientation or introducing gaps.
03
Report
Calculate by the specified method and retain individual values, mean, range and sample construction.
Interpret opacity with the end use in mind
Opacity can change with basis weight, thickness, filler, coating, fibre formation, white-ink film and lamination. A sheet can be very white yet insufficiently opaque, or opaque but visibly tinted. Control color, whiteness and opacity separately. For packaging, test whether the chosen laboratory backing represents the filled product or label application. Continue with paper whiteness, packaging color and instrument selection.
Last reviewed 22/09/2026 · Level: Intermediate
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