Reflectance Protocol
Laboratory determination of paper and board opacity is standardized through an internationally harmonized method based on diffuse reflectance measurements over black and white backings. Published by the International Organization for Standardization, ISO 2471 establishes the formal procedure for calculating the contrast ratio of single sheets using integrating sphere geometry. The standard applies universally to white and near-white pulp, paper, and paperboard grades, governing how optical opacity is measured across printing and writing papers.
Its operational validity terminates when testing coloured stocks, fluorescent grades containing elevated levels of optical brightening agents, or translucent papers with opacities falling below fifty percent.
Instrumental Geometry
Specimen analysis mandates spectrophotometers or reflectometers conforming to d/0 diffuse illumination and normal observation conditions, incorporating an integrating sphere to diffuse light across the sample plane. Measurements require the blue-filtered spectral distribution characterized under CIE illuminant C conditions, with the ultraviolet component adjusted or trimmed using optical filters to prevent false fluorescence readings. The protocol directs the operator to measure the luminous reflectance factor of a single sheet backed by an intensely black cavity, followed immediately by a measurement of the same sheet backed by an opaque pad of identical paper sheets.
Opacity appears as the percentage ratio between the reflectance over the black backing and the intrinsic reflectance of the infinite pad. Exact conditioning of specimens at twenty-three degrees Celsius and fifty percent relative humidity is mandatory to maintain baseline moisture levels, as water content alters the internal scattering surfaces between cellulose fibres.
Mill Implementation
High-speed offset presses running lightweight coated publication papers rely on values generated under ISO 2471 to verify that print show-through remains within contract specifications. Mills run frequent quality checks against this protocol during furnish refinement and filler loading, because adding mineral pigments like precipitated calcium carbonate elevates the opacity calculation while reducing overall mechanical tensile strength. Calendering operations also track the standard closely, since excessive nip pressures flatten internal sheet voids, reducing light scattering and dropping the recorded opacity reading below contractual delivery limits.
Packaging converters require verified test certificates under this procedure when qualifying folding carton top plies to guarantee that grey recycled under-plies do not show through post-print bleached surfaces.
