Opacity (ISO 2471)

Opacity is the fraction of light blocked by a sheet. A perfectly opaque sheet transmits nothing; a sheet with 80% opacity transmits 20% of incident light. In printing, low opacity = show-through: the ink or image from the reverse side bleeds visibly through the sheet. For double-sided printing, high opacity is essential.

The ISO 2471 method

ISO 2471 defines printing opacity as a contrast ratio:

Opacity (%) = (R₀ / R∞) × 100

Where R₀ is the reflectance of a single sheet backed by a black body (zero reflectance), and R∞ is the intrinsic reflectance of an infinitely thick pad of the same paper. Practically: R∞ is measured with a thick stack; R₀ is a single sheet over black.

A result of 90% means the single sheet blocks 90% of what the thick stack would show. The measurement wavelength is 457 nm (same as brightness).

Show-through in practice

Show-through is a visual problem that doesn't map cleanly to a single threshold. Variables include:

• Ink coverage and darkness, heavy black coverage shows through more than a light tint
• Caliper, a thicker sheet at the same gsm (higher bulk) will typically have better opacity
• Fillers, calcium carbonate and clay increase opacity; they scatter light at the filler-fiber boundary
• Formation, uneven formation creates opacity variation across the sheet, visible as "cloud" when backlit

For book printing, a practical minimum is 88-90% opacity for pages that will carry moderate ink coverage on both sides. Below 85%, show-through becomes visible even with single-sided content at normal reading distances.

Opacity ranges by grade

| Grade | Typical opacity range | Notes | |---|---|---| | Newsprint | 80-88% | Low, single-sided reading expected; show-through acceptable | | Uncoated offset (80 g/m²) | 90-94% | Standard office paper target | | Coated art (115-135 g/m²) | 93-97% | Heavier basis gives headroom | | Bible / thin printing | 78-90% | 40 g/m² bible paper trades opacity for low bulk | | Tissue | 55-75% | Translucency is inherent; opacity is not a selling point |

Standard newsprint WPI-g-000869 operates at the low end, intentionally. Readers don't flip the page mid-sentence; the web is read in one direction, and the print economics of 45 g/m² outweigh the opacity penalty.

Bible paper at 20-50 g/m² (WPI-g-000769) is the archetypal thin-sheet challenge: the entire grade exists to minimize bulk while preserving sufficient opacity for double-sided printing. Fillers (often precipitated calcium carbonate, PCC) and tight formation are the primary tools. Some bible papers still fall below 88% and rely on the printed layout, wide margins, modest ink coverage, to keep show-through acceptable.

Contested opacity values

The WPI WPI index preserves source-level disagreements. Some grades carry opacity values from multiple sources that do not reconcile: one source may quote brightness-referenced opacity (ISO 2471), another a blue-light opacity (T425), and a third a simple transmission measurement without a standard citation. When opacity matters for your specification, demand the test standard alongside the number.

Increasing opacity in practice

• Increase GSM, more mass = more light-scattering fibers and fillers
• Add fillers, PCC and talc both scatter light effectively; chalk adds whiteness as a bonus
• Improve formation, even fiber distribution eliminates transparent spots
• Reduce calendering, heavy calender rolls compress the sheet and reduce scattering cross-section

For questions on how caliper interacts with opacity and bulk, see caliper.