Opacity (ISO 2471)
How much light passes through the sheet. Why show-through ruins thin paper. How it is measured and what ranges to expect.
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 canonical 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 in the canonical graph
The WPI canonical graph 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.