Overview
ISO 6846:1992 specifies an internationally recognized method for determining ISO speed and ISO range of black-and-white continuous‑tone (silver‑halide) photographic papers used for printing from film negatives. The standard applies to conventional contact and enlarging papers (including variable‑contrast papers) and defines an exposure‑processing‑measurement workflow that produces a sensitometric curve from which speed and range are calculated. Annexes A–C explain relationships between paper range, negative effective density range, log exposure range (LER), contrast and average gradient.
Key topics and requirements
- Scope and exclusions: Applies to conventional silver‑halide printing papers; excludes non‑silver papers, high‑contrast graphic arts papers and direct positive reversal papers.
- Test principle: Expose and process samples, measure reflection densities, plot density vs. log10(exposure) to obtain a sensitometric curve.
- Measurements:
- Determine Dmin, Dmax and maximum net density (DN).
- Calculate log exposure range (LER) and derive ISO range.
- Locate the exposure producing 0.60 above Dmin to determine ISO speed.
- Equipment & conditions:
- Use a non‑intermittent illuminance‑scale sensitometer with spectral power distribution equivalent to CIE illuminant A (2856 K).
- Conditioning and processing specified: samples at 23 °C ±2 °C and (50 ±5)% RH; process completion between 1 min and 2 h after exposure.
- Densitometry per ISO 5‑3 and ISO 5‑4 geometric/spectral requirements.
- Rounding and labelling: Raw values are converted to ISO speed and ISO range scales (e.g., “ISO P100”, “ISO R140”); processing conditions should be stated when publishing values.
- Limitations: Results are process‑dependent; values apply to papers processed according to the manufacturer’s recommended procedures.
Practical applications and users
- Paper manufacturers use ISO 6846 to classify and publish comparable speed/range data across product lines.
- Photographic labs and QA teams apply the method for product acceptance, process control and batch verification.
- Professional photographers and printmakers use ISO range and LER guidance to select paper grade or filters (variable‑contrast papers) to match the effective density range of negatives for optimal tone reproduction.
- Equipment and chemical suppliers reference the standard for compatibility and calibration specifications.
Related standards
- ISO 5‑3 (density measurements - spectral conditions)
- ISO 5‑4 (density measurements - geometric conditions)
- ISO 554 (conditioning atmospheres)
- ISO 6728 (camera lens spectral modification)
- ISO/CIE 10526 (CIE standard colorimetric illuminants)
ISO 6846:1992 is a practical standard for consistent evaluation and comparison of black‑and‑white printing papers, helping users match paper characteristics to negative density and achieve predictable print results.