Overview
ISO 12645:1998 - Graphic technology - Certified reference material for opaque area calibration of transmission densitometers defines requirements for half‑tone reference materials used to calibrate transmission densitometers and colorimeters for determining the opaque area percentage in graphic‑arts half‑tone control patches. It specifies material, pattern, dimensional and metrological requirements, reporting and traceability rules for a reference material that can serve as a Certified Reference Material (CRM) when ISO 15790 requirements are also met.
Key topics and technical requirements
- Purpose: Provide a check on tone‑value measurements (Murray–Davies approach) and correct for opal‑type interreflection effects that cause discrepancies between densitometer tone values and true opaque area percentages.
- Materials & construction: Clear substrate with opaque half‑tone dots on one side; reflectance (front and second surface) ≤ 0.05 at normal incidence.
- Thickness: Nominal 0.1 mm to 0.8 mm; variation across the CRM ≤ 0.01 mm.
- Half‑tone pattern:
- Screen ruling: 50 to 70 cm−1 (constant across the CRM).
- Patch sizes: >5 mm diameter (10 mm preferred).
- Required opaque area percentages: circular opaque dots at 20 or 25% and 50%; circular clear dots at 75 or 80%; plus 0% and 100% (2% and 98% recommended). ±1% allowed deviation on nominal values.
- Optical specs: Fringe width ≤ 2 μm; solid (opaque) ISO status A density > 4.00; half‑tone core densities > 3.00; clear substrate diffuse density ≤ 0.04.
- Accuracy & reporting: Report measured opaque area percentages with standard uncertainties ≤ 0.6%. Deviation between measured and labeled value ≤ 1% (for 5–95%) and ≤ 0.6% outside this range. Local uniformity: ≤ 0.6% difference between any two 2 mm areas.
- Traceability: Results must be traceable to a national standardizing laboratory. Test methods are given in informative annex A (microscopy geometry and integrating‑sphere transmittance formulas); annex B gives a recommended densitometer calibration method (zeroing on a 50% patch).
Applications and users
- Quality control in prepress, film duplication, image‑setting and platemaking workflows.
- Calibration and verification of transmission densitometers, spectrophotometric colorimeters and spectrophotometers used in graphic arts.
- Manufacturers of test equipment, printing houses, color‑management labs, and national metrology institutes seeking traceable CRM for half‑tone process control.
- Useful where tone‑value consistency and reproducible dot gain/transfer across the process chain are required.
Related standards
- ISO 12647‑1 (process control for half‑tone separations and measurement methods)
- ISO 5‑2 (standard diffuse density instruments)
- ISO 15790 (requirements for Certified Reference Materials)
- CIE 17.4 (definitions used)
Keywords: ISO 12645:1998, certified reference material, opaque area calibration, transmission densitometer, colorimeter, half‑tone, graphic arts, tone value, Murray‑Davies, integrating sphere, CRM, traceability.
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