Overview
EN ISO 787-24:1995 (ISO 787/24:1985) specifies photometric test methods for comparing the tinting strength of coloured pigments and the scattering (opacity) power of white pigments. It provides an instrumental alternative to the visual procedures in ISO 787-16 and ISO 787-17 by measuring reflectivity and calculating K/S (Kubelka–Munk) values. The method gives relative tinting strength for coloured pigments and relative scattering power for white pigments, enabling objective, repeatable pigment performance comparisons.
Key topics and technical requirements
- Principle: Prepare dispersions of equal masses of test and reference pigments in a consistent paste, measure reflectivity, then compute K/S ratios to give relative tinting or scattering values.
- Coloured pigments: Test pigment and agreed reference pigment are each dispersed in the same white pigment paste; reflectivity measured photometrically across visible wavelengths; K/S ratios used to compute relative tinting strength.
- White pigments: Test pigment and agreed reference pigment are each dispersed in the same black pigment paste; reflectivity measured at 550 nm (or using a Y-filter); K/S ratios give relative scattering power (expressed as a percentage).
- Optical model: Uses Kubelka–Munk analysis (K = absorption, S = scattering) and reflectance-to-K/S conversion to quantify colouring and scattering effects.
- Sample preparation & materials: Specifies preparation of white and black pigment pastes (examples include titanium dioxide-based white paste and carbon-black-based black paste), use of an alkyd resin binder, and grinding/dispersion procedures.
- Apparatus: Spectrophotometer (400–700 nm) or tristimulus calorimeter, triple-roll mill, automatic muller (ground glass plates), paste film holders/drawdowns and fineness-of-grind control.
- Reporting: Method variants and any deviations must be documented in the test report; when multiple accepted procedures exist, the chosen method should be stated.
Applications and who uses it
- Paint, coatings and ink manufacturers use EN ISO 787-24 for product development, QC and batch-to-batch consistency checks of pigments and extenders.
- Pigment producers and raw‑materials labs apply the method to characterize and certify tinting strength and opacity compared with agreed references.
- Quality assurance and R&D teams use photometric methods to replace subjective visual matching, enabling faster and more reproducible pigment evaluations.
- Suppliers and standards bodies reference the standard in pigment specifications and supplier declarations.
Related standards
- ISO 787 series (general methods for pigments and extenders) - notably Part 16 (visual tinting strength) and Part 17 (lightening power of white pigments)
- ISO 591 (titanium dioxide test methods), ISO 1524 (fineness of grind), ISO 4652 (carbon black surface area)
- Kubelka–Munk theory referenced for K/S calculations
Keywords: EN ISO 787-24, ISO 787/24, tinting strength, scattering power, pigments, extenders, photometric methods, K/S, spectrophotometer, paint testing.