ISO 6504-1:2019

Overview - ISO 6504-1:2019 (Kubelka‑Munk method for white and light‑coloured paints)

ISO 6504-1:2019 specifies a standardized laboratory method to determine the hiding power (the spreading rate required to give 98% hiding) of white and light‑coloured paints using the Kubelka‑Munk (K‑M) optical model. The procedure applies to paint films with a CIE tristimulus value Y ≥ 70 and hiding power > 80%. It is not applicable to fluorescent or metallic paints. The method converts measured reflectance values into scattering and absorption coefficients and derives the wet film thickness that corresponds to 98% opacity (hiding).

Key technical topics and requirements

• Principle: Uses the Kubelka‑Munk equations to relate film reflectance to scattering (S) and absorption (a, b) coefficients and to predict hiding at a target reflectance ratio (Rfilm on black / R∞ ≈ 0.98).
• Measurements required:
  • Reflectance of paint film on a black substrate (R_B).
  • Reflectivity (R∞) - reflectance of an effectively infinitely thick film, typically measured on a white substrate.
  • CIE tristimulus Y values (Y/100 used as reflectance input).
• Apparatus and materials:
  • Substrates: polished black glass or polyester film and white/grey charts (ceramic or glass alternative).
  • Film applicators (bar or wire‑wound) producing uniform wet films (typical 40–150 µm).
  • Photoelectric reflectometer with spectral response approximating D65 × y(λ), accuracy ~0.1% full scale; 8°/diffuse geometry recommended in disputes.
  • Templates and controlled application procedure.
• Calculations and output:
  • Compute K‑M parameters (a, b) and St product from measured reflectances.
  • Derive scattering coefficient S (S = St / t) and calculate hiding power V (V = 1000 / t for the film thickness t that yields 98% hiding).
• Limitations & precision: Not suitable for fluorescent or metallic paints; standard includes repeatability and reproducibility guidance and example graphs/tables (Annexes A–C) to simplify computation.

Practical applications and who uses this standard

• Quality control and batch release testing in paint and coatings manufacturing
• R&D and formulation optimization to compare pigment loading and extender efficiency
• Third‑party testing laboratories and certification bodies verifying opacity claims
• Specifiers, architects and procurement teams assessing coating performance
• Suppliers evaluating pigment/binder tradeoffs for cost vs. hiding efficiency

Benefits include objective, reproducible hiding‑power values, reduced extrapolation compared with full spreading‑rate tests, and direct linkage to optical parameters used in color/appearance engineering.

Related standards

Normative and complementary references used in ISO 6504-1:2019:

• ISO 6504 series (other parts, e.g., ISO 6504‑3 referenced)
• ISO 1513 - sample preparation
• ISO 2808 - film thickness determination
• ISO 2811‑1 - density (pycnometer)
• ISO 3251 - non‑volatile content
• ISO 4618 - terms and definitions
• ISO 15528 - sampling

Keywords: ISO 6504-1:2019, hiding power, Kubelka‑Munk, paints and varnishes, reflectance measurement, scattering coefficient, paint opacity, white paints, light‑coloured paints.