EN ISO 18314 Analytical Color Measurement

The “EN ISO 18314 Analytical color measurement” standard, developed by the International Organization for Standardization (ISO) and later published as a European standard within the scope of harmonization efforts, consists of the following sub-sections published under the same title:

• The standard “EN ISO 18314-1 Part 1: Practical color measurement” describes the method for determining the color coordinates of a paint film. This method is applicable to paint films that appear monochromatic when viewed under normal conditions. Paint films that do not completely cover an opaque surface represent an opaque system and can be measured using the procedure described in this standard. Glow-in-the-dark paint films, transparent paint films, translucent paint films (e.g., for display or lamp glass), retroreflective paint films (e.g., for traffic signs), and metallic paint films are outside the scope of this standard.

These tests utilize a spectrophotometer. Measurements should preferably be performed using spectrophotometers. The spectral measurement range should be at least between 400 nm and 700 nm. The spectral step width and optical bandwidth should be a maximum of 20 nm (preferably between 5 nm and 10 nm). A tristimulus colorimeter is also used in the tests. These can be used for applications that do not require monitoring of metamerism.

This standard specifies a practical method for determining the color coordinates of a paint film using instrumental color measurement. It applies particularly to paint films that appear uniformly monochromatic (single-color) when viewed with normal human vision. It covers measurements of opaque systems, including cases where the paint film does not completely obscure an opaque substrate (although still considered effectively opaque under the measurement procedure).

This method cannot be applied to more complex or specialized paint film types, including:

• Shimmering paint films
• Transparent paint films
• Translucent paint films (for example, for screen or lamp glass)
• Reflective paint films (e.g., traffic signs)
• Metallic paint films

These require different approaches due to their optical behavior (e.g., light propagation, transmission, or metallic effects).

• The standard “EN ISO 18314-2 Part 2: Saunderson correction, solutions of the Kubelka-Munk equation, tinting power, opacity” describes the Saunderson correction for different measurement geometries and the solutions of the Kubelka-Munk equation for opaque and transparent layers. This standard also specifies methods for calculating tinting power and opacity, including residual color difference, using different criteria. Procedures for preparing samples for these measurements are not part of this standard. These shall be agreed upon between the contracting parties or described in other national or international standards.

For color measurement calculations, it is necessary to account for surface phenomena in order to obtain valid results. The formulas are derived assuming an ideal surface, a perfect opaque layer, and perfectly diffused light scattering within the sample. Any deviation from these assumptions leads to an evaluation of the usefulness of the calculations.

The Kubelka-Munk theory describes the reflection of a pigmented layer using two constants: absorption and scattering. It is based on the following assumptions:

• Ideally, diffuse radiation distribution is used in the irradiation process.
• Ideally, the radiation distribution within the layer is diffuse.
• Failure to consider surface phenomena resulting from discontinuities in the refractive index.

This standard goes beyond basic instrumental measurement (EN ISO 18314-1) by offering advanced, computational methods for color evaluation in paints, pigments, and coatings. It defines precise computational procedures for correcting and interpreting reflectance data from paint films and pigment systems. Using classical theories in pigment optics, it focuses on opaque and transparent layers.

The key elements mentioned are as follows:

• Saunderson correction: Adjusts the measured reflection values ​​to account for surface reflection effects (specular and diffuse components) depending on the instrument geometry (e.g., d/8 degrees, 45/0 degrees). This correction is necessary for accurate internal reflection values ​​used in subsequent calculations.
• Solutions to the Kubelka-Munk equation: It provides mathematical solutions for both opaque and transparent layers. Kubelka-Munk theory models light scattering and absorption in cloudy media (such as paints) and relates reflection to absorption and scattering coefficients.
• Coloring power: Methods for calculating the coloring power of a pigment when mixed with a reference (e.g., white base), including evaluation through residual color difference based on various criteria (e.g., based on different CIE color spaces or shade depth mapping).
• Shadow depth: This relates to color intensity depending on concentration.
• Coverage power (or opacity): Procedures for determining how effectively a coating obscures the substrate; typically using contrast ratios or the minimum film thickness required for complete coverage.
• The standard “EN ISO 18314-3 Part 3: Special Indices” describes different methods for calculating special indices, which are generally used to define the lightness or darkness of samples, including chromaticity or tone within a single color coordinate. This standard applies to tristimulus values ​​and chromaticity coordinates calculated using color matching functions of the CIE 1931 (2 degrees) or CIE 1964 (10 degrees) standard colorimetric systems. It is used to specify color stimuli perceived as belonging to a reflective or transmitting object where a one-dimensional value is required.

The indexes described in this standard are as follows:

• CIE whiteness index
• Jaundice index
• Blackness values
• Blackness value independent of color tone.
• Blackness value depending on color tone.
• The absolute contribution of color tone
• Gray scale values
• Gray scale value independent of color tone.
• Grayness value depending on color tone.
• The absolute contribution of color tone
• Flop index

This standard specifies methods for calculating proprietary indices that provide a single numerical value to describe the perceived color quality of nearly monochromatic samples (e.g., deep blacks, intense reds, or pure whites). These indices go beyond standard CIELAB coordinates, focusing on specific characteristics such as lightness or darkness, often including subtle chromaticity or tonal effects. They apply to:

• Triple stimulus values ​​(X, Y, Z) and chromaticity coordinates.
• Reflective or transmitting objects perceived as belonging to a single color coordinate.
• Color stimuli requiring a one-dimensional descriptor.

These methods are particularly useful when a simple, interpretable number is needed for quality control or specification, rather than multidimensional color differences.

• The standard “EN ISO 18314-4 Part 4: Index of metamerism for pairs of samples depending on the change in lighting source” describes a formalism for calculating the illumination metamerism of solid surface colors. This standard cannot be applied to the colors of effect coatings without metric adaptation.

This standard covers the phenomenon of metamerism due to lighting variation, which is of paramount importance in practical application. It provides recommendations on what corrective measures should be taken when the chromatic coordinates of a pair of samples do not exactly match under reference conditions. In relation to color reproduction, the metamerism index is used as a quality measure to determine tolerances for color differences between a color sample and a color match under different lighting conditions.

Quantification of the illumination metamerism of sample pairs is formally performed with a color difference assessment, where tolerances commonly used in evaluating color differences can be employed.

This standard specifies a formalism (standardized calculation method) for determining the luminescence metamerism index (luminescence exchange metamerism) between pairs of solid surface color samples.

Metamerism occurs when two colors appear to match (or very similar) under one light source/illuminator but differ significantly under another light source/illuminator.

Key features:

• This applies to solid, effect-free surface colors (uniform, non-metallic, non-pearlescent coatings).
• This is not applicable to effect coatings (e.g., metallic, pearlescent, interference pigments) without specific metric customization.
• It calculates a metamerism index that measures the degree of mismatch when illuminators are changed.
• It is based on spectral reflectance data and CIE colorimetry principles.
• The standard “EN ISO 18314-5 Part 5: Procedure for the colorimetric determination of color differences of object colors according to equidistant color spaces” describes the procedure and test report for determining small color differences of object colors according to equidistant color spaces. The appendices to this standard include three proposed criteria for quantitatively determining small color differences of non-luminous colors. These examples relate to three color space models:
• Appendix A: A common modern uniform color difference formula (e.g., based on CIEDE2000 or similar advanced criteria).
• Appendix B: Another alternative equidistant approach.
• Appendix C: A third method recommended for specific applications.

These criteria improve accuracy for small color differences where the traditional CIELAB color difference might be perceptually less uniform (especially in neutral, dark, or saturated regions).

This standard applies to the evaluation of pigments, dyes, and fillers in coatings, plastics, and printing applications that are assessed in pairs for color match and have only minor, perceptible color differences.

To determine subtle color differences between object colors according to equidistant color spaces, one of the three procedures described in the appendices should be used. Three suggested metrics for quantitatively determining subtle color differences in unlit colors are detailed in these appendices for your information. The calculation metric used for color difference should be stated in the test report.

When the formulas described in the appendices are used for large color differences (greater than approximately 10 CIELAB units), larger discrepancies between calculated color differences and visual assessments should be expected. This is due to the nonlinear relationship between small and large color differences. If the use of formulas within this range is intended, it should be explicitly agreed upon between the relevant parties.

This standard defines standardized procedures and requirements for test reporting when calculating and reporting subtle color differences between object colors (non-luminous, surface colors such as paints and coatings). It emphasizes the use of equidistant color spaces, where equidistant colors correspond more closely to perceived color differences. The document provides three recommended criteria for such calculations (detailed in the informative appendices):

• The standard “EN ISO 18314-6 Part 6: Conditions for measuring the colors of objects” is still under development.

For many years, our organization, with its competent and experienced staff, has been understanding the needs of its clients, providing the advanced testing services they require, and assisting in the establishment, implementation, and improvement of management systems. Within this framework, we also provide testing services in accordance with the "EN ISO 18314 Analytical color measurement" standard.