DIN 6175 Colour Tolerances for Automotive Coatings - Solid and Effect Coatings

The standard "DIN 6175 Colour tolerances for automotive coatings - Solid and effect coatings", developed by the German Institute for Standardization (DIN), specifies the requirements for acceptable tolerances for solid and effect coatings, such as the largest acceptable colour difference between a colour sample and its realisation, i.e. between these coatings.

Essentially, this standard consists of two parts published under the same title:

• In the standard “DIN 6175-1 Part 1: Uni-paints” a distinction is made between tolerances for the receipt of delivered paint products, different areas of a car after varnishing and repair varnish of areas or parts of cars that are assessed according to the original varnish.
• The standard “DIN 6175-2 Part 2: Goniochromatic dyes” describes the requirements for determining the characteristic properties of goniochromatic dyes. A dichroic material is a material that has a reflection color that is different from its transmission color. Goniochromatic property is the phenomenon where a material exhibits a different color as a function of the viewing geometry. The aim is to deepen the knowledge of the goniochromatic properties of dichroic material, especially when used with artificial light.

The color of special effect coatings is highly dependent on the illumination and detection geometry, which gives them a very attractive appearance. As a result, these coatings have become very popular in the automotive industry and other applications such as cosmetics and security inks. Effect coatings consist of a transparent substrate containing conventional absorption pigments and flake effect pigments. The layered structure gives rise to interferences, which are the source of this special perceived effect.

To fully characterize the wide color gamut of these coatings under any illuminant and for any illumination/observation geometry, the spectral bidirectional reflectance distribution function must be measured for a large number of measurement geometries, thus providing all the information needed to characterize the color shift. However, such a measurement is not easy to perform and can only be done in very well-equipped laboratories. Moreover, the amount of data contained in a spectral bidirectional reflectance distribution function measurement is so large that it is not an easy task to obtain rapid information from them. However, due to the increasing popularity of these coatings, the development of new techniques and tools to characterize the spectral reflectance and color as a function of different irradiance/imaging geometric configurations is urgently required.

Characterization of the color of special effect coatings requires specification of geometric variables that directly affect the variation of the spectral bidirectional reflectance distribution function. A physical approach to the optics of effect coatings shows that the so-called flake-based parameters (the inclination of the interference pigments and the angle of incidence on them) are better candidates than spherical coordinates referenced to the coating surface (traditional geometric variables). The main advantage of using these geometric variables is that spherical coordinates referenced to the coating surface, unlike the polar angles, are constant as long as the values ​​of the spectral bidirectional reflectance distribution function are held constant. Therefore, characterization of the color of special effect coatings should be based on selection of measurement geometries with well-distributed values ​​of the flake-based parameters.

Although the flake-based approach has proven to be important in understanding and characterizing such coatings, for practical questions the connection between flake-based coordinates and traditional geometric variables must be preserved.

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