The standard "EN ISO 2813 Paints and varnishes - Determination of gloss value at 20 degrees, 60 degrees and 85 degrees", which is based on a European standard and subsequently published by the International Organization for Standardization (ISO), describes a method for determining the gloss of coatings using three geometries of 20 degrees, 60 degrees or 85 degrees. This method is suitable for measuring the gloss of non-woven coatings on flat, opaque substrates. Comparative gloss measurements are possible on test samples other than those specified above. However, it is not possible to be sure that the gloss values obtained correspond to the visual perception of gloss.
For the purposes of this standard, the following definitions apply in addition to the terms and definitions given in ISO 4618:
In principle, the gloss values on coated surfaces are determined by a reflectometric device in relation to the visual gloss perception. In this context, the ratio between the gloss of the coating and the gloss of a polished flat glass sheet with the specified reference refractive index is obtained. The gloss measurement method is determined by the following parameters:
Gloss is a visual perception caused by looking at surfaces. Specular reflection of objects is more pronounced when light is reflected from a surface in a more directional manner. Incident light is reflected only in the main specular direction on surfaces with high gloss. On matt surfaces, light is not only reflected in the main specular direction, but is also scattered diffusely at all solid angles. The more uniformly the light is distributed in space, the smaller the intensity of the directed component and the substance of the surface.
The principle of gloss measurement is based on the measurement of directionally reflected light. In this context, its intensity is measured in a defined angular area around the reflection angle. The intensity of the reflected light depends on the surface material and the incident angle. Accordingly, more light is reflected on coated surfaces with increasing incident angle. The remaining light is refracted at the air/coating interface and light is scattered or absorbed within the coating.
The gloss values are related to the reflection properties of a black, polished glass standard of the specified refractive index, not to the intensity of the incident light. The perception of gloss can vary greatly for different viewing angles. For example, matt surfaces may appear glossy under scattered light and low viewing angles because under these reflection conditions a lot of light is reflected directionally and also because the scattered intensity is low. The figure given in the standard shows the relationship between the gloss perception of visually equally graded coating samples (matte to glossy) and the gloss values assigned for the measurement angles of 20 degrees, 60 degrees and 85 degrees.
Due to the non-linearity of the characteristic curves in the figure, the brightness for each measuring angle can only be distinguished within a certain range. In practice, the following measuring angles have been proven, depending on the degree of brightness of the sample surface:
The geometry must be specified for each brightness value.
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