ISO 25178 Areal Surface Texture Parameters for Optical Inspection
ISO 25178 defines 3D areal surface parameters like Sa and Sdr, replacing 2D profiles to qualify optical non-contact inspection on textured tool steel and mouldings.
Str Parameter
Meaning
Spatial texture aspect ratio calculations evaluate spatial isotropy to determine whether surface features exhibit directional lay or uniform random distribution across a surface. When assessing polished optical molds or textured interior trim, the Str parameter yields a quantitative ratio between zero and one that identifies directional tool marks or isotropic surface finishes. The calculation stops applying when spatial autocorrelation functions drop below defined threshold values.
Spatial Isotropy
Autocorrelation decay rates in orthogonal directions reveal spatial patterns on machined or molded surfaces. Within areal surface metrology, the Str parameter evaluates the ratio of fastest to slowest autocorrelation decay distances. Values close to one indicate fully isotropic surfaces like bead-blasted mold cavities, whereas values approaching zero identify strong directional lay produced by parallel milling or lathe turning lines.
Identifying surface directionality helps engineers optimize mold release forces and reduce optical glare on clear plastic lenses. Unidirectional textures cause anisotropic friction during part demolding.
Tooling Directionality
Machining marks impart distinct directional patterns on injection mold inserts. A low Str parameter confirms the presence of linear machining grooves that can cause liquid flow channeling or localized stress concentration.
Pattern Metric
Quality control laboratories track spatial isotropy parameters to ensure uniform grit-blasting or laser-texturing across complex tool geometries. Specifying an Str parameter target on part drawings ensures that cosmetic plastic components maintain consistent visual appearance regardless of illumination angle. Mold polishers use spatial parameters to confirm complete removal of directional cutter marks.
Isotropic surface structures provide uniform light diffusion across translucent lighting covers.