Standard ISO 25178 Parameters for Optical Profilometry Inspection
ISO 25178 areal optical profilometry replaces subjective 2D stylus traces with quantitative 3D parameters that accurately govern tool wear and part demoulding.
Sal
Meaning
Spatial surface parameters measure the horizontal distance over which surface texture patterns maintain mathematical autocorrelation above a threshold value. Metrology technicians use Sal, the autocorrelation length parameter, to identify the dominant spatial grain size on textured mould cavities and plastic components. A high Sal value indicates a surface dominated by long wavelength components, while a low value indicates fine high frequency texture.
Datasheet amplitude parameters like Ra provide no information regarding this horizontal spatial scale.
Autocorrelation Analysis
Mathematical processing shifts spatial datasets across themselves to calculate correlation values as a function of distance. The shortest distance where autocorrelation drops to point two defines the characteristic spatial length. Directional surface lay produces varying correlation lengths when measured across different spatial orientations.
Isotropic textures show uniform autocorrelation decay regardless of measurement direction.
Texture Identification
Short autocorrelation lengths describe fine, densely packed micro-structures created by bead blasting or laser ablation. High values reflect coarse machining marks or wide tool feed passes on cavity walls. Resin melt viscosity affects how accurately molten polymer replicates fine spatial lengths during injection moulding.
Regrind polymer blending alters melt flow behavior, leading to incomplete filling of low Sal surface features.
Functional Boundary
Spatial length measurements require sufficient sample area to capture multiple repeating texture cycles. Inadequate optical field size under-samples long spatial components, leading to falsely reduced parameter values. Quality specifications combine spatial length data with height parameters to control part friction and visual finish.