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.
Diffuse Scattering
Meaning
Optical radiation deflection in non-specular directions caused by surface roughness, microscopic phase boundaries, or internal fillers within a polymer substrate redistributes light across multiple angles. Engineers measure diffuse scattering to quantify opacity, haze, and surface texture in injection moulded lens covers or diffuse light sheets. Irregularities smaller than light wavelengths disperse incoming rays unevenly across three-dimensional space.
Polymer matrix inhomogeneities like glass beads or mineral fillers intensify this light distribution phenomenon.
Photometric Reflection
Reflected beam intensity variations across off-specular angles provide detailed quantitative data regarding mold surface finish quality. Elevated diffuse scattering signals high micro-roughness on cavity walls, which transfers directly onto cooled poly-methyl methacrylate parts during injection cycles. Spectrophotometers map these angular intensity profiles.
Tool polishers use light reflection distributions to judge polish quality.
Optical Transmission
Transmitted light distribution through filled thermoplastic films determines backlight diffusion performance in display panels. Controlling diffuse scattering ensures uniform illumination without visible hot spots from underlying light-emitting diodes. Polycarbonate resins blended with light-scattering additives yield consistent haze values.
Processing temperatures alter filler dispersion and optical performance.
Surface Defect
Unintended surface roughing from improper mould venting or sink marks causes localized light diffusion that ruins cosmetic finishes. Excess diffuse scattering indicates localized shrinkage or severe sink marks across thick rib sections in aesthetic housings. Moulding technicians adjust pack pressure to smooth surface topography.
Consistent pack pressure reduces unwanted light dispersion.