Optical Profilometry Parameter Verification for Hardened Steel Tool Cavities
Verify hardened steel tool cavity topography using ISO 25178 areal parameters and filtered coherence scanning interferometry to guarantee polymer part release.
Batwing Artifacts
Meaning
Non physical spikes appearing in optical surface profiles at steep geometric transitions occur when light diffracts at the sharp edges of a measured object and interferes with the principal reflection. Batwing artifacts are specific measurement errors found in interference microscopy where the reconstructed surface height shows false peaks at step edges. In the production of plastic films and precision moulded optics, these errors distort the true topography of the part.
The phenomenon stops affecting the data once the slope of the surface becomes shallower than the numerical aperture of the objective lens allows. Measurement professionals identify batwing artifacts by looking for symmetrical height errors that do not match the physical reality of the tool or the part. These spikes can lead to the false rejection of parts that are actually within the specified tolerance.
Geometric Cause
Optical interference between the light reflected from the top of a step and the light reflected from the bottom creates the batwing artifacts. When the step height is smaller than the coherence length of the light source, the waves overlap in a way that the phase reconstruction algorithm cannot resolve. This confusion results in a calculated height that is either too high or too low at the very edge of the feature.
Sharp corners on a moulding tool, such as those used for microfluidic channels or Fresnel lenses, are particularly prone to this effect. The physics of diffraction ensures that as long as light acts as a wave, these artifacts will appear on surfaces with vertical or near vertical transitions.
Measurement Distortion
Data accuracy suffers when batwing artifacts inflate the values of peak to valley roughness parameters. If a quality control system uses the maximum height of a profile to judge the quality of a mould, these false spikes can make a smooth surface appear unacceptably rough. The error often looks like a pair of wings extending upward or downward from the edge, hence the descriptive name.
In the sourcing of high precision polymer components, failing to account for these artifacts leads to disputes between the moulder and the metrology lab. One party might see a surface that meets the finish requirement while the other sees a surface covered in narrow peaks.
Mitigation Strategy
Reducing the impact of these errors involves choosing the correct optical setup and processing the data with advanced algorithms. Using a light source with a shorter coherence length or a higher numerical aperture lens can narrow the width of the artifact. Software filters designed for surface metrology can also identify and remove the false spikes by comparing the measured profile to the known limitations of the optical system.
Moulders must specify how these artifacts are handled during the inspection of precision parts to ensure that the reported values reflect the actual physical geometry. By selecting a metrology process that accounts for diffraction limits, the production team maintains better control over the final quality of the plastic parts. This approach ensures that the reported roughness reflects the true performance of the moulded surface.