Coherence Scanning Calibration Procedures for High Aspect Ratio Tool Steel Sidewalls
Calibrating optical coherence scanners on tool steel sidewalls requires specular tilt mapping, transfer function correction, and noise floor subtractive adjustment.
Fringe Contrast
Meaning
Optical interferometry determines surface heights by measuring the intensity modulation of superimposed light waves. In this metrology domain, fringe contrast represents the difference between the maximum and minimum intensities of the interference pattern. When the modulation decreases, the signal-to-noise ratio drops and height errors increase.
Signal Quality
The intensity pattern arises from the phase difference between light reflected from the sample and light from a reference mirror. For a given profiler, the fringe contrast determines the clarity of the resulting fringe pattern. Clearer fringes yield more precise phase extraction and more reliable height values.
In contrast, weak modulation generates high phase noise and surface data gaps.
Modulation Decay
Several variables degrade the signal quality on molded polymer parts. Surface slopes that exceed the acceptance angle of the objective lens disperse the reflected light and weaken the interference pattern. Furthermore, surface roughness or high local absorption of the polymer resin can suppress the signal strength.
Low reflectivity of transparent plastics reduces the reflected intensity, which lowers the fringe contrast and impairs the profile measurement.
Surface Limitation
Highly polished molds produce excellent modulation, while matte finishes or textured molding surfaces reduce the signal strength. Moulders use antireflective coatings or temporary gold sputtering to enhance the fringe contrast on difficult polymer targets. When the polymer material exhibits high scatter, the profiling system fails to reconstruct the three-dimensional geometry.