Low Coherence Interferometry

Surface Inspection

Polymer thickness monitoring requires precise control over the beam spot size and incidence angle to avoid signal saturation during the scan. Injection moulded parts often exhibit birefringence near the gate area, which distorts the interference pattern and complicates the automated detection of delamination. Operators adjust the scan speed to maintain a signal-to-noise ratio sufficient for identifying voids located beneath the outer skin of the component.

A stable thermal environment prevents drift in the reference mirror position, which would otherwise introduce errors into the calculation of wall thickness.

Material Precision

Datasheet values for refractive index provide the primary calibration constant for calculating distance from raw delay times. Moulders must account for batch-to-batch variations in additive concentration that alter the optical density of the resin, as these changes shift the apparent position of subsurface features. Recycled content increases the population of scattering centers that degrade signal clarity and limit the maximum penetration depth of the system.

Validating the accuracy of a measurement requires periodic comparison against physical gauge blocks that share similar refractive properties to the processed material.

Process Verification

Tooling performance evaluations use these scans to map the consistency of wall thickness across complex geometries. Deviations in cooling rates throughout the cavity lead to non-uniform density distributions that appear as shifts in the interferometric data during quality audits. High pressure packing stages minimize internal micro-voids, creating a clearer optical path that allows the technique to distinguish between true structural features and transient artifacts.

Consistency in the refractive index throughout the final part determines the absolute reliability of the non-destructive measurement.