Optical Coherence Measurement Fundamentals for Deep Tooling Cavities
Optical coherence measurement uses broad-bandwidth infrared interference to capture deep cavity topographies without mechanical probe contact or lens clipping.
Multi Path Ghost Artifact
Meaning
Secondary reflections create false features in a reconstructed image when light travels multiple routes between the source and the sensor. A multi path ghost artifact often appears when measuring transparent or highly reflective moulded parts using optical metrology. The sensor misinterprets the delayed signal as a physical surface at a different location.
This leads to errors in the reported geometry of the part.
Signal Interference
Light bouncing off the back wall of a clear plastic component creates a second pulse. This multi path ghost artifact overlaps with the primary signal from the front surface. The resulting data point is an average of the two positions, which is physically incorrect.
Specialized software filters are needed to distinguish the true surface from these echoes to maintain accuracy.
Geometry Correlation
Complex shapes with deep recesses or internal ribs are more prone to these errors. The multi path ghost artifact occurs because the light scatters within the cavity before returning to the detector. Understanding the part geometry helps in positioning the sensor to minimize these stray reflections.
Proper alignment reduces the noise in the final point cloud.
Error Mitigation
Coating the part with a fine matte spray is a common way to eliminate these reflections during the prototyping phase. In a production environment, adjusting the incident angle or using polarized light can reduce the multi path ghost artifact. These techniques ensure that the metrology system only captures the direct reflection.
Accurate data is essential for validating the tolerances of optical grade components.