Optical Coherence Scanning Measurement Limit Boundaries on Vertical Tooling Cavity Sidewalls
Side-viewing prisms and polarized optical coherence gating extend sidewall slope scanning limits beyond eighty-five degrees in deep tooling cavities.
Coherence Gating
Meaning
Optical depth discrimination isolates backscattered light signals originating from specific planar depths within a semi-transparent medium. In polymer film extrusion and multilayer barrier film inspection, coherence gating measures individual layer thicknesses without physical contact or sample destruction. The technique operates by matching the path length of a reference light beam to light reflected from internal polymer interfaces.
Application boundaries occur in heavily pigmented resins or filled compounds that scatter light beyond the coherent detection limit.
Interferometric Resolution
Path length matching creates temporal interference fringes only when optical paths fall within the coherence length of the light source. Low coherence light sources yield narrow gate windows that resolve thin coextruded barrier layers down to sub-micron scales. Broadband superluminescent diodes provide the spectral width necessary to separate reflections from adjacent polymer interfaces with minimal axial crosstalk.
Signal processing algorithms extract layer boundary positions from the envelope of the resulting interference signal.
Layer Detection
Multi-layer film structures present distinct refractive index steps at internal resin interfaces. Interface detection relies on Fresnel reflection intensity driven by the optical density contrast between adjacent polymers.
Refractive Index
Calibration requires accurate values for the wavelength dependent refractive index of each coextruded polymer layer. Inaccurate index values introduce proportional errors into the calculated physical thickness of internal layers.