Meaning
Interferometric measurement techniques divide incoming light beams into displaced paths to detect phase gradients and refractive index variations across transparent media. In quality inspection of transparent moulded plastics, optical path shearing creates an interference pattern by laterally shifting a wavefront relative to itself across a test specimen. The resulting shear fringe map reveals localized variations in thickness and material density within plastic films and optical lenses.
This metrology method governs non-destructive optical characterization, applying to transparent or translucent polymers and stopping where opaque fillers extinguish light transmission.
Differential Metrology
Shearing interferometers evaluate light wavefronts without requiring an external reference flat. Implementing optical path shearing allows metrology technicians to map spatial derivatives of optical path length across precision injection moulded windows. The technique isolates local refractive index variations caused by non-uniform polymer melt cooling inside the mould cavity.
Birefringence Analysis
Anisotropic molecular orientation in moulded parts creates polarization-dependent refractive index differences. Light passing through stressed polymer regions splits into distinct polarized wavefronts that display diagnostic shear fringe shifts.
Defect Detection
Quality control laboratories deploy shearing interferometry to identify internal flow lines and weld line stress concentrations in optical components. Small changes in cavity pressure profiles alter the magnitude of wavefront displacement visible in the sheared interference image. Process engineers tune holding pressure and barrel temperature settings until optical path gradients stabilize across production runs.
